Showing 1–50 of 204 results for author: Leja, J

Ionizing Photon Production Efficiencies and Chemical Abundances at Cosmic Dawn Revealed by Ultra-Deep Rest-Frame Optical Spectroscopy of JADES-GS-z14-0

Authors: Jakob M. Helton, Jane E. Morrison, Kevin N. Hainline, Francesco D'Eugenio, George H. Rieke, Stacey Alberts, Stefano Carniani, Joel Leja, Yijia Li, Pierluigi Rinaldi, Jan Scholtz, Meredith Stone, Christopher N. A. Willmer, Zihao Wu, William M. Baker, Andrew J. Bunker, Stephane Charlot, Jacopo Chevallard, Nikko J. Cleri, Mirko Curti, Emma Curtis-Lake, Eiichi Egami, Daniel J. Eisenstein, Peter Jakobsen, Zhiyuan Ji , et al. (18 additional authors not shown)

Abstract: JWST has discovered an early period of galaxy formation that was more vigorous than expected, which has challenged our understanding of the early Universe. In this work, we present the longest spectroscopic integration ever acquired by JWST/MIRI. This spectrum covers the brightest rest-frame optical nebular emission lines for the luminous galaxy JADES-GS-z14-0 at $z > 14$. Most notably, we detect… ▽ More JWST has discovered an early period of galaxy formation that was more vigorous than expected, which has challenged our understanding of the early Universe. In this work, we present the longest spectroscopic integration ever acquired by JWST/MIRI. This spectrum covers the brightest rest-frame optical nebular emission lines for the luminous galaxy JADES-GS-z14-0 at $z > 14$. Most notably, we detect $[\mathrm{OIII}] λλ4959,5007$ at $\approx 11 σ$ and $\mathrm{H}α$ at $\approx 4 σ$ with these ultra-deep observations. These lines reveal that JADES-GS-z14-0 has low dust attenuation with a recent star-formation rate of $\mathrm{SFR} \approx 10 \pm 2\ M_{\odot} / \mathrm{yr}$, star-formation rate surface density of $Σ_{\mathrm{SFR}} \approx 23 \pm 5\ M_{\odot}/\mathrm{yr}/\mathrm{kpc}^{2}$, and ionizing photon production efficiency of $ξ_{\mathrm{ion}} \approx 10^{25.3 \pm 0.1}\ \mathrm{Hz/erg}$. Using standard strong-line diagnostics, we infer a gas-phase oxygen abundance of $[\mathrm{O/H}] \approx -1.1 \pm 0.4$ ($\approx 10\%\ Z_{\odot}$), carbon-to-oxygen ratio of $[\mathrm{C/O}] \approx -0.4 \pm 0.4$, ionization parameter of $\mathrm{log}_{10}(U) \gtrsim -2.4$, and density of $n_{\mathrm{H}} \approx 720 \pm 210\ \mathrm{cm}^{-3}$. Using detailed photoionization modeling, we instead derive $[\mathrm{O/H}] \approx -0.3_{-0.4}^{+0.4}$ ($\approx 50\%\ Z_{\odot}$) and $\mathrm{log}_{10}(U) \approx -1.5_{-0.4}^{+0.3}$. The inferred properties of JADES-GS-z14-0 are similar to those measured for similarly luminous galaxies at $z > 10$ with previous MIRI/Spectroscopy, such as GHZ2/GLASSz12, GN-z11, and MACS0647-JD1. Existing simulations are unable to reproduce the empirical and inferred properties of JADES-GS-z14-0. This work demonstrates an important step toward understanding the formation of the first stars and heavy elements in the Universe. [Abridged] △ Less

Submitted 22 December, 2025; originally announced December 2025.

Comments: Submitted to ApJL; main text has 23 pages, 8 figures, and 2 tables; comments are welcome!

The LEGA-C galaxy survey: multiple quenching channels for quiescent galaxies at $z\sim1$

Authors: Angelos Nersesian, Yasha Kaushal, Marco Martorano, Arjen van der Wel, Po-Feng Wu, Rachel Bezanson, Eric F. Bell, Francesco D'Eugenio, Anna R. Gallazzi, Joel Leja, Stefano Zibetti, Sandro Tacchella

Abstract: We analyzed the sizes and star-formation histories (SFHs) of 2908 galaxies with $M_\star \geq 10^9$ M$_\odot$ at $0.6 < z < 1.0$, drawn from the LEGA-C survey. The goal is to investigate the connection between galaxy sizes with SFH, stellar age, and metallicity. SFHs were derived with Prospector by fitting the high signal-to-noise, high spectral resolution spectroscopy drawn from the LEGA-C DR3 to… ▽ More We analyzed the sizes and star-formation histories (SFHs) of 2908 galaxies with $M_\star \geq 10^9$ M$_\odot$ at $0.6 < z < 1.0$, drawn from the LEGA-C survey. The goal is to investigate the connection between galaxy sizes with SFH, stellar age, and metallicity. SFHs were derived with Prospector by fitting the high signal-to-noise, high spectral resolution spectroscopy drawn from the LEGA-C DR3 together with the broadband photometry from the UltraVISTA catalog. Galaxy sizes were measured by fitting a 2D S{é}rsic profile to the HST ACS~F814W images. We find diverse SFHs and quenching timescales ($τ_\rm{q}$). The main quiescent population quenched over $τ_\rm{q}=1.23\pm0.04$ Gyr, whereas compact post-starburst galaxies (PSBs) quenched much faster, $τ_\rm{q}=0.13\pm0.03$ Gyr. At fixed stellar mass, smaller quiescent galaxies quenched more rapidly than larger ones; at fixed size, the dependence on stellar mass is weak. Larger quiescent galaxies are marginally younger, quenched more slowly, and have near-solar metallicities, while compact quiescent galaxies are older, metal-rich, and quenched faster. PSBs formed half their mass later ($z_\rm{form}\sim1.9$) and quenched on the shortest timescales. The general trends with galaxy size, $Z_\star$, and $z_\rm{form}$ for the quiescent populations remain consistent regardless of the method used to derive the stellar properties. We conclude that compact quiescent galaxies are consistent with both early, moderately fast quenching and with more rapid, late quenching. While this may suggest the existence of multiple quenching channels, our data are also compatible with a continuous distribution of quenching timescales. These findings suggest that different physical mechanisms may drive quenching across galaxy populations, potentially leading to similar morphological outcomes despite differing evolutionary histories. △ Less

Submitted 15 December, 2025; v1 submitted 11 December, 2025; originally announced December 2025.

Comments: 16 pages, 12 figures, accepted

LEGA-C stellar populations scaling relations. I: Chemo-archaeological downsizing trends at z~0.7

Authors: Anna R. Gallazzi, Stefano Zibetti, Arjen van der Wel, Angelos Nersesian, Yasha Kaushal, Rachel Bezanson, Francesco D'Eugenio, Eric F. Bell, Joel Leja, Laura Scholz-Diaz, Po-Feng Wu, Camilla Pacifici, Michael Maseda, Daniele Mattolini

Abstract: We analyze stellar population properties of 552 galaxies at redshift 0.6<z<0.77 from the LEGA-C spectroscopic survey. This first paper in a series presents the catalog of revised absorption indices for LEGA-C DR3 and inferred physical parameters, and derives benchmark scaling relations for the general massive galaxy population at intermediate redshift. We estimate light-weighted mean ages and stel… ▽ More We analyze stellar population properties of 552 galaxies at redshift 0.6<z<0.77 from the LEGA-C spectroscopic survey. This first paper in a series presents the catalog of revised absorption indices for LEGA-C DR3 and inferred physical parameters, and derives benchmark scaling relations for the general massive galaxy population at intermediate redshift. We estimate light-weighted mean ages and stellar metallicities by interpreting key stellar absorption features and rizYJ photometry in a Bayesian framework with a comprehensive library of model spectra based on stochastic star formation and metallicity histories and dust attenuations. We discuss systematic uncertainties within our method and compared to other spectral fitting approaches. We derive volume-weighted scaling relations of light-weighted mean ages and stellar metallicities with stellar mass for the general galaxy population at <z>=0.7 and masses >10^10Msun. The downsizing trends observed locally were already in place 6 Gyr ago. We observe bimodal age distribution as a function of mass, transitioning around 10^11Msun. No bimodality appears in the stellar metallicity-mass relation, which changes from steep to flat across 10^10.8Msun. Similar trends emerge for age and metallicity with velocity dispersion, but with sharper transition from young to old around log(sigma)=2.3. Differences with respect to trens with stellar mass suggest that age primarily depends on velocity dispersion below and above the transition regime, while both stellar mass and velocity dispersion contribute to stellar metallicity. The catalogs of revised absorption index measurements for LEGA-C DR3 and inferred stellar population physical parameters will be released to public repositories. (Abridged) △ Less

Submitted 8 December, 2025; originally announced December 2025.

Comments: 20 pages of main text, 10 pages Appendices with discussion on systematics, 10 figures in main text, 8 figures in Appendices; Resubmitted to A&A after revision. This is the first paper of a series, accompanying arXiv:2511.11805

The Mass-Metallicity Relation and its Observational Effects at z~3-6

Authors: Zach Lewis, Michael V. Maseda, Anna de Graaff, Joel Leja, Bingjie Wang, Hans-Walter Rix, Ian McConachie, Nikko J. Cleri, Rachel Bezanson, Leindert A. Boogaard, Gabriel Brammer, Jenny E. Greene, Michaela Hirschmann, Harley Katz, Ivo Labbe, Jorryt Matthee, Tim B. Miller, Rohan P. Naidu, Pascal A. Oesch, David J. Setton, Katherine A. Suess, Andrea Weibel, Katherine E. Whitaker, Christina C. Williams

Abstract: The correlation between galaxy stellar mass and gas-phase metallicity, known as the mass-metallicity relation (MZR), gives key insights into the processes that govern galaxy evolution. However, unquantified observational and selection biases can result in systematic errors in attempts to recover the intrinsic MZR, particularly at higher redshifts. We characterize the MZR at z~3-6 within a fully Ba… ▽ More The correlation between galaxy stellar mass and gas-phase metallicity, known as the mass-metallicity relation (MZR), gives key insights into the processes that govern galaxy evolution. However, unquantified observational and selection biases can result in systematic errors in attempts to recover the intrinsic MZR, particularly at higher redshifts. We characterize the MZR at z~3-6 within a fully Bayesian framework using JWST NIRSpec spectra of 193 galaxies from the RUBIES survey. We forward model the observed mass-metallicity surface using prospector-generated spectra to account for two selection biases: the survey selection function and success in observing high signal-to-noise emission lines. We demonstrate that the RUBIES selection function, based on F444W magnitude and F150W-F444W color, has a negligible effect on our measured MZR. A correct treatment of the non-Gaussian metallicity uncertainties from strong-line calibrations lowers the derived MZR normalization by 0.2 dex and flattens the slope by ~20%; forward-modeling the effect of emission line observability steepens the slope by ~15%. Both of these biases must be taken into account in order to properly measure the intrinsic MZR. This novel forward modeling process motivates careful consideration of selection functions in future surveys, and paves the way for robust, high-redshift chemical enrichment studies that trace the evolution of the mass-metallicity relation across cosmic time. △ Less

Submitted 2 December, 2025; originally announced December 2025.

Comments: 16 pages, 8 figures, submitted to ApJ

Little Red Dots host Black Hole Stars: A unified family of gas-reddened AGN revealed by JWST/NIRSpec spectroscopy

Authors: Anna de Graaff, Raphael E. Hviding, Rohan P. Naidu, Jenny E. Greene, Tim B. Miller, Joel Leja, Jorryt Matthee, Gabriel Brammer, Harley Katz, Rachel Bezanson, Leindert A. Boogaard, Sownak Bose, John Chisholm, Nikko J. Cleri, Pratika Dayal, Robert Feldmann, Yoshinobu Fudamoto, Seiji Fujimoto, Lukas J. Furtak, Karl Glazebrook, Rashmi Gottumukkala, Kasper E. Heintz, Vasily Kokorev, Ivo Labbe, Michael V. Maseda , et al. (12 additional authors not shown)

Abstract: We use the DAWN JWST Archive to construct and characterise a sample of 116 little red dots (LRDs) across 2.3<z<9.3, selecting all sources with v-shaped UV-optical continua from NIRSpec/PRISM spectra and compact morphologies in NIRCam/F444W imaging. We show that LRD continuum spectra are ubiquitously well described by modified blackbodies across ~$0.4-1.0μ$m, with typical T~5000K or $λ_{peak}$~… ▽ More We use the DAWN JWST Archive to construct and characterise a sample of 116 little red dots (LRDs) across 2.3<z<9.3, selecting all sources with v-shaped UV-optical continua from NIRSpec/PRISM spectra and compact morphologies in NIRCam/F444W imaging. We show that LRD continuum spectra are ubiquitously well described by modified blackbodies across ~$0.4-1.0μ$m, with typical T~5000K or $λ_{peak}$~$0.65μ$m across 2 dex in luminosity, and a tail toward T~2000K. LRDs therefore trace a locus in the Hertzsprung-Russell diagram that is directly analogous to stars on the Hayashi track, strongly supporting the picture that LRDs are AGN embedded in thermalised dense gas envelopes in approximate hydrostatic equilibrium. Hotter LRDs with $λ_{peak}<0.65μ$m typically have strong Balmer breaks, redder UV slopes and high optical luminosities; other LRDs show weak or no Balmer breaks, and wide variety in $β_{UV}$ and $L_{5100}$. Crucially, we demonstrate that the UV-optical continuum shapes and luminosities are strongly linked to the $Hα,\ Hβ$, [OIII] and OI line properties. There is a tight linear relation between the H$α$ and optical continuum luminosities, as well as H$α$ and OI$_{8446}$, indicating that Balmer, OI and optical emission must primarily be powered by the same source. The Balmer decrement increases strongly toward higher $L_{Hα}$, $L_{5100}$ and Balmer break strength, providing key evidence for luminosity-dependent effects of collisional (de-)excitation and resonant scattering in the gaseous envelopes. In contrast, we show that [OIII] emission likely originates from star-forming host galaxies, and that its strong correlation with Balmer break strength arises naturally from variation in the AGN-to-host ratio. Our work presents an empirical description of the nature and structure of LRDs, defining a new benchmark for ongoing LRD model developments. △ Less

Submitted 26 November, 2025; originally announced November 2025.

Comments: 22 pages, 18 figures, submitted to MNRAS. LRD sample and measurements are publicly available at https://doi.org/10.5281/zenodo.17665942

LEGA-C stellar populations scaling relations. II: Dissecting mass-complete archaeological trends and their evolution since z~0.7 with LEGA-C and SDSS

Authors: Anna R. Gallazzi, Stefano Zibetti, Arjen van der Wel, Angelos Nersesian, Yasha Kaushal, Rachel Bezanson, Daniele Mattolini, Eric F. Bell, Laura Scholz-Diaz, Joel Leja, Francesco D'Eugenio, Po-Feng Wu, Camilla Pacifici, Michael Maseda

Abstract: With a sample of 552 galaxies at z~0.7 from the LEGA-C survey, we investigate how current star formation influences light-weighted mean stellar ages and metallicities, and their median trends with stellar mass or velocity dispersion. The bimodality in the global age-mass relation stems from the different age distributions in the quiescent (Q) and star-forming (SF) populations. A bimodality is not… ▽ More With a sample of 552 galaxies at z~0.7 from the LEGA-C survey, we investigate how current star formation influences light-weighted mean stellar ages and metallicities, and their median trends with stellar mass or velocity dispersion. The bimodality in the global age-mass relation stems from the different age distributions in the quiescent (Q) and star-forming (SF) populations. A bimodality is not observed in the stellar metallicity-mass relation, although Q and SF galaxies have different distributions in this parameter space. We identify a high-metallicity sequence populated by both Q and weakly SF galaxies. At masses below logM/Msun=10.8 the median stellar metallicity-mass relation of SF galaxies steepens, as a consequence of increasing scatter toward lower stellar metallicities for galaxies with increasing specific star formation rate at fixed mass. With a consistent analysis of SDSS DR7 spectra, accounting for aperture corrections, we quantify the evolution of the stellar age and stellar metallicity scaling relations between z=0.7 and the present. We find negligible evolution in the stellar metallicity-mass relation of Q galaxies and for logM/Msun>11 galaxies in general. Lower mass SF galaxies, instead, have typically lower metallicities than their local counterparts, indicating significant enrichment since z~0.7 in the low-mass regime. The median of the stellar ages of both the general population and Q galaxies has changed by only 2 Gyr between z=0.7 and z=0.1, less than expected from cosmic aging. Some Q galaxies must evolve passively to reach the old boundary of the local population. However, in order to explain the evolution of the median trends, both individual evolution, through rejuvenation and/or minor merging impacting the outer galaxy regions, and population evolution, through quenching of massive, metal-rich star-forming galaxies, are required. (Abridged) △ Less

Submitted 14 November, 2025; originally announced November 2025.

Comments: 23 pages including Appendices, 7 figures in main text, 3 figures in Appendices. Accepted for publication on A&A. This is the second paper of a series. Paper I will be posted soon

JWST/NIRSpec Reveals a Small Population of Dominant Dust-Obscured Ionizing Sources in Galaxies at 1 < z < 3

Authors: Si-Rui Ge, Nikko J. Cleri, Joel Leja, Antonello Calabro, Vital Fernandez

Abstract: Rest-frame optical emission line diagnostics are often used to help classify ionizing sources within galaxies. However, rest-frame optical tracers can miss sources with high dust attenuation, leading to misclassification of the dominant ionizing source. Longer wavelength tracers, such as those in the near-infrared, carry the power to diagnose ionizing sources while being more robust than optical t… ▽ More Rest-frame optical emission line diagnostics are often used to help classify ionizing sources within galaxies. However, rest-frame optical tracers can miss sources with high dust attenuation, leading to misclassification of the dominant ionizing source. Longer wavelength tracers, such as those in the near-infrared, carry the power to diagnose ionizing sources while being more robust than optical tracers to the presence of dust. The diagnostics used in this work employ the ratios of bright near-infrared emission lines [S III] 9530, [Fe II] 12566 and [Fe II] 16443 to Paschen lines in publicly-available JWST/NIRSpec MSA medium-resolution spectroscopy of 55 galaxies at z < 3. We compare the rest-frame near-infrared and rest-frame optical diagnostics and find that ~90% of our sample have consistent classifications across wavelengths (49/55), while the remaining sources can be explained through ionizing radiation obscured by dust and/or elevated N/Fe abundances. We identify three objects classified as star-forming in the rest frame optical and as active galactic nuclei (AGN) in the rest-frame near-infrared, which we interpret as obscured AGN. We also identify three objects which are classified as AGN in the rest-frame optical and star forming in the rest-frame near-infrared. We interpret two of these objects as AGN with obscured star formation and the other with elevated N/Fe. We discuss how future spatially-resolved and/or mid-infrared spectroscopy can test the relative contributions of AGN and stars to the ionizing photon budgets of these disagreeing sources. △ Less

Submitted 11 November, 2025; originally announced November 2025.

Comments: 9 pages + appendix and references, 4 figures, submitted to ApJ

Where Galaxies Go to Die: The Environments of Massive Quiescent Galaxies at $3<z<5$

Authors: Ian McConachie, Anna de Graaff, Michael V. Maseda, Joel Leja, Yunchong Zhang, David J. Setton, Rachel Bezanson, Leindert A. Boogaard, Gabriel Brammer, Nikko J. Cleri, Olivia R. Cooper, Karl Glazebrook, Rashmi Gottumukkala, Jenny E. Greene, Andy D. Goulding, Michaela Hirschmann, Ivo Labbe, Zach Lewis, Jorryt Matthee, Tim B. Miller, Rohan P. Naidu, Pascal A. Oesch, Sedona H. Price, Themiya Nanayakkara, Katherine A. Suess , et al. (3 additional authors not shown)

Abstract: At low redshift, massive quiescent galaxies (MQGs) are most frequently found in massive, rich galaxy clusters, but at high redshift the trend is less clear. Here, we present spectroscopic evidence of the effects of environment on the formation and assembly of high-redshift MQGs. We identify 25 (5) $\log (M_*/\mathrm{M_\odot}\geq10.5$ ($10.0\leq\log (M_*/\mathrm{M_\odot}<10.5$) spectroscopically-co… ▽ More At low redshift, massive quiescent galaxies (MQGs) are most frequently found in massive, rich galaxy clusters, but at high redshift the trend is less clear. Here, we present spectroscopic evidence of the effects of environment on the formation and assembly of high-redshift MQGs. We identify 25 (5) $\log (M_*/\mathrm{M_\odot}\geq10.5$ ($10.0\leq\log (M_*/\mathrm{M_\odot}<10.5$) spectroscopically-confirmed quiescent galaxies in the UDS and EGS fields at $3<z<5$ with NIRSpec PRISM spectroscopy from RUBIES and other public JWST NIRSpec programs. We measure the density contrast in these fields by applying a Monte Carlo Voronoi Tesselation density mapping technique to photometric and spectroscopic redshifts of $m_\mathrm{F444W}<27.5$ sources. We robustly detect 12 massive overdense peaks with $\log (M_\mathrm{Peak}/\mathrm{M_\odot})\geq13$ and six extended massive protoclusters ($\log (M_\mathrm{Struct}/\mathrm{M_\odot})\geq13.85$). We observe that MQGs are preferentially found in these massive peaks and within these massive structures: $\approx50\%$ of MQGs are found in massive peaks, compared to $\approx20\%$ of massive star forming galaxies (MSFGs) and $\approx15\%$ of the overall spectroscopically-confirmed population. We also find an apparent dependence on both quiescent galaxy mass and environment, with $75\%$ of the most massive ($\log (M_*/\mathrm{M_\odot}\geq10.75$) residing inside overdense peaks. We compare the star formation histories (SFHs) of the MQGs with the high-redshift galaxy stellar mass function from observations and simulated quiescent galaxies at $z>5$, finding that the masses from the inferred MQG SFHs regularly exceed either observed or simulated high-redshift galaxies, which suggests indicates that mergers and ex-situ star formation play a key role in the mass assembly of MQGs in overdense environments. △ Less

Submitted 28 October, 2025; originally announced October 2025.

Comments: Submitted to ApJ; 27 pages, 14 figures, 4 tables. Comments welcome! Animation and interactive figure hosted on Google Drive at https://drive.google.com/drive/folders/1LkQTPyNo83WAVEVw4-NUTvzmTtD0NVVb?usp=share_link

Rapid, out of equilibrium metal enrichment indicated by a flat mass-metallicity relation at z~6 from NIRCam grism spectroscopy

Authors: Gauri Kotiwale, Jorryt Matthee, Daichi Kashino, Aswin P. Vijayan, Alberto Torralba, Claudia Di Cesare, Edoardo Iani, Rongmon Bordoloi, Joel Leja, Michael V. Maseda, Sandro Tacchella, Irene Shivaei, Kasper E. Heintz, A. Lola Danhaive, Sara Mascia, Ivan Kramarenko, Benjamín Navarrete, Ruari Mackenzie, Rohan P. Naidu, David Sobral

Abstract: We aim to characterise the mass-metallicity relation (MZR) and the 3D correlation between stellar mass, metallicity and star-formation rate (SFR) known as the fundamental metallicity relation (FMR) for galaxies at $5<z<7$. Using $\sim800$ [O III] selected galaxies from deep NIRCam grism surveys, we present our stacked measurements of direct-$T\rm_e$ metallicities, which we use to test recent stron… ▽ More We aim to characterise the mass-metallicity relation (MZR) and the 3D correlation between stellar mass, metallicity and star-formation rate (SFR) known as the fundamental metallicity relation (FMR) for galaxies at $5<z<7$. Using $\sim800$ [O III] selected galaxies from deep NIRCam grism surveys, we present our stacked measurements of direct-$T\rm_e$ metallicities, which we use to test recent strong-line metallicity calibrations. Our measured direct-$T\rm_e$ metallicities ($0.1$-$0.2\,\rm Z_\odot$ for M$_\star$ $\approx5\times10^{7-9}$ M$_{\odot}$, respectively) match recent JWST/NIRSpec-based results. However, there are significant inconsistencies between observations and hydrodynamical simulations. We observe a flatter MZR slope than the SPHINX$^{20}$ and FLARES simulations, which cannot be attributed to selection effects. With simple models, we show that the effect of an [O III] flux-limited sample on the observed shape of the MZR is strongly dependent on the FMR. If the FMR is similar to the one in the local Universe, the intrinsic high-redshift MZR should be even flatter than observed. In turn, a 3D relation where SFR correlates positively with metallicity at fixed mass would imply an intrinsically steeper MZR. Our measurements indicate that metallicity variations at fixed mass show little dependence on the SFR, suggesting a flat intrinsic MZR. This could indicate that the low-mass galaxies at these redshifts are out of equilibrium and that metal enrichment occurs rapidly in low-mass galaxies. However, being limited by our stacking analysis, we are yet to probe the scatter in the MZR and its dependence on SFR. Large carefully selected samples of galaxies with robust metallicity measurements can put tight constraints on the high-redshift FMR and, help to understand the interplay between gas flows, star formation and feedback in early galaxies. △ Less

Submitted 1 December, 2025; v1 submitted 22 October, 2025; originally announced October 2025.

Comments: 18 pages, 11 figures, 5 Tables. Main result: Fig. 6 (direct-Te MZR) and 9 (Impact of selection effects). Accepted for publication in A&A

The slope and scatter of the star forming main sequence at z~5 : reconciling observations with simulations

Authors: Claudia Di Cesare, Jorryt Matthee, Rohan P. Naidu, Alberto Torralba, Gauri Kotiwale, Ivan G. Kramarenko, Jeremy Blazoit, Joakim Rosdahl, Joel Leja, Edoardo Iani, Angela Adamo, Alba Covelo-Paz, Lukas J. Furtak, Kasper E. Heintz, Sara Mascia, Benjamín Navarrete, Pascal A. Oesch, Michael Romano, Irene Shivaei, Sandro Tacchella

Abstract: Galaxies exhibit a tight correlation between their star-formation rate and stellar mass over a wide redshift range known as the star-forming main sequence (SFMS). With JWST, we can now investigate the SFMS at high redshifts down to masses of $\sim10^6$ M$_{\odot}$, using sensitive star-formation rate tracers such as H$α$ emission -- which allow us to probe the variability in star formation histori… ▽ More Galaxies exhibit a tight correlation between their star-formation rate and stellar mass over a wide redshift range known as the star-forming main sequence (SFMS). With JWST, we can now investigate the SFMS at high redshifts down to masses of $\sim10^6$ M$_{\odot}$, using sensitive star-formation rate tracers such as H$α$ emission -- which allow us to probe the variability in star formation histories. We present inferences of the SFMS based on 316 H$α$-selected galaxies at $z\sim4$-$5$ with $\log(\rm M_\star/M_\odot) = 6.4$ -$10.6$. These galaxies were identified behind the Abell 2744 lensing cluster with NIRCam grism spectroscopy from the ``All the Little Things'' (ALT) survey. At face value, our data suggest a shallow slope of the SFMS (SFR $\propto \mathrm{M}_\star^α$, with $α=0.45$). After correcting for the H$α$-flux limited nature of our survey using a Bayesian framework, the slope steepens to $α= 0.59^{+0.10}_{-0.09}$, whereas current data on their own are inconclusive on the mass dependence of the scatter. These slopes differ significantly from the slope of $\approx1$ expected from the observed evolution of the galaxy stellar mass function and from simulations. When fixing the slope to $α=1$, we find evidence for a decreasing intrinsic scatter with stellar mass (from $\approx 0.5$ dex at M$_\star=10^8$ M$_\odot$ to $0.4$ dex at M$_\star=10^{10}$ M$_\odot$). This tension might be explained by a (combination of) luminosity-dependent SFR(H$α$) calibration, a population of (mini)-quenched low-mass galaxies, or underestimated dust attenuation in high-mass galaxies. Future deep observations across facilities can quantify these processes, enabling better insights into the variability of star formation histories. △ Less

Submitted 21 October, 2025; originally announced October 2025.

Comments: Main text 15 pages with 11 figures. Figure 7 shows the key measurements and Fig 8 model inference. Submitted to A&A. Comments welcome

Discovery of red galaxy candidates at z ~ 12: Early dust growth or significant nebular emission with high-temperature stars?

Authors: Ikki Mitsuhashi, Katherine A. Suess, Joel Leja, Pratika Dayal, Robert Feldmann, Seiji Fujimoto, Harley Katz, Themiya Nanayakkara, Desika Narayanan, Sedona H. Price, John R. Weaver, Christina C. Williams, Ivo Labbe, Rachel Bezanson, Hakim Atek, Gabriel Brammer, Sam E. Cutler, Lukas J. Furtak, Richard Pan, Bingjie Wang, Katherine E. Whitaker

Abstract: We report the discovery of two z ~ 12 galaxy candidates with unusually red UV slopes (betaUV ~> -1.5), and probe the origin of such colors at cosmic dawn. From Prospector fits to the UNCOVER/MegaScience dataset -- deep JWST/NIRCam imaging of Abell 2744 in 20 broad- and medium-bands -- we identify several new z > 10 galaxies. Medium-band data improve redshift estimates, revealing two lensed (mu ~ 3… ▽ More We report the discovery of two z ~ 12 galaxy candidates with unusually red UV slopes (betaUV ~> -1.5), and probe the origin of such colors at cosmic dawn. From Prospector fits to the UNCOVER/MegaScience dataset -- deep JWST/NIRCam imaging of Abell 2744 in 20 broad- and medium-bands -- we identify several new z > 10 galaxies. Medium-band data improve redshift estimates, revealing two lensed (mu ~ 3.3) z ~ 12 galaxies in a close pair with beta_UV ~> -1.5 at an UV absolute magnitude of M_UV ~ -19 mag, lying away from typical scatter on previously known MUV-betaUV relations. SED fitting with Prospector, Bagpipes, and EAZY support their high-z nature, with probability of low-z interlopers of p(z < 7) < 10%. The potential low-z interlopers are z ~ 3 quiescent galaxies (QGs), but unexpected to be detected at the given field of view unless z ~ 3 QG stellar mass function has a strong turn up at log Mstar/Msun ~ 9. Unlike typical blue high-redshift candidates (beta_UV ~< -2.0), these red slopes require either dust or nebular continuum reddening. The dust scenario implies Av ~ 0.8 mag, which is larger than theoretical predictions, but is consistent with a dust-to-stellar mass ratio (log M_dust/M_star ~ -3). The nebular scenario demands dense gas (log nH /cm^3 ~ 4.0) around hot stars (log Teff [K] ~ 4.9). Spectroscopic follow-up is essential to determine their true nature and reveal missing galaxies at the cosmic dawn. △ Less

Submitted 15 October, 2025; originally announced October 2025.

The Blue Jay Survey: Deep JWST Spectroscopy for a Representative Sample of Galaxies at Cosmic Noon

Authors: Sirio Belli, Letizia Bugiani, Minjung Park, J. Trevor Mendel, Rebecca L. Davies, Amir H. Khoram, Benjamin D. Johnson, Joel Leja, Sandro Tacchella, Vanessa Brown, Charlie Conroy, Razieh Emami, Yijia Li, Caterina Liboni, Gabriel Maheson, Elijah P. Mathews, Rohan P. Naidu, Erica J. Nelson, Bryan A. Terrazas, Rainer Weinberger

Abstract: We present the Blue Jay survey, a Cycle-1 JWST program aimed at studying the stellar and gas content of galaxies at Cosmic Noon. The survey consists of deep spectroscopy for 153 targets observed over two pointings in the COSMOS field using the NIRSpec micro-shutter assembly (MSA). We employ the three medium-resolution gratings G140M, G235M, and G395M, with exposure times of 13 hours, 3.2 hours, an… ▽ More We present the Blue Jay survey, a Cycle-1 JWST program aimed at studying the stellar and gas content of galaxies at Cosmic Noon. The survey consists of deep spectroscopy for 153 targets observed over two pointings in the COSMOS field using the NIRSpec micro-shutter assembly (MSA). We employ the three medium-resolution gratings G140M, G235M, and G395M, with exposure times of 13 hours, 3.2 hours, and 1.6 hours, respectively. We thus obtain full coverage of the 1-5 micron range, corresponding to the entire rest-frame optical wavelength range. The sample is carefully selected to provide a census of galaxies over the redshift range 1.7 < z < 3.5 above a redshift-dependent minimum stellar mass that ranges from 10^8.7 Msun to 10^9.3 Msun.The Blue Jay sample is representative of the entire galaxy population at these redshifts, without strong biases in color, star formation rate, or other properties. The sizes of massive galaxies at these redshifts are comparable to the NIRSpec shutters, which requires custom strategies for designing and reducing the observations. Since the standard A-B nod subtraction leads to flux self-subtraction, we construct a master background from empty shutters and subtract it from each of the science spectra. This, in turn, allows for the use of shorter slitlets consisting of only two shutters per galaxy instead of the usual three, with a substantial increase in the multiplexing of the NIRSpec MSA. We measure multi-band photometry using archival JWST and HST observations in two different ways: in a large elliptical aperture encompassing the entire source and from the exact area in the sky where the NIRSpec 1D spectrum is extracted. This enables self-consistent fits of spectroscopic and photometric data. The Blue Jay dataset, which we publicly release, represents the ideal sample for studying the stellar populations, neutral gas, and ionized gas in Cosmic Noon galaxies. △ Less

Submitted 13 October, 2025; originally announced October 2025.

Comments: 21 pages, 19 figures, submitted

pop-cosmos: Star formation over 12 Gyr from generative modelling of a deep infrared-selected galaxy catalogue

Authors: Sinan Deger, Hiranya V. Peiris, Stephen Thorp, Daniel J. Mortlock, Gurjeet Jagwani, Justin Alsing, Boris Leistedt, Joel Leja

Abstract: We study star formation over 12 Gyr using pop-cosmos, a generative model trained on 26-band photometry of 420,000 COSMOS2020 galaxies (IRAC Ch.1 $<26$). The model learns distributions over 16 SPS parameters via score-based diffusion, matching observed colours and magnitudes. We compute the star formation rate density (SFRD) to $z=3.5$ by directly integrating individual galaxy SFRs. The SFRD peaks… ▽ More We study star formation over 12 Gyr using pop-cosmos, a generative model trained on 26-band photometry of 420,000 COSMOS2020 galaxies (IRAC Ch.1 $<26$). The model learns distributions over 16 SPS parameters via score-based diffusion, matching observed colours and magnitudes. We compute the star formation rate density (SFRD) to $z=3.5$ by directly integrating individual galaxy SFRs. The SFRD peaks at $z=1.3\pm0.1$, with peak value $0.08\pm0.01$ M$_{\odot}$ yr$^{-1}$ Mpc$^{-3}$. We classify star-forming (SF) and quiescent (Q) galaxies using specific SFR $<10^{-11}$ yr$^{-1}$, comparing with $NUVrJ$ colour selection. The sSFR criterion yields up to 20% smaller quiescent fractions across $0<z<3.5$, with $NUVrJ$-selected samples contaminated by galaxies with sSFR up to $10^{-9}$ yr$^{-1}$. Our sSFR-selected stellar mass function shows a negligible number density of low-mass ($<10^{9.5}$ M$_\odot$) Q galaxies at $z\sim1$, where colour-selection shows a prominent increase. Non-parametric star formation histories around the SFRD peak reveal distinct patterns: SF galaxies show gradually decreasing SFR correlations with lookback time ($r\sim1$ to $r\sim0$ over 13 Gyr), implying increasingly stochastic star formation toward early epochs. Q galaxies exhibit full correlation ($r>0.95$) during the most recent $\sim$300 Myr, then sharp decorrelation with earlier star-forming epochs, marking clear quenching transitions. Massive ($10<\log_{10}(M_*/$M$_{\odot})<11$) galaxies quench on a time-scale of $\sim1$ Gyr, with mass assembly concentrated in their first 3.5 Gyr. Finally, AGN activity (infrared luminosity) peaks as massive ($\sim10^{10.5}$ M$_\odot$) galaxies approach the transition between star-forming and quiescent states, declining sharply once quiescence is established. This provides evidence that AGN feedback operates in a critical regime during the $\sim1$ Gyr quenching transition. △ Less

Submitted 24 September, 2025; originally announced September 2025.

Comments: 23 pages, 15 figures, for submission to MNRAS

What you see is what you get: empirically measured bolometric luminosities of Little Red Dots

Authors: Jenny E. Greene, David J. Setton, Lukas J. Furtak, Rohan P. Naidu, Marta Volonteri, Pratika Dayal, Ivo Labbe, Pieter van Dokkum, Rachel Bezanson, Gabriel Brammer, Sam E. Cutler, Karl Glazebrook, Anna de Graaff, Michaela Hirschmann, Raphael E. Hviding, Vasily Kokorev, Joel Leja, Hanpu Liu, Yilun Ma, Jorryt Matthee, Themiya Nanayakkara, Pascal A. Oesch, Richard Pan, Sedona H. Price, Justin S. Spilker , et al. (5 additional authors not shown)

Abstract: New populations of red active galactic nuclei (known as ``Little Red Dots'') discovered by JWST exhibit remarkable spectral energy distributions. Leveraging X-ray through far-infrared observations of two of the most luminous known Little Red Dots, we directly their bolometric luminosities. We find evidence that more than half of the bolometric luminosity likely emerges in the rest-frame optical, w… ▽ More New populations of red active galactic nuclei (known as ``Little Red Dots'') discovered by JWST exhibit remarkable spectral energy distributions. Leveraging X-ray through far-infrared observations of two of the most luminous known Little Red Dots, we directly their bolometric luminosities. We find evidence that more than half of the bolometric luminosity likely emerges in the rest-frame optical, with $L_{\rm bol}/L_{5100} = 5$, roughly half the value for ``standard'' Active Galactic Nuclei. Meanwhile, the X-ray emitting corona, UV-emitting black-body, and reprocessed mid to far-infrared emission are all considerably sub-dominant, assuming that the far-infrared luminosity is well below current measured limits. We present new bolometric corrections that dramatically lower inferred bolometric luminosities by a factor of ten compared to published values in the literature. These bolometric corrections are in accord with expectations from models in which gas absorption and reprocessing are responsible for the red rest-frame optical colors of Little Red Dots. We discuss how this lowered luminosity scale suggests a lower mass scale for the population by at least an order of magnitude {\bf (e.g., $\sim 10^5-10^7~{\rm M_{\odot}}$ black holes, and $\sim 10^8~{\rm M_{\odot}}$ galaxies)}, alleviating tensions with clustering, overmassive black holes, and the integrated black hole mass density in the Universe. △ Less

Submitted 5 September, 2025; originally announced September 2025.

Comments: 11 pages, 3 figures, 1 table, posted following referee comments

The Diversity and Evolution of Dust Attenuation Curves from Redshift z ~ 1 to 9

Authors: Irene Shivaei, Rohan P. Naidu, Francisco Rodríguez Montero, Kosei Matsumoto, Joel Leja, Jorryt Matthee, Benjamin D. Johnson, Pascal A. Oesch, Jacopo Chevallard, Angela Adamo, Sarah Bodansky, Andrew J. Bunker, Alba Covelo Paz, Claudia Di Cesare, Eiichi Egami, Lukas J. Furtak, Kasper E. Heintz, Ivan Kramarenko, Romain A. Meyer, Naveen A. Reddy, Pierluigi Rinaldi, Sandro Tacchella, Alberto Torralba, Joris Witstok, Michael A. Wozniak , et al. (1 additional authors not shown)

Abstract: The UV-optical dust attenuation curve is key to interpreting the intrinsic properties of galaxies and provides insights into the nature of dust grains and their geometry relative to stars. In this work, we constrain the UV-optical slope of the stellar attenuation curve using a spectroscopic-redshift sample of ~3300 galaxies at z~1-9, to characterize the diversity and redshift evolution of stellar… ▽ More The UV-optical dust attenuation curve is key to interpreting the intrinsic properties of galaxies and provides insights into the nature of dust grains and their geometry relative to stars. In this work, we constrain the UV-optical slope of the stellar attenuation curve using a spectroscopic-redshift sample of ~3300 galaxies at z~1-9, to characterize the diversity and redshift evolution of stellar attenuation curves and to gain insight into dust production and evolution at high redshifts. The sample is constructed from three JWST/NIRCam grism surveys in GOODS and A2744 fields, with a wealth of JWST/NIRCam and HST photometry. With constraints from spectroscopic redshifts and emission line fluxes, we use the Prospector SED fitting code with a flexible dust model. We find that the attenuation curve slope varies strongly with Av at all redshifts, becoming flatter at higher attenuation. We find no strong correlation between attenuation curve slope and size or axis ratio, and the trends with stellar mass and star-formation rate are largely driven by their correlation with Av. We find strong evidence that at fixed Av, the curve becomes flatter with increasing redshift. On average, the attenuation curves derived here are shallower than those at z~0 and than the SMC curve. The highest redshift galaxies at z=7-9 (124 galaxies, a significantly larger sample than in previous studies) show slopes even flatter than the Calzetti curve, implying reduced UV obscuration and lower IR luminosities than expected from an SMC dust curve, by as large as an order of magnitude. Hydrodynamical simulations that couple dust growth to gas chemical enrichment successfully reproduce the different loci of high- and low-redshift galaxies in the slope-Av diagram, suggesting that dust in high-redshift galaxies is increasingly dominated by large grains produced in supernova ejecta with limited ISM processing at early times. △ Less

Submitted 1 September, 2025; originally announced September 2025.

Comments: Sumbitted to A&A

The Missing Hard Photons of Little Red Dots: Their Incident Ionizing Spectra Resemble Massive Stars

Authors: Bingjie Wang, Joel Leja, Harley Katz, Kohei Inayoshi, Nikko J. Cleri, Anna de Graaff, Raphael E. Hviding, Pieter van Dokkum, Jenny E. Greene, Ivo Labbé, Jorryt Matthee, Ian McConachie, Rohan P. Naidu, Erica J. Nelson

Abstract: The nature of Little Red Dots (LRDs) has largely been investigated through their continuum emission, with lines assumed to arise from a broad-line region. In this paper, we instead use recombination lines to infer the intrinsic properties of the central engine of LRDs. Our analysis first reveals a tension between the ionizing properties implied from H$α$ and HeII$\,λ$4686. The high H$α$ EWs requir… ▽ More The nature of Little Red Dots (LRDs) has largely been investigated through their continuum emission, with lines assumed to arise from a broad-line region. In this paper, we instead use recombination lines to infer the intrinsic properties of the central engine of LRDs. Our analysis first reveals a tension between the ionizing properties implied from H$α$ and HeII$\,λ$4686. The high H$α$ EWs require copious H-ionizing photons, more than the bluest AGN ionizing spectra can provide. In contrast, HeII emission is marginally detected, and its low EW is, at most, consistent with the softest AGN spectra. The low HeII/H$β$ ($\sim10^{-2}$, $<20\times$ local AGN median) further points to an unusually soft ionizing spectrum. We extend our analysis to dense gas environments (the ``black-hole star'' hypothesis), and find that hydrogen recombination lines become optically thick and lose diagnostic power, but HeII remains optically thin and a robust tracer. Photoionization modeling with Cloudy rules out standard AGN accretion disk spectra. Alternative explanations include: exotic AGN with red rest-optical emission; a very high {\it average} optical depth ($>10$) from gas/dust; and/or soft ionizing spectra with abundant H-ionizing photons, consistent with e.g., a cold accretion disk or a composite of AGN and stars. The latter is an intriguing scenario since high hydrogen densities are highly conducive for star formation, and nuclear star clusters are found in the immediate vicinity of local massive black holes. While previous studies have mostly focused on features dominated by the absorbing hydrogen cloud, the HeII-based diagnostic proposed here represents a crucial step toward understanding the central engine of LRDs. △ Less

Submitted 25 August, 2025; originally announced August 2025.

Comments: Submitted to ApJ; 25 pages, 14 figures, 1 table

RUBIES spectroscopically confirms the high number density of quiescent galaxies from $\mathbf{2<z<5}$

Authors: Yunchong Zhang, Anna de Graaff, David J. Setton, Sedona H. Price, Rachel Bezanson, Claudia del P. Lagos, Sam E. Cutler, Ian McConachie, Nikko J. Cleri, Olivia R. Cooper, Rashmi Gottumukkala, Jenny E. Greene, Michaela Hirschmann, Gourav Khullar, Ivo Labbe, Joel Leja, Michael V. Maseda, Jorryt Matthee, Tim B. Miller, Themiya Nanayakkara, Katherine A. Suess, Bingjie Wang, Katherine E. Whitaker, Christina C. Williams

Abstract: We present the number density of massive ($ \mathrm{ log (M_{*}/M_{\odot}) > 10.3} $) quiescent galaxies at $2<z<5$ using JWST NIRSpec PRISM spectra. This work relies on spectra from RUBIES, which provides excellent data quality and an unparalleled, well-defined targeting strategy to robustly infer physical properties and number densities. We identify quiescent galaxy candidates within RUBIES thro… ▽ More We present the number density of massive ($ \mathrm{ log (M_{*}/M_{\odot}) > 10.3} $) quiescent galaxies at $2<z<5$ using JWST NIRSpec PRISM spectra. This work relies on spectra from RUBIES, which provides excellent data quality and an unparalleled, well-defined targeting strategy to robustly infer physical properties and number densities. We identify quiescent galaxy candidates within RUBIES through principal component analysis and construct a final sample using star formation histories derived from spectro-photometric fitting of the NIRSpec PRISM spectra and NIRCam photometry. By inverting the RUBIES selection function, we correct for survey incompleteness and calculate the number density of massive quiescent galaxies at these redshifts, providing the most complete spectroscopic estimates prior to cosmic noon to date. We find that early massive quiescent galaxies are surprisingly common ($\gtrsim 10^{-5}$ Mpc$^{-3}$ by $4<z<5$), which is consistent with previous studies based on JWST photometry alone and/or in smaller survey areas. We compare our number densities with predictions from six state-of-the-art cosmological galaxy formation simulations. At $z>3$, most simulations fail to produce enough massive quiescent galaxies, suggesting the treatment of feedback and/or the channels for early efficient formation are incomplete in most galaxy evolution models. △ Less

Submitted 11 August, 2025; originally announced August 2025.

Comments: 27 pages, 11 figures

MINERVA: A NIRCam Medium Band and MIRI Imaging Survey to Unlock the Hidden Gems of the Distant Universe

Authors: Adam Muzzin, Katherine A. Suess, Danilo Marchesini, Luke Robbins, Chris J. Willott, Stacey Alberts, Jacqueline Antwi-Danso, Yoshihisa Asada, Gabriel Brammer, Sam E. Cutler, Kartheik G. Iyer, Ivo Labbe, Nicholas S. Martis, Tim B. Miller, Ikki Mitsuhashi, Alexandra Pope, Anna Sajina, Ghassan T. E. Sarrouh, Monu Sharma, Mauro Stefanon, Katherine E. Whitaker, Roberto Abraham, Hakim Atek, Marusa Bradac, Samantha Berek , et al. (59 additional authors not shown)

Abstract: We present an overview of the MINERVA survey, a 259.8 hour (prime) and 127 hour (parallel) Cycle 4 treasury program on the James Webb Space Telescope (JWST). MINERVA is obtaining 8 filter NIRCam medium band imaging (F140M, F162M, F182M, F210M, F250M, F300M, F360M, F460M) and 2 filter MIRI imaging (F1280W, F1500W) in four of the five CANDELS Extragalactic fields: UDS, COSMOS, AEGIS and GOODS-N. The… ▽ More We present an overview of the MINERVA survey, a 259.8 hour (prime) and 127 hour (parallel) Cycle 4 treasury program on the James Webb Space Telescope (JWST). MINERVA is obtaining 8 filter NIRCam medium band imaging (F140M, F162M, F182M, F210M, F250M, F300M, F360M, F460M) and 2 filter MIRI imaging (F1280W, F1500W) in four of the five CANDELS Extragalactic fields: UDS, COSMOS, AEGIS and GOODS-N. These fields were previously observed in Cycle 1 with 7 - 9 NIRCam filters by the PRIMER, CEERS and JADES programs. MINERVA reaches a 5$σ$ depth of 28.1 mag in F300M and covers $\sim$ 542 arcmin$^2$, increasing the area of existing JWST medium-band coverage in at least 8 bands by $\sim$ 7$\times$. The MIRI imaging reaches a 5$σ$ depth of 23.9 mag in F1280W and covers $\sim$ 275 arcmin$^2$ in at least 2 MIRI filters. When combined with existing imaging, these data will provide a photometric catalog with 20-26 JWST filters (depending on field) and 26-35 filters total, including HST. This paper presents a detailed breakdown of the filter coverage, exposure times, and field layout relative to previous observations, as well as an overview of the primary science goals of the project. These include uncovering the physics of enigmatic sources hiding in current broadband catalogs, improving systematics on stellar mass functions and number densities by factors of $\gtrsim$ 3, and resolved mapping of stellar mass and star formation at 1 $< z <$ 6. When complete, MINERVA will become an integral part of the treasury deep field imaging datasets, significantly improving population studies with well-understood completeness, robust photometric redshifts, stellar masses, and sizes, and facilitating spectroscopic follow up for decades to come. △ Less

Submitted 25 July, 2025; originally announced July 2025.

Comments: 29 pages, 12 Figures, submitted to ApJS. MINERVA Website: https://jwst-minerva.github.io/

Breaking Through the Cosmic Fog: JWST/NIRSpec Constraints on Ionizing Photon Escape in Reionization-Era Galaxies

Authors: Emma Giovinazzo, Pascal A. Oesch, Andrea Weibel, Romain A. Meyer, Callum Witten, Aniket Bhagwat, Gabriel Brammer, John Chisholm, Anna de Graaff, Rashmi Gottumukkala, Michelle Jecmen, Harley Katz, Joel Leja, Rui Marques-Chaves, Michael Maseda, Irene Shivaei, Maxime Trebitsch, Anne Verhamme

Abstract: The escape fraction of Lyman continuum photons (fesc(LyC)) is the last key unknown in our understanding of cosmic reionization. Directly estimating the escape fraction (fesc) of ionizing photons in the epoch of reionization (EoR) is impossible, due to the opacity of the intergalactic medium (IGM). However, a high fesc leaves clear imprints in the spectrum of a galaxy, due to reduced nebular line a… ▽ More The escape fraction of Lyman continuum photons (fesc(LyC)) is the last key unknown in our understanding of cosmic reionization. Directly estimating the escape fraction (fesc) of ionizing photons in the epoch of reionization (EoR) is impossible, due to the opacity of the intergalactic medium (IGM). However, a high fesc leaves clear imprints in the spectrum of a galaxy, due to reduced nebular line and continuum emission, which also leads to bluer UV continuum slopes (betaUV). Here, we exploit the large archive of deep JWST/NIRSpec spectra from the DAWN JWST Archive to analyze over 1'400 galaxies at 5 < zspec < 10 and constrain their fesc based on SED fitting enhanced with a picket fence model. We identify 71 high-confidence sources with significant fesc based on Bayes factor analysis strongly favouring fesc > 0 over fesc = 0 solutions. We compare the characteristics of this high-escape subset against both the parent sample and established diagnostics including betaUV slope, O32, and SFR surface density (SigmaSFR). For the overall sample, we find that most sources have a low escape fraction (<1%), however, a small subset of sources seems to emit a large number of their ionizing photons into the IGM, such that the average fesc is found to be ~10%, as needed for galaxies to drive reionization. Although uncertainties remain regarding recent burstiness and the intrinsic stellar ionizing photon output at low metallicities, our results demonstrate the unique capability of JWST/NIRSpec to identify individual LyC leakers, measure average fesc and thus constrain the drivers of cosmic reionization. △ Less

Submitted 1 July, 2025; originally announced July 2025.

Comments: Submitted to A&A

Even redder than we knew: color and $A_{\mathrm{V}}$ evolution up to $z=2.5$ from JWST/NIRCam photometry

Authors: A. van der Wel, M. Martorano, D. Marchesini, S. Wuyts, E. F. Bell, S. E. Meidt, A. Gebek, G. Brammer, K. Whitaker, R. Bezanson, E. J. Nelson, G. Rudnick, M. Kriek, J. Leja, J. S. Dunlop, C. Casey, J. Kartaltepe

Abstract: JWST/NIRCam provides rest-frame near-IR photometry of galaxies up to $z=2.5$ with exquisite depth and accuracy. This affords an unprecedented view of the evolution of the UV-optical-near-IR color distribution and its interpretation in terms of the evolving dust attenuation, $A_{\mathrm{V}}$. We use the value-added data products (photometric redshift, stellar mass, rest-frame $U-V$ and $V-J$ colors… ▽ More JWST/NIRCam provides rest-frame near-IR photometry of galaxies up to $z=2.5$ with exquisite depth and accuracy. This affords an unprecedented view of the evolution of the UV-optical-near-IR color distribution and its interpretation in terms of the evolving dust attenuation, $A_{\mathrm{V}}$. We use the value-added data products (photometric redshift, stellar mass, rest-frame $U-V$ and $V-J$ colors, and $A_{\rm V}$) provided by the public DAWN JWST Archive. This data product derives from fitting the spectral energy distributions obtained from multiple NIRCam imaging surveys, augmented with pre-existing HST imaging data. Our sample consists of a stellar mass complete sample of $\approx 28,000$ $M_\star> 10^{9}~M_\odot$ galaxies in the redshift range $0.5<z<2.5$. The $V-J$ color distribution of star-forming galaxies evolves strongly, in particular for high-mass galaxies ($M_\star>3\times 10^{10}~M_\odot$), which have a pronounced tail of very red galaxies reaching $V-J> 2.5$ at $z>1.5$ that does not exist at $z<1$. Such red $V-J$ can only be explained by dust attenuation, with typical values for $M_\star \approx 10^{11}~M_\odot$ galaxies in the range $A_{\mathrm{V}}\approx 1.5-3.5$ at $z\approx 2$. This redshift evolution went largely unnoticed before because the photometric redshift estimates for the reddest ($V-J>2.5$), most attenuated galaxies has markedly improved thanks to the new, precise photometry. Despite the increased attenuation, $U-V$ colors across the entire mass range are slightly bluer at higher $z$. In conclusion, whereas the rest-frame UV-optical color distribution evolves remarkably little from $z=0.5$ to $z=2.5$, the rest-frame optical-near-IR color distribution evolves strongly, primarily due to a very substantial increase with redshift in dust attenuation for massive galaxies. (Abbr.) △ Less

Submitted 2 July, 2025; v1 submitted 30 June, 2025; originally announced June 2025.

Comments: Accepted for publication in A&A. This second version has minor corrections to adhere to A&A style

Journal ref: A&A 701, A30 (2025)

Optical Strong Line Ratios Cannot Distinguish Between Stellar Populations and Accreting Black Holes at High Ionization Parameters and Low Metallicities

Authors: Nikko J. Cleri, Grace M. Olivier, Bren E. Backhaus, Joel Leja, Casey Papovich, Jonathan R. Trump, Pablo Arrabal Haro, Veronique Buat, Denis Burgarella, Emilie Burnham, Antonello Calabro, Jonathan H. Cohn, Justin W. Cole, Kelcey Davis, Mark Dickinson, Steven L. Finkelstein, Ray Garner III, Michaela Hirschmann, Weida Hu, Taylor A. Hutchison, Dale D. Kocevski, Anton M. Koekemoer, Rebecca L. Larson, Zach J. Lewis, Michael V. Maseda , et al. (2 additional authors not shown)

Abstract: High-redshift observations from JWST indicate that optical strong line ratios do not carry the same constraining power as they do at low redshifts. Critically, this prevents a separation between stellar- and black hole-driven ionizing radiation, thereby obscuring both active galactic nuclei demographics and star formation rates. To investigate this, we compute a large suite of photoionization mode… ▽ More High-redshift observations from JWST indicate that optical strong line ratios do not carry the same constraining power as they do at low redshifts. Critically, this prevents a separation between stellar- and black hole-driven ionizing radiation, thereby obscuring both active galactic nuclei demographics and star formation rates. To investigate this, we compute a large suite of photoionization models from Cloudy powered by stellar populations and accreting black holes over a large grid of ages, metallicities, initial mass functions, binarity, ionization parameters, densities, and black hole masses. We use these models to test three rest-frame optical strong line ratio diagnostics which have been designed to separate ionizing sources at low redshifts: the [NII]-BPT, VO87, and OHNO diagrams. We show that the position of a model in these diagrams is strongly driven by the ionization parameter (log U) and the gas-phase metallicity, often more so than the ionizing spectrum itself; in particular, there is significant overlap between stellar population and accreting black hole models at high log U and low Z. We show that the OHNO diagram is especially susceptible to large contamination of the AGN region defined at z=1 for stellar models with high log U and low Z, consistent with many observed JWST spectra at high redshift. We show that the optical line ratio diagnostics are most sensitive to the shape of the <54 eV ionizing continuum, and that the derived ionizing sources for a given set of optical strong line ratios can be highly degenerate. Finally, we demonstrate that very high ionization (>54 eV) emission lines that trace ionizing sources harder than normal stellar populations help to break the degeneracies present when using the strong line diagnostics alone, even in gas conditions consistent with those at high redshifts. △ Less

Submitted 26 June, 2025; originally announced June 2025.

Comments: 22 pages, 10 figures, animations are available at https://github.com/njcleri/AR_05558_modeling/tree/main/figures/animations

Taking a Break at Cosmic Noon: Continuum-selected Low-mass Galaxies Require Long Burst Cycles

Authors: Abby Mintz, David J. Setton, Jenny E. Greene, Joel Leja, Bingjie Wang, Emilie Burnham, Katherine A. Suess, Hakim Atek, Rachel Bezanson, Gabriel Brammer, Sam E. Cutler, Pratika Dayal, Robert Feldmann, Lukas J. Furtak, Karl Glazebrook, Gourav Khullar, Vasily Kokorev, Ivo Labbé, Michael V. Maseda, Tim B. Miller, Ikki Mitsuhashi, Themiya Nanayakkara, Richard Pan, Sedona H. Price, John R. Weaver , et al. (1 additional authors not shown)

Abstract: While bursty star formation in low-mass galaxies has been observed in local populations and reproduced in simulations, the dormant phase of the burst cycle has not been well studied beyond the local Universe due to observational limitations. We present a unique sample of 41 JWST PRISM spectra of low-mass galaxies ($M_\star < 10^{9.5}\,M_\odot$) at cosmic noon ($1<z<3$), uniformly selected on F200W… ▽ More While bursty star formation in low-mass galaxies has been observed in local populations and reproduced in simulations, the dormant phase of the burst cycle has not been well studied beyond the local Universe due to observational limitations. We present a unique sample of 41 JWST PRISM spectra of low-mass galaxies ($M_\star < 10^{9.5}\,M_\odot$) at cosmic noon ($1<z<3$), uniformly selected on F200W magnitude and precise photometric redshifts enabled by 20-band JWST photometry from the UNCOVER and MegaScience surveys. The spectra reveal numerous strong Balmer breaks, which are negatively correlated with the galaxies' H$α$ equivalent width. By comparing these observations to synthetic samples of spectra generated using a simple parametrization of bursty star formation histories, we show that star formation in low-mass galaxies at cosmic noon is likely dominated by burst cycles with long timescales ($\gtrsim 100$ Myr) and large deviations below the star-forming main sequence ($\gtrsim 0.8$ dex). Our results suggest that galaxies in this population--at least those within our detection limits--should not be classified solely by their current star formation rates, but instead viewed as a unified population undergoing dynamic movement above and below the star-forming main sequence. The derived constraints demonstrate that long-timescale fluctuations are important for this class of galaxies, indicating that galaxy-scale gas cycles--rather than molecular-cloud-scale stochasticity--are the primary regulators of star formation variability in low-mass galaxies at cosmic noon. △ Less

Submitted 19 June, 2025; originally announced June 2025.

Comments: 24 pages, 11 figures. Submitted to ApJ

pop-cosmos: Insights from generative modeling of a deep, infrared-selected galaxy population

Authors: Stephen Thorp, Hiranya V. Peiris, Gurjeet Jagwani, Sinan Deger, Justin Alsing, Boris Leistedt, Daniel J. Mortlock, Anik Halder, Joel Leja

Abstract: We present an extension of the pop-cosmos model for the evolving galaxy population up to redshift $z\sim6$. The model is trained on distributions of observed colors and magnitudes, from 26-band photometry of $\sim420,000$ galaxies in the COSMOS2020 catalog with Spitzer IRAC $\textit{Ch. 1}<26$. The generative model includes a flexible distribution over 16 stellar population synthesis (SPS) paramet… ▽ More We present an extension of the pop-cosmos model for the evolving galaxy population up to redshift $z\sim6$. The model is trained on distributions of observed colors and magnitudes, from 26-band photometry of $\sim420,000$ galaxies in the COSMOS2020 catalog with Spitzer IRAC $\textit{Ch. 1}<26$. The generative model includes a flexible distribution over 16 stellar population synthesis (SPS) parameters, and a depth-dependent photometric uncertainty model, both represented using score-based diffusion models. We use the trained model to predict scaling relationships for the galaxy population, such as the stellar mass function, star-forming main sequence, and gas-phase and stellar metallicity vs. mass relations, demonstrating reasonable-to-excellent agreement with previously published results. We explore the connection between mid-infrared emission from active galactic nuclei (AGN) and star-formation rate, finding high AGN activity for galaxies above the star-forming main sequence at $1\lesssim z\lesssim 2$. Using the trained population model as a prior distribution, we perform inference of the redshifts and SPS parameters for 429,669 COSMOS2020 galaxies, including 39,588 with publicly available spectroscopic redshifts. The resulting redshift estimates exhibit minimal bias ($\text{median}[Δ_z]=-8\times10^{-4}$), scatter ($σ_\text{MAD}=0.0132$), and outlier fraction ($6.19\%$) for the full $0<z<6$ spectroscopic compilation. These results establish that pop-cosmos can achieve the accuracy and realism needed to forward-model modern wide--deep surveys for Stage IV cosmology. We publicly release pop-cosmos software, mock galaxy catalogs, and COSMOS2020 redshift and SPS parameter posteriors. △ Less

Submitted 17 September, 2025; v1 submitted 13 June, 2025; originally announced June 2025.

Comments: 47 pages, 34 figures. Accepted for publication in ApJ. Mock galaxy catalogs available at https://doi.org/10.5281/zenodo.15757717 . MCMC chains available at https://doi.org/10.5281/zenodo.15850536 . Software available at https://github.com/Cosmo-Pop

Journal ref: ApJ 993, 240 (2025)

Impact of redshift distribution uncertainties on Lyman-break galaxy cosmological parameter inference

Authors: Francesco Petri, Boris Leistedt, Daniel J. Mortlock, Joel Leja, Stephen Thorp, Justin Alsing, Hiranya V. Peiris, Sinan Deger

Abstract: A significant number of Lyman-break galaxies (LBGs) with redshifts 3 < z < 5 are expected to be observed by the upcoming Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST). This will enable us to probe the universe at higher redshifts than is currently possible with cosmological galaxy clustering and weak lensing surveys. However, accurate inference of cosmological parameters require… ▽ More A significant number of Lyman-break galaxies (LBGs) with redshifts 3 < z < 5 are expected to be observed by the upcoming Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST). This will enable us to probe the universe at higher redshifts than is currently possible with cosmological galaxy clustering and weak lensing surveys. However, accurate inference of cosmological parameters requires precise knowledge of the redshift distributions of selected galaxies, where the number of faint objects expected from LSST alone will make spectroscopic based methods of determining these distributions extremely challenging. To overcome this difficulty, it may be possible to leverage the information in the large volume of photometric data alone to precisely infer these distributions. This could be facilitated using forward models, where in this paper we use stellar population synthesis (SPS) to estimate uncertainties on LBG redshift distributions for a 10 year LSST (LSSTY10) survey. We characterise some of the modelling uncertainties inherent to SPS by introducing a flexible parameterisation of the galaxy population prior, informed by observations of the galaxy stellar mass function (GSMF) and cosmic star formation density (CSFRD). These uncertainties are subsequently marginalised over and propagated to cosmological constraints in a Fisher forecast. Assuming a known dust attenuation model for LBGs, we forecast constraints on the sigma8 parameter comparable to Planck cosmic microwave background (CMB) constraints. △ Less

Submitted 6 June, 2025; originally announced June 2025.

Journal ref: Mon Not R Astron Soc (2025)

Probing neutral outflows in z ~ 2 galaxies using JWST observations of Ca II H and K absorption lines

Authors: Caterina Liboni, Sirio Belli, Letizia Bugiani, Rebecca Davies, Minjung Park, Charlie Conroy, Razieh Emami, Benjamin D. Johnson, Amir H. Khoram, Joel Leja, Gabriel Maheson, Matteo Sapori, Trevor Mendel, Sandro Tacchella, Rainer Weinberger

Abstract: Using deep JWST/NIRSpec spectra from the Blue Jay survey, we perform the first systematic investigation of neutral gas content in massive galaxies at Cosmic Noon based on the Ca II H, K absorption lines. We analyze a sample of 9 galaxies at 1.8 < z < 2.8 with stellar masses > 10.6, for which we detect neutral gas absorption both in Ca II and in Na I. After removing the stellar continuum using the… ▽ More Using deep JWST/NIRSpec spectra from the Blue Jay survey, we perform the first systematic investigation of neutral gas content in massive galaxies at Cosmic Noon based on the Ca II H, K absorption lines. We analyze a sample of 9 galaxies at 1.8 < z < 2.8 with stellar masses > 10.6, for which we detect neutral gas absorption both in Ca II and in Na I. After removing the stellar continuum using the best-fit model obtained with Prospector, we fit the excess absorption due to neutral gas in the Ca II H, K doublet and in the Na I D doublet, together with nearby emission lines produced by ionized gas. We measure covering fractions between 0.2 and 0.9 from the Ca II H and K lines, which are spectrally well resolved in the NIRSpec R ~ 1000 observations, unlike the absorption lines in the Na I D doublet. We measure the velocity shift, velocity dispersion, and column density separately for Ca II and Na I. About half of the galaxies present blueshifted Ca II, indicative of an outflow of neutral gas, consistent with previous results based on Na I. The velocity shift and the column density measured from Ca II are correlated with those measured from Na I, implying that these absorption lines trace gas in similar physical conditions. However, the column densities are not in a 1:1 relation, meaning that the relative amount of Ca II and Na I atoms along the line of sight varies with the gas column density. After discussing possible reasons for this behavior, we derive an empirical relation between the column density of Ca II and the column density of Na I and, in a more indirect way, of neutral hydrogen H I. This calibration offers a new way to estimate the outflow mass and the mass outflow rate for the neutral phase from current and future JWST observations of massive galaxies at Cosmic Noon and beyond △ Less

Submitted 5 June, 2025; originally announced June 2025.

Comments: 12 pages, 9 figures, Submitted to A&A

RUBIES: A Spectroscopic Census of Little Red Dots; All V-Shaped Point Sources Have Broad Lines

Authors: Raphael E. Hviding, Anna de Graaff, Tim B. Miller, David J. Setton, Jenny E. Greene, Ivo Labbé, Gabriel Brammer, Rachel Bezanson, Leindert A. Boogaard, Nikko J. Cleri, Joel Leja, Michael V. Maseda, Ian McConachie, Jorryt Matthee, Rohan P. Naidu, Pascal A. Oesch, Bingjie Wang, Katherine E. Whitaker, Christina Williams

Abstract: The physical nature of Little Red Dots (LRDs) - a population of compact, red galaxies revealed by JWST - remains unclear. Photometric samples are constructed from varying selection criteria with limited spectroscopic follow-up available to test intrinsic spectral shapes and prevalence of broad emission lines. We use the RUBIES survey, a large spectroscopic program with wide color-morphology covera… ▽ More The physical nature of Little Red Dots (LRDs) - a population of compact, red galaxies revealed by JWST - remains unclear. Photometric samples are constructed from varying selection criteria with limited spectroscopic follow-up available to test intrinsic spectral shapes and prevalence of broad emission lines. We use the RUBIES survey, a large spectroscopic program with wide color-morphology coverage and homogeneous data quality, to systematically analyze the emission-line kinematics, spectral shapes, and morphologies of $\sim$1500 galaxies at $z > 3.1$. We identify broad Balmer lines via a novel fitting approach that simultaneously models NIRSpec/PRISM and G395M spectra, yielding 80 broad-line sources with 28 (35%) at $z > 6$. A large subpopulation naturally emerges from the broad Balmer line sources, with 36 exhibiting `v-shaped' UV-to-optical continua and a dominant point source component in the rest-optical; we define these as spectroscopic LRDs, constituting the largest such sample to date. Strikingly, the spectroscopic LRD population is largely recovered when either a broad line or rest-optical point source is required in combination with a v-shaped continuum, suggesting an inherent link between these three defining characteristics. We compare the spectroscopic LRD sample to published photometric searches. Although these selections have high accuracy, down to $\rm F444W<26.5$, only 50-62% of the RUBIES LRDs were previously identified. The remainder were missed due to a mixture of faint rest-UV photometry, comparatively blue rest-optical colors, or highly uncertain photometric redshifts. Our findings highlight that well-selected spectroscopic campaigns are essential for robust LRD identification, while photometric criteria require refinement to capture the full population. △ Less

Submitted 5 June, 2025; originally announced June 2025.

Comments: Main text: 17 pages, 10 figures. Appendix: 7 pages, 3 figures. Submitted to A&A

Journal ref: A&A 702, A57 (2025)

A Cosmic Miracle: A Remarkably Luminous Galaxy at $z_{\rm{spec}}=14.44$ Confirmed with JWST

Authors: Rohan P. Naidu, Pascal A. Oesch, Gabriel Brammer, Andrea Weibel, Yijia Li, Jorryt Matthee, John Chisholm, Clara L. Pollock, Kasper E. Heintz, Benjamin D. Johnson, Xuejian Shen, Raphael E. Hviding, Joel Leja, Sandro Tacchella, Arpita Ganguly, Callum Witten, Hakim Atek, Sirio Belli, Sownak Bose, Rychard Bouwens, Pratika Dayal, Roberto Decarli, Anna de Graaff, Yoshinobu Fudamoto, Emma Giovinazzo , et al. (21 additional authors not shown)

Abstract: JWST has revealed a stunning population of bright galaxies at surprisingly early epochs, $z>10$, where few such sources were expected. Here we present the most distant example of this class yet -- MoM-z14, a luminous ($M_{\rm{UV}}=-20.2$) source in the COSMOS legacy field at $z_{\rm{spec}}=14.44^{+0.02}_{-0.02}$ that expands the observational frontier to a mere 280 million years after the Big Bang… ▽ More JWST has revealed a stunning population of bright galaxies at surprisingly early epochs, $z>10$, where few such sources were expected. Here we present the most distant example of this class yet -- MoM-z14, a luminous ($M_{\rm{UV}}=-20.2$) source in the COSMOS legacy field at $z_{\rm{spec}}=14.44^{+0.02}_{-0.02}$ that expands the observational frontier to a mere 280 million years after the Big Bang. The redshift is confirmed with NIRSpec/prism spectroscopy through a sharp Lyman-$α$ break and $\approx3σ$ detections of five rest-UV emission lines. The number density of bright $z_{\rm{spec}}\approx14-15$ sources implied by our "Mirage or Miracle" survey spanning $\approx350$ arcmin$^{2}$ is $>100\times$ larger ($182^{+329}_{-105}\times$) than pre-JWST consensus models. The high EWs of UV lines (${\approx}15{-}35$ Å) signal a rising star-formation history, with a ${\approx}10\times$ increase in the last 5 Myr ($\rm{SFR_{\rm{5Myr}}}/\rm{SFR_{\rm{50Myr}}}=9.9^{+3.0}_{-5.8}$). The source is extremely compact (circularized $r_{\rm{e}} = 74^{+15}_{-12}$ pc), and yet resolved, suggesting an AGN is not the dominant source of light. The steep UV slope ($β=-2.5^{+0.2}_{-0.2}$) implies negligible dust attenuation and a young stellar population. The absence of a strong damping wing may indicate that the immediate surroundings of MoM-z14 are partially ionized at a redshift where virtually every reionization model predicts a $\approx100\%$ neutral fraction. The nitrogen emission and highly super-solar [N/C]$>1$ hint at an abundance pattern similar to local globular clusters that may have once hosted luminous supermassive stars. Since this abundance pattern is also common among the most ancient stars born in the Milky Way, we may be directly witnessing the formation of such stars in dense clusters, connecting galaxy evolution across the entire sweep of cosmic time. △ Less

Submitted 16 May, 2025; originally announced May 2025.

Comments: Submitted to the Open Journal of Astrophysics. Comments greatly appreciated and warmly welcomed!

Measuring Emission Lines with JWST-MegaScience Medium-Bands: A New Window into Dust and Star Formation at Cosmic Noon

Authors: Brian Lorenz, Katherine A. Suess, Mariska Kriek, Sedona H. Price, Joel Leja, Erica Nelson, Hakim Atek, Rachel Bezanson, Gabriel Brammer, Sam E. Cutler, Pratika Dayal, Anna de Graaff, Jenny E. Greene, Lukas J. Furtak, Ivo Labbé, Danilo Marchesini, Michael V. Maseda, Tim B. Miller, Abby Mintz, Ikki Mitsuhashi, Richard Pan, Natalia Porraz Barrera, Bingjie Wang, John R. Weaver, Christina C. Williams , et al. (1 additional authors not shown)

Abstract: We demonstrate the power of JWST-NIRCam medium-band photometry to measure emission line fluxes and study dust and star formation properties of galaxies at cosmic noon. In this work, we present photometric emission line measurements and spatially-resolved maps of H$α$ and Pa$β$ for a sample of 14 galaxies at $1.3\leq z\leq 2.4$, observed by the MegaScience medium-band survey and the UNCOVER deep sp… ▽ More We demonstrate the power of JWST-NIRCam medium-band photometry to measure emission line fluxes and study dust and star formation properties of galaxies at cosmic noon. In this work, we present photometric emission line measurements and spatially-resolved maps of H$α$ and Pa$β$ for a sample of 14 galaxies at $1.3\leq z\leq 2.4$, observed by the MegaScience medium-band survey and the UNCOVER deep spectroscopic survey. We measure line fluxes directly from the medium-band photometry and compare with spectroscopic measurements from UNCOVER. We find reasonable agreement between the photometric and spectroscopic emission line fluxes for both H$α$ and Pa$β$, with scatter $<0.15$ dex down to emission line equivalent widths of $10$Å. We also make a nebular dust measurement from the ratio Pa$β$ / H$α$, finding an average nebular A$_\mathrm{V}$ of 1.4. Our photometric A$_\mathrm{V}$ measurements show a slightly larger scatter of $0.5$ magnitudes when compared to spectroscopic measurements; however, this scatter may be partially caused by aperture effects. Finally, we produce spatially resolved maps of H$α$ emission, Pa$β$ emission, and stellar continuum. We find that offsets in H$α$ and Pa$β$ emission are common, especially for galaxies with the highest A$_\mathrm{V}$, indicating dusty sub-structures. Furthermore, the correlation between H$α$ and continuum emission decreases with increasing A$_\mathrm{V}$, suggesting that the dustiest objects have clumpy dust and star formation distributions. Our study demonstrates the power of medium-band photometry to directly probe emission line strengths, star formation, and dust attenuation for hundreds of galaxies in UNCOVER and thousands of galaxies in upcoming JWST medium-band surveys. △ Less

Submitted 15 May, 2025; originally announced May 2025.

Comments: 13 pages, 4 figures

Ages and metallicities of quiescent galaxies: confronting broadband ($UVJ$) colours with stellar absorption lines

Authors: Chloe M. Cheng, Mariska Kriek, Aliza G. Beverage, Martje Slob, Rachel Bezanson, Marijn Franx, Joel Leja, Pavel E. Mancera Piña, Katherine A. Suess, Arjen van der Wel, Jesse van de Sande, Pieter G. van Dokkum

Abstract: For decades, studying quiescent galaxies beyond $z\sim1$ has been challenging due to the reliance on photometric spectral energy distributions, which are highly susceptible to degeneracies between age, metallicity, dust, and star-formation history. Only recently has deep, rest-frame, optical spectroscopy made robust metallicity and age measurements possible, allowing us to empirically assess their… ▽ More For decades, studying quiescent galaxies beyond $z\sim1$ has been challenging due to the reliance on photometric spectral energy distributions, which are highly susceptible to degeneracies between age, metallicity, dust, and star-formation history. Only recently has deep, rest-frame, optical spectroscopy made robust metallicity and age measurements possible, allowing us to empirically assess their effects on continuum shapes. To this end, we measure ages and metallicities of $\sim700$ massive ($10.2\lesssim\log(M_*/M_\odot)\lesssim11.8$), quiescent galaxies at $0.6\lesssim z\lesssim1.0$ from the Large Early Galaxy Astrophysics Census (LEGA-C) via continuum-normalized, absorption-line spectra, and compare with independent rest-frame $U-V$ and $V-J$ colours. Age increases along the quiescent sequence as both colours redden, consistent with stellar population synthesis (SPS) model predictions. Metallicity increases perpendicularly to the age trend, with higher metallicities at redder $U-V$ and bluer $V-J$ colours. Thus, age and metallicity behave differently in the $UVJ$ diagram. Moreover, this trend conflicts with SPS model expectations of increasing metallicity approximately along the quiescent sequence. Independent dynamical mass-to-light ratio trends also differ dramatically from SPS model predictions. These results demonstrate that relying on model fits to continuum shapes alone may lead to systematic biases in ages, metallicities, and stellar masses. The cause of these data-model disparities may stem from non-solar abundance patterns in quiescent galaxies or the treatment of evolved stellar phases in the models. Resolving these discrepancies is crucial, as photometric data remain central even with JWST. △ Less

Submitted 26 May, 2025; v1 submitted 13 May, 2025; originally announced May 2025.

Comments: 17 pages, 7 figures, accepted for publication in MNRAS; minor typesetting corrections after copyediting

Big, Dusty Galaxies in Blue Jay: Insights into the Relationship Between Morphology and Dust Attenuation at Cosmic Noon

Authors: Gabriel Maheson, Sandro Tacchella, Sirio Belli, Minjung Park, A. Lola Danhaive, Letizia Bugiani, Rebecca Davies, Razieh Emami, Amir H. Khoram, Laurence Lam, Joel Leja, Trevor Mendel, Erica June Nelson

Abstract: The dust attenuation of galaxies is highly diverse and closely linked to stellar population properties and the star dust geometry, yet its relationship to galaxy morphology remains poorly understood. We present a study of 141 galaxies ($9<\log(\rm M_{\star}/\rm M_{\odot})<11.5$) at $1.7<z<3.5$ from the Blue Jay survey combining deep JWST/NIRCam imaging and $R\sim1000$ JWST/NIRSpec spectra. Using \… ▽ More The dust attenuation of galaxies is highly diverse and closely linked to stellar population properties and the star dust geometry, yet its relationship to galaxy morphology remains poorly understood. We present a study of 141 galaxies ($9<\log(\rm M_{\star}/\rm M_{\odot})<11.5$) at $1.7<z<3.5$ from the Blue Jay survey combining deep JWST/NIRCam imaging and $R\sim1000$ JWST/NIRSpec spectra. Using \texttt{Prospector} to perform a joint analysis of these data with non-parametric star-formation histories and a two-component dust model with flexible attenuation laws, we constrain stellar and nebular properties. We find that the shape and strength of the attenuation law vary systematically with optical dust attenuation ($A_V$), stellar mass, and star formation rate (SFR). $A_V$ correlates strongly with stellar mass for starbursts, star-forming galaxies and quiescent galaxies. The inclusion of morphological information tightens these correlations: attenuation correlates more strongly with stellar mass and SFR surface densities than with the global quantities. The Balmer decrement-derived nebular attenuation for 67 of these galaxies shows consistent trends with the stellar continuum attenuation. We detect a wavelength-dependent size gradient: massive galaxies ($\rm M_{\star}\gtrsim 10^{10}~M_{\odot}$) appear $\sim30\%$ larger in the rest-optical than in the rest-NIR, driven by central dust attenuation that flattens optical light profiles. Lower-mass systems exhibit more diverse size ratios, consistent with either inside-out growth or central starbursts. These results demonstrate that dust attenuation significantly alters observed galaxy structure and highlight the necessity of flexible attenuation models for accurate physical and morphological inference at cosmic noon. △ Less

Submitted 21 April, 2025; originally announced April 2025.

Comments: 18 pages, 13 figures (+Appendix 1 page, 1 figure), submitted to MNRAS, comments welcome

Population Models for Star Formation Timescales in Early Galaxies: The First Step Towards Solving Outshining in Star Formation History Inference

Authors: Bingjie Wang, Joel Leja, Hakim Atek, Rachel Bezanson, Emilie Burnham, Pratika Dayal, Robert Feldmann, Jenny E. Greene, Benjamin D. Johnson, Ivo Labbe, Michael V. Maseda, Themiya Nanayakkara, Sedona H. Price, Katherine A. Suess, John R. Weaver, Katherine E. Whitaker

Abstract: JWST have revealed temporarily-quenched and ultraviolet-luminous galaxies in the early universe, suggesting enhanced star formation stochasticity. Verifying this hypothesis is critical, yet challenging; outshining, wherein light from young stars dominates the spectral energy distribution, represents perhaps the greatest challenge in inferring the formation histories of unresolved galaxies. In this… ▽ More JWST have revealed temporarily-quenched and ultraviolet-luminous galaxies in the early universe, suggesting enhanced star formation stochasticity. Verifying this hypothesis is critical, yet challenging; outshining, wherein light from young stars dominates the spectral energy distribution, represents perhaps the greatest challenge in inferring the formation histories of unresolved galaxies. In this paper, we take a simple model of burstiness and show that state-of-the-art inference methods with flexible star formation histories (SFHs) and neutral priors, while recovering average star formation rates (SFRs; $\sim0.1$ dex median offset), fail to recover the complexities of fluctuations on tens of Myr timescales, and typically underestimate masses in bursty systems ($\sim0.15$ dex). Surprisingly, detailed SFH recovery is still sensitive to priors even when data quality is optimal, e.g., including high signal-to-noise ($\rm20~pixel^{-1}$) spectroscopy with wide coverage (rest-frame $0.12-1.06~μ$m). Crucially, however, refitting the same data with a prior correctly encoding the bursty expectation eliminates these biases: median offsets in mass and SFRs decrease to $\sim 0.04$ dex and $\sim 0.05$ dex, respectively. Under the assumption that current population burstiness predicts past SFH, the solution to outshining in modeling statistical samples is empirically measuring recent galaxy SFHs with population modeling. A prototype is H$α$/UV: while helpful, it is insufficient to constrain the expected complex burstiness. To this end, we introduce a more complete, quantitative population-level approach and demonstrate that it promises to recover the typical amplitude, timescale, and slope of the recent SFH to high accuracy. This approach thus has the strong potential to solve outshining using observations from JWST. △ Less

Submitted 26 May, 2025; v1 submitted 21 April, 2025; originally announced April 2025.

Comments: Accepted for publication in ApJ; 27 pages, 17 figures

Journal ref: The Astrophysical Journal, 987, 184 (2025)

The Structure and Formation Histories of Low-Mass Quiescent Galaxies in the Abell 2744 Cluster Environment

Authors: Sam E. Cutler, John R. Weaver, Katherine E. Whitaker, Jenny E. Greene, David J. Setton, Zach J. Webb, Ayesha Abdullah, Aubrey Medrano, Rachel Bezanson, Gabriel Brammer, Robert Feldmann, Lukas J. Furtak, Karl Glazebrook, Ivo Labbe, Joel Leja, Danilo Marchesini, Tim B. Miller, Ikki Mitsuhashi, Themiya Nanayakkara, Erica J. Nelson, Richard Pan, Sedona H. Price, Katherine A. Suess, Bingjie Wang

Abstract: Low-mass quiescent galaxies are thought to predominantly reside in overdense regions, as environmental effects are often invoked to explain their shutdown of star formation. These longer-timescale quenching mechanisms - such as interactions with hot gas in the intracluster medium and dynamical encounters with other cluster galaxies - leave imprints on galaxy morphologies, emphasizing the importanc… ▽ More Low-mass quiescent galaxies are thought to predominantly reside in overdense regions, as environmental effects are often invoked to explain their shutdown of star formation. These longer-timescale quenching mechanisms - such as interactions with hot gas in the intracluster medium and dynamical encounters with other cluster galaxies - leave imprints on galaxy morphologies, emphasizing the importance of quantifying the structures of low-mass quiescent galaxies in galaxy clusters at $z<0.5$. Using spectrophotometric data from the UNCOVER and MegaScience programs, we present the first measurement of the quiescent size-mass relation between $7<\log(M_\star/M_\odot)<10$ using JWST imaging, based on a sample of 1531 galaxies in the $z=0.308$ Abell 2744 galaxy cluster. The resulting size-mass relation has a significantly higher scatter than similar-redshift field samples, despite comparable best-fit relations in both the dwarf and intermediate-mass regimes. Both "progenitor bias", where larger, diskier low-mass galaxies enter the cluster at later epochs, and a general expansion of galaxy structure from dynamical interactions could be at play. This evolutionary framework is further supported by the tentative evidence that older low-mass quiescent galaxies in the cluster are more spheroidal. The star-formation histories derived for our cluster sample imply their formation and quenching occurs relatively late, at $z<1.5$. In this scenario, the progenitor population would have disky axis-ratio distributions at cosmic noon, in agreement with recent observations. While this leaves ample time for dynamical interactions to maintain quiescence and drive the observed subsequent morphological evolution post-quenching, the data disfavors an onset of quenching due to the environment. △ Less

Submitted 12 August, 2025; v1 submitted 14 April, 2025; originally announced April 2025.

Comments: 21 pages, 8 figures. Submitted to ApJ

UNCOVER/MegaScience: No Evidence of Environmental Quenching in a z$\sim$2.6 Proto-cluster

Authors: Richard Pan, Katherine A. Suess, Danilo Marchesini, Bingjie Wang, Joel Leja, Sam E. Cutler, Katherine E. Whitaker, Rachel Bezanson, Sedona H. Price, Lukas J. Furtak, John R. Weaver, Ivo Labbé, Gabriel Brammer, Yunchong Zhang, Pratika Dayal, Robert Feldmann, Karl Glazebrook, Jenny E. Greene, Tim B. Miller, Ikki Mitsuhashi, Adam Muzzin, Themiya Nanayakkara, Erica J. Nelson, David J. Setton, Adi Zitrin

Abstract: Environmental quenching -- where interactions with other galaxies and/or the intra-cluster medium (ICM) suppress star formation in low-mass galaxies -- has been well-established as the primary driver behind the formation of the red sequence for low-mass galaxies within clusters at low redshift ($z<1$). However, it remains unclear whether these mechanisms are active at higher-redshifts in proto-clu… ▽ More Environmental quenching -- where interactions with other galaxies and/or the intra-cluster medium (ICM) suppress star formation in low-mass galaxies -- has been well-established as the primary driver behind the formation of the red sequence for low-mass galaxies within clusters at low redshift ($z<1$). However, it remains unclear whether these mechanisms are active at higher-redshifts in proto-cluster environments that are not yet fully virialized. In large part, this regime has remained unexplored due to observational limitations; however, JWST has recently opened a new window into the role of environmental quenching on low-mass (log(M$_{\star}$/M$_{\odot}$$<$9.0) galaxies at cosmic noon ($2 < z < 3$). Here, we leverage the deep imaging and R$\sim$15 spectrophotometry enabled by the 20 band JWST/NIRCam data from the UNCOVER and MegaScience programs to examine environmental quenching in a newly discovered $z\approx2.58$ proto-cluster. We compare the star formation histories (SFHs) of 19 low-mass quiescent galaxies in the proto-cluster to a matched sample of 18 in the field, and find no significant differences. This similarity extends to galaxy sizes and quenched fractions, which also show no significant differences between the two environments across the full stellar mass range (8.5$<$log(M$_{\star}$/M$_{\odot}$$\leq$11.0). This indicates that the proto-cluster has not yet accelerated quenching relative to the field and is consistent with expectations that $z>2$ proto-clusters have yet to virialize and develop a dense enough environment required to efficiently quench low-mass galaxies. △ Less

Submitted 22 August, 2025; v1 submitted 8 April, 2025; originally announced April 2025.

Comments: 15 pages, 7 figures, Submitted to ApJL

A remarkable Ruby: Absorption in dense gas, rather than evolved stars, drives the extreme Balmer break of a Little Red Dot at $z=3.5$

Authors: Anna de Graaff, Hans-Walter Rix, Rohan P. Naidu, Ivo Labbe, Bingjie Wang, Joel Leja, Jorryt Matthee, Harley Katz, Jenny E. Greene, Raphael E. Hviding, Josephine Baggen, Rachel Bezanson, Leindert A. Boogaard, Gabriel Brammer, Pratika Dayal, Pieter van Dokkum, Andy D. Goulding, Michaela Hirschmann, Michael V. Maseda, Ian McConachie, Tim B. Miller, Erica Nelson, Pascal A. Oesch, David J. Setton, Irene Shivaei , et al. (3 additional authors not shown)

Abstract: The origin of the rest-optical emission of compact, red, high-redshift sources known as `little red dots' (LRDs) poses a major puzzle. If interpreted as starlight, it would imply that LRDs would constitute the densest stellar systems in the Universe. However, alternative models suggest active galactic nuclei (AGN) may instead power the rest-optical continuum. Here, we present JWST/NIRSpec, NIRCam… ▽ More The origin of the rest-optical emission of compact, red, high-redshift sources known as `little red dots' (LRDs) poses a major puzzle. If interpreted as starlight, it would imply that LRDs would constitute the densest stellar systems in the Universe. However, alternative models suggest active galactic nuclei (AGN) may instead power the rest-optical continuum. Here, we present JWST/NIRSpec, NIRCam and MIRI observations from the RUBIES and PRIMER programs of The Cliff: a bright LRD at $z=3.55$ with an exceptional Balmer break, twice as strong as that of any high-redshift source previously observed. The spectra also reveal broad Hydrogen (H$α \rm FWHM\sim1500$km/s) and He I emission, but no significant metal lines. We demonstrate that massive evolved stellar populations cannot explain the observed spectrum, even when considering unusually steep and strong dust attenuation, or reasonable variations in the initial mass function. Moreover, the formally best-fit stellar mass and compact size ($M_*\sim10^{10.5}\,M_\odot,\ r_{e}\sim40\,$pc) would imply densities at which near-monthly stellar collisions might lead to significant X-ray emission. We argue that the Balmer break, emission lines, and H$α$ absorption line are instead most plausibly explained by a `black hole star' (BH*) scenario, in which dense gas surrounds a powerful ionising source. In contrast to recently proposed BH* models of dust-reddened AGN, we show that spectral fits in the rest UV to near-infrared favour an intrinsically redder continuum over strong dust reddening. This may point to a super-Eddington accreting massive black hole or, possibly, the presence of (super)massive stars in a nuclear star cluster. The Cliff is the clearest evidence to date that at least some LRDs are not ultra-dense, massive galaxies, and are instead powered by a central ionising source embedded in dense, absorbing gas. △ Less

Submitted 14 July, 2025; v1 submitted 20 March, 2025; originally announced March 2025.

Comments: main text 19 pages, 9 figures; accepted for publication in A&A

Journal ref: A&A 701, A168 (2025)

A "Black Hole Star" Reveals the Remarkable Gas-Enshrouded Hearts of the Little Red Dots

Authors: Rohan P. Naidu, Jorryt Matthee, Harley Katz, Anna de Graaff, Pascal Oesch, Aaron Smith, Jenny E. Greene, Gabriel Brammer, Andrea Weibel, Raphael Hviding, John Chisholm, Ivo Labbé, Robert A. Simcoe, Callum Witten, Hakim Atek, Josephine F. W. Baggen, Sirio Belli, Rachel Bezanson, Leindert A. Boogaard, Sownak Bose, Alba Covelo-Paz, Pratika Dayal, Yoshinobu Fudamoto, Lukas J. Furtak, Emma Giovinazzo , et al. (26 additional authors not shown)

Abstract: The physical processes that led to the formation of billion solar mass black holes within the first 700 million years of cosmic time remain a puzzle. Several theoretical scenarios have been proposed to seed and rapidly grow black holes, but direct observations of these mechanisms remain elusive. Here we present a source 660 million years after the Big Bang that displays singular properties: among… ▽ More The physical processes that led to the formation of billion solar mass black holes within the first 700 million years of cosmic time remain a puzzle. Several theoretical scenarios have been proposed to seed and rapidly grow black holes, but direct observations of these mechanisms remain elusive. Here we present a source 660 million years after the Big Bang that displays singular properties: among the largest Hydrogen Balmer breaks reported at any redshift, broad multi-peaked H$β$ emission, and Balmer line absorption in multiple transitions. We model this source as a "black hole star" (BH*) where the Balmer break and absorption features are a result of extremely dense, turbulent gas forming a dust-free "atmosphere" around a supermassive black hole. This source may provide evidence of an early black hole embedded in dense gas -- a theoretical configuration proposed to rapidly grow black holes via super-Eddington accretion. Radiation from the BH* appears to dominate almost all observed light, leaving limited room for contribution from its host galaxy. We demonstrate that the recently discovered "Little Red Dots" (LRDs) with perplexing spectral energy distributions can be explained as BH*s embedded in relatively brighter host galaxies. This source provides evidence that black hole masses in the LRDs may be over-estimated by orders of magnitude -- the BH* is effectively dust-free contrary to the steep dust corrections applied while modeling LRDs, and the physics that gives rise to the complex line shapes and luminosities may deviate from assumptions underlying standard scaling relations. △ Less

Submitted 20 March, 2025; originally announced March 2025.

Comments: Submitted. NIRSpec prism spectra featured in this work from JWST program GO-5224 ("Mirage or Miracle", PIs: Oesch & Naidu) are publicly available at https://zenodo.org/records/15059215 . See De Graaff et al. in today's arXiv posting for a z=3.5 BH*. Comments greatly appreciated and warmly welcomed!

A confirmed deficit of hot and cold dust emission in the most luminous Little Red Dots

Authors: David J. Setton, Jenny E. Greene, Justin S. Spilker, Christina C. Williams, Ivo Labbe, Yilun Ma, Bingjie Wang, Katherine E. Whitaker, Joel Leja, Anna de Graaff, Stacey Alberts, Rachel Bezanson, Leindert A. Boogaard, Gabriel Brammer, Sam E. Cutler, Nikko J. Cleri, Olivia R. Cooper, Pratika Dayal, Seiji Fujimoto, Lukas J. Furtak, Andy D. Goulding, Michaela Hirschmann, Vasily Kokorev, Michael V. Maseda, Ian McConachie , et al. (11 additional authors not shown)

Abstract: Luminous broad H$α$ emission and red rest-optical SEDs are the hallmark of compact Little Red Dots (LRDs), implying highly attenuated dusty starbursts and/or obscured active galactic nuclei. However, the lack of observed FIR emission has proved difficult to reconcile with the implied attenuated luminosity in these models. Here, we utilize deep new ALMA imaging, new and existing JWST/MIRI imaging,… ▽ More Luminous broad H$α$ emission and red rest-optical SEDs are the hallmark of compact Little Red Dots (LRDs), implying highly attenuated dusty starbursts and/or obscured active galactic nuclei. However, the lack of observed FIR emission has proved difficult to reconcile with the implied attenuated luminosity in these models. Here, we utilize deep new ALMA imaging, new and existing JWST/MIRI imaging, and archival Spitzer/Herschel imaging of two of the rest-optically brightest LRDs ($z=3.1$ and $z=4.47$) to place the strongest constraints on the IR luminosity in LRDs to date. The detections at $λ_\mathrm{rest}=1-4 \ μ$m imply flat slopes in the rest-IR, ruling out a contribution from hot ($T\gtrsim500$ K) dust. Similarly, FIR non-detections rule out any appreciable cold ($T\lesssim75$ K) dust component. Assuming energy balance, these observations are inconsistent with the typical FIR dust emission of dusty starbursts and quasar torii, which usually show a mixture of cold and hot dust. Additionally, our [$\mathrm{C}_{II}$] non-detections rule out typical dusty starbursts. We compute empirical maximum IR SEDs and find that both LRDs must have $\log(L_\mathrm{IR}/L_\odot) \lesssim 12.2$ at the $3σ$ level. These limits are in tension with the predictions of rest-optical spectrophotometric fits, be they galaxy only, AGN only, or composite. It is unlikely that LRDs are highly dust-reddened intrinsically blue sources with a dust temperature distribution that conspires to avoid current observing facilities. Rather, we favor an intrinsically redder LRD SED model that alleviates the need for strong dust attenuation. △ Less

Submitted 3 March, 2025; originally announced March 2025.

Comments: 16 pages, 5 figures, 3 tables. Submitted to ApJ Letters. Comments welcome!

Unveiling the origin of fast radio bursts by modeling the stellar mass and star formation distributions of their host galaxies

Authors: Nick Loudas, Dongzi Li, Michael A. Strauss, Joel Leja

Abstract: Diverse formation channels have been proposed to explain the emergence of fast radio bursts (FRBs), yet their origin remains elusive. With improved localization precision, roughly 90 FRBs are now associated with host galaxies. SED fitting to the host galaxy photometry reveals their stellar masses ($M_\star$) and star formation rates (SFRs), enabling discrimination between various formation channel… ▽ More Diverse formation channels have been proposed to explain the emergence of fast radio bursts (FRBs), yet their origin remains elusive. With improved localization precision, roughly 90 FRBs are now associated with host galaxies. SED fitting to the host galaxy photometry reveals their stellar masses ($M_\star$) and star formation rates (SFRs), enabling discrimination between various formation channels. We conduct an extensive comparison of the stellar mass, SFR and z distributions of 51 FRB hosts and mock-generated galaxy samples to test whether FRBs trace SFR or $M_\star$. We incorporate a mass-to-light ratio prescription to address optical selection biases. In line with Sharma et al. (2024), we provide evidence in favor of FRB progenitors tracking SF rather than stellar mass. We show that the shape of the assumed $(M_\star/L_r)_{obs}$ distribution affects the predictions, bringing the low mass end of the stellar mass distribution closer to the data when accounting for the $\mathrm{SFR} - (M_\star/L_r)_{obs}$ correlation. The K-correction effect in the $r-$band is minimal for galaxies at $z \lesssim 0.7$. Even if FRBs trace SF, up to $\sim 6\%$ of a flux-limited FRB host sample can reside below the star-forming main sequence. Finally, we examine a hybrid model in which a fraction of FRBs tracks stellar mass rather than SF. This fraction can be as large as $\sim(40-50)\%$, suggesting that multiple formation channels are still consistent with observations. The toolkit developed in this work is publicly available (GALFRB code), offering a straightforward way to generate mock galaxy samples suitable for direct comparisons with future FRB host galaxy data. △ Less

Submitted 2 April, 2025; v1 submitted 21 February, 2025; originally announced February 2025.

Comments: 23 pages, 12 figures, 1 table. Submitted to the Astrophysical Journal (ApJ); Updated Figures 5, 9, and 11, and revised methodology of sampling SFR values

Investigating photometric and spectroscopic variability in the multiply-imaged Little Red Dot A2744-QSO1

Authors: Lukas J. Furtak, Amy R. Secunda, Jenny E. Greene, Adi Zitrin, Ivo Labbé, Miriam Golubchik, Rachel Bezanson, Vasily Kokorev, Hakim Atek, Gabriel B. Brammer, Iryna Chemerynska, Sam E. Cutler, Pratika Dayal, Robert Feldmann, Seiji Fujimoto, Karl Glazebrook, Joel Leja, Yilun Ma, Jorryt Matthee, Rohan P. Naidu, Erica J. Nelson, Pascal A. Oesch, Richard Pan, Sedona H. Price, Katherine A. Suess , et al. (3 additional authors not shown)

Abstract: JWST observations have uncovered a new population of red, compact objects at high redshifts dubbed `Little Red Dots' (LRDs), which typically show broad emission lines and are thought to be dusty Active Galactic Nuclei (AGN). Some of their other features, however, challenge the AGN explanation, such as prominent Balmer breaks and extremely faint or even missing metal high-ionization lines, X-ray, o… ▽ More JWST observations have uncovered a new population of red, compact objects at high redshifts dubbed `Little Red Dots' (LRDs), which typically show broad emission lines and are thought to be dusty Active Galactic Nuclei (AGN). Some of their other features, however, challenge the AGN explanation, such as prominent Balmer breaks and extremely faint or even missing metal high-ionization lines, X-ray, or radio emission, including in deep stacks. Time variability is another, robust, test of AGN activity. Here, we exploit the $z=7.045$ multiply-imaged LRD A2744-QSO1, which offers a particularly unique test of variability due to lensing-induced time delays between the three images spanning 22 yr (2.7 yr in the rest-frame), to investigate its photometric and spectroscopic variability. We find the equivalent widths (EWs) of the broad H$α$ and H$β$ lines, which are independent of magnification and other systematics, to exhibit significant variations, up to $18\pm3$ % for H$α$ and up to $22\pm8$ % in H$β$, on a timescale of 875 d (2.4 yr) in the rest-frame. This suggests that A2744-QSO1 is indeed an AGN. We find no significant photometric variability beyond the limiting systematic uncertainties, so it currently cannot be determined whether the EW variations are due to line-flux or continuum variability. These results are consistent with a typical damped random walk (DRW) variability model for an AGN like A2744-QSO1 ($M_{\mathrm{BH}}=4\times10^7 \mathrm{M}_{\odot}$) given the sparse sampling of the light-curve with the available data. Our results therefore support the AGN interpretation of this LRD, and highlight the need for further photometric and spectroscopic monitoring in order to build a detailed and reliable light-curve. △ Less

Submitted 7 May, 2025; v1 submitted 11 February, 2025; originally announced February 2025.

Comments: Accepted for publication in A&A. v3 updated to match the accepted version

Journal ref: A&A 698, A227 (2025)

More is better: Strong constraints on the stellar properties of LEGA-C z ~ 1 galaxies with Prospector

Authors: Angelos Nersesian, Arjen van der Wel, Anna R. Gallazzi, Yasha Kaushal, Rachel Bezanson, Stefano Zibetti, Eric F. Bell, Francesco D'Eugenio, Joel Leja, Marco Martorano, Po-Feng Wu

Abstract: We present the stellar properties of 2908 galaxies at 0.6 < z < 1.0 from the LEGA-C survey. We emphasize the importance of high signal-to-noise, high spectral resolution spectroscopy in the inference of stellar population properties of galaxies. We estimate the galaxy properties with the SED fitting code Prospector, by fitting spectroscopy and broadband photometry together, drawn from the LEGA-C D… ▽ More We present the stellar properties of 2908 galaxies at 0.6 < z < 1.0 from the LEGA-C survey. We emphasize the importance of high signal-to-noise, high spectral resolution spectroscopy in the inference of stellar population properties of galaxies. We estimate the galaxy properties with the SED fitting code Prospector, by fitting spectroscopy and broadband photometry together, drawn from the LEGA-C DR3 and UltraVISTA catalogs respectively. We report a positive correlation between light-weighted ages and stellar velocity dispersion ($σ_\star$). The trend with $σ_\star$ is weaker for the mass-weighted ages and stellar metallicity ($Z_\star$). On average, quiescent galaxies are characterized by high $Z_\star$, they are \sim 1.1 Gyr older, less dusty, with steeper dust attenuation slopes compared to star-forming galaxies. Conversely, star-forming galaxies are characterized by significantly higher dust optical depths and shallower (grayer) attenuation slopes. Low mass (high mass) star-forming galaxies have lower (higher) $Z_\star$, while their stellar populations are on average younger (older). A key pragmatic result of our study is that a linear-space metallicity prior is preferable to a logarithmic-space one when using photometry alone, as the latter biases the posteriors downward. Spectroscopy greatly improves stellar population measurements and is required to provide meaningful constraints on age, metallicity, and other properties. Pairing spectroscopy with photometry helps resolving the dust-age-metallicity degeneracy, yielding more accurate mass- and light-weighted ages, with ages inferred from photometry alone suffering such large uncertainties. Stellar metallicities are constrained by our spectroscopy, but precise measurements remain challenging (and impossible with photometry alone), particularly in the absence of Mg and Fe lines redward of 5000 $Å$ in the observed spectrum. △ Less

Submitted 12 February, 2025; v1 submitted 5 February, 2025; originally announced February 2025.

Comments: Accepted, 24 pages, 17 figures

Journal ref: A&A 695, A86 (2025)

Discovery of Ancient Globular Cluster Candidates in The Relic, a Quiescent Galaxy at z=2.5

Authors: Katherine E. Whitaker, Sam E. Cutler, Rupali Chandar, Richard Pan, David J. Setton, Lukas J. Furtak, Rachel Bezanson, Ivo Labbé, Joel Leja, Katherine A. Suess, Bingjie Wang, John R. Weaver, Hakim Atek, Gabriel B. Brammer, Robert Feldmann, Natascha M. Förster Schreiber, Karl Glazebrook, Anna de Graaff, Jenny E. Greene, Gourav Khullar, Danilo Marchesini, Michael V. Maseda, Tim B. Miller, Houjun Mo, Lamiya A. Mowla , et al. (9 additional authors not shown)

Abstract: Globular clusters (GCs) are some of the oldest bound structures in the Universe, holding clues to the earliest epochs of star formation and galaxy assembly. However, accurate age measurements of ancient clusters are challenging due to the age-metallicity degeneracy. Here, we report the discovery of 42 compact stellar systems within the 'Relic', a massive, quiescent galaxy at $z=2.53$. The Relic re… ▽ More Globular clusters (GCs) are some of the oldest bound structures in the Universe, holding clues to the earliest epochs of star formation and galaxy assembly. However, accurate age measurements of ancient clusters are challenging due to the age-metallicity degeneracy. Here, we report the discovery of 42 compact stellar systems within the 'Relic', a massive, quiescent galaxy at $z=2.53$. The Relic resides in an over-density behind the Abell 2744 cluster, with a prominent tidal tail extending towards two low-mass companions. Using deep data from the UNCOVER/MegaScience JWST Surveys, we find that clusters formed in age intervals ranging from 8 Myr up to $\sim2$ Gyr, suggesting a rich formation history starting at $z\sim10$. While the cluster-based star formation history is broadly consistent with the high past star formation rates derived from the diffuse host galaxy light, one potential discrepancy is a tentative $\sim2-3\times$ higher rate in the cluster population for the past Gyr. Taken together with the spatial distribution and low inferred metallicities of these young-to-intermediate age clusters, we may be seeing direct evidence for the accretion of star clusters in addition to their early in-situ formation. The cluster masses are high, $\sim10^6-10^7~M_{\odot}$, which may explain why we are able to detect them around this likely post-merger galaxy. Overall, the Relic clusters are consistent with being precursors of the most massive present-day GCs. This unique laboratory enables the first connection between long-lived, high-redshift clusters and local stellar populations, offering insights into the early stages of GC evolution and the broader processes of galaxy assembly. △ Less

Submitted 13 January, 2025; originally announced January 2025.

Comments: 19 pages, 7 figures, submitted to Astrophysical Journal on January 9, 2025 (comments welcome!)

JWST UNCOVERs the Optical Size - Stellar Mass Relation at $4<z<8$: Rapid Growth in the Sizes of Low Mass Galaxies in the First Billion Years of the Universe

Authors: Tim B. Miller, Katherine A. Suess, David J. Setton, Sedona H. Price, Ivo Labbe, Rachel Bezanson, Gabriel Brammer, Sam E. Cutler, Lukas J. Furtak, Joel Leja, Richard Pan, Bingjie Wang, John R. Weaver, Katherine E. Whitaker, Pratika Dayal, Anna de Graaff, Robert Feldmann, Jenny E. Greene, S. Fujimoto, Michael V. Maseda, Themiya Nanayakkara, Erica J. Nelson, Pieter van Dokkum, Adi Zitrin

Abstract: We study the rest-frame optical and ultraviolet morphology of galaxies in the first billion years of the Universe. Using JWST data from the UNCOVER and MegaScience surveys targeting the lensing cluster Abell 2744 we present multi-band morphological measurements for a sample of 995 galaxies selected using 20-band NIRCam photometry and 35 using NIRSpec Prism spectroscopy over the redshift range of… ▽ More We study the rest-frame optical and ultraviolet morphology of galaxies in the first billion years of the Universe. Using JWST data from the UNCOVER and MegaScience surveys targeting the lensing cluster Abell 2744 we present multi-band morphological measurements for a sample of 995 galaxies selected using 20-band NIRCam photometry and 35 using NIRSpec Prism spectroscopy over the redshift range of $4<z<8$. The wavelength-dependent morphology is measured using pysersic by simultaneously modeling the images in 6 NIRCam wide filters covering the rest-frame UV to optical. The joint modeling technique increases the precision of measured radii by 50\%. Galaxies in our sample show a wide range of Sersic indices, with no systematic difference between optical and UV morphology. We model the size-mass relation in a Bayesian manner using a continuity model to directly fit the redshift evolution while accounting for observational uncertainties. We find the average size of galaxies at $\log M_*/M_\odot=8.5$ grows rapidly, from 400 pc at $z=8$ to 830 pc at $z=4$. This is faster evolution than expected from power law scalings of the Hubble parameter or scale factor that describe well previous results at $z<2$. This suggests that different and/or much stronger processes affect low mass systems during the epoch of reionization. The measured logarithmic slope (0.25) and scatter (0.23 dex) are non-evolving. We discuss the remarkable consistency of the slope and scatter over cosmic time in the context of the galaxy-halo connection. △ Less

Submitted 16 June, 2025; v1 submitted 9 December, 2024; originally announced December 2024.

Comments: Accepted for publication in ApJ

An unambiguous AGN and a Balmer break in an Ultraluminous Little Red Dot at z=4.47 from Ultradeep UNCOVER and All the Little Things Spectroscopy

Authors: Ivo Labbe, Jenny E. Greene, Jorryt Matthee, Helena Treiber, Vasily Kokorev, Tim B. Miller, Ivan Kramarenko, David J. Setton, Yilun Ma, Andy D. Goulding, Rachel Bezanson, Rohan P. Naidu, Christina C. Williams, Hakim Atek, Gabriel Brammer, Sam E. Cutler, Iryna Chemerynska, Aidan P. Cloonan, Pratika Dayal, Anna de Graaff, Yoshinobu Fudamoto, Seiji Fujimoto, Lukas J. Furtak, Karl Glazebrook, Kasper E. Heintz , et al. (15 additional authors not shown)

Abstract: We present a detailed exploration of the most optically-luminous Little Red Dot ($L_{Hα}=10^{44}$erg/s, $L_V=10^{45}$erg/s, F444W=22AB) found to date. Located in the Abell 2744 field, source A744-45924 was observed by NIRSpec/PRISM with ultradeep spectroscopy reaching SNR$\sim$100pix$^{-1}$, high-resolution 3-4 micron NIRCam/Grism spectroscopy, and NIRCam Medium Band imaging. The NIRCam spectra re… ▽ More We present a detailed exploration of the most optically-luminous Little Red Dot ($L_{Hα}=10^{44}$erg/s, $L_V=10^{45}$erg/s, F444W=22AB) found to date. Located in the Abell 2744 field, source A744-45924 was observed by NIRSpec/PRISM with ultradeep spectroscopy reaching SNR$\sim$100pix$^{-1}$, high-resolution 3-4 micron NIRCam/Grism spectroscopy, and NIRCam Medium Band imaging. The NIRCam spectra reveal high rest-frame EW $W_{Hα,0,broad}>800$Å, broad H$α$ emission (FWHM$\sim$4500 km/s), on top of narrow, complex absorption. NIRSpec data show exceptionally strong rest-frame UV to NIR Fe II emission ($W_{FeII-UV,0}\sim$340Å), N IV]$λλ$1483,1486 and N III]$λ$1750, and broad NIR O I $λ$8446 emission. The spectra unambiguously demonstrate a broad-line region associated with an inferred $M_{BH}\sim10^9M_\odot$ supermassive black hole embedded in dense gas, which might explain a non-detection in ultradeep Chandra X-ray data (>$10\times$ underluminous relative to broad $L_{Hα}$). Strong UV Nitrogen lines suggest supersolar N/O ratios due to rapid star formation or intense radiation near the AGN. The continuum shows a clear Balmer break at rest-frame 3650Å, which cannot be accounted for by an AGN power-law alone. A stellar population model produces an excellent fit with a reddened Balmer break and implying a massive ($M_*\sim8\times10^{10}M_\odot$), old $\sim$500 Myr, compact stellar core, among the densest stellar systems known ($ρ\sim3\times10^6M_\odot$/pc$^2$ for $R_{e,opt}=70\pm10$ pc), and AGN emission with extreme intrinsic EW $W_{Hα,0}\gg$1000Å. However, although high $M_*$ and $M_{BH}$ are supported by evidence of an overdensity containing 40 galaxies at $z=4.41-4.51$, deep high-resolution spectroscopy is required to confirm stellar absorption and rule out that dense gas around the AGN causes the Balmer break instead. △ Less

Submitted 5 December, 2024; originally announced December 2024.

Comments: 28 pages,10 figures, submitted to ApJ

Simultaneous explanation of XTE J1814-338 and HESS J1731-347 objects using ${K^{-}}$ and ${\bar{K}^{0}}$ condensates

Authors: M. Veselsky, V. Petousis, P. S. Koliogiannis, Ch. C. Moustakidis, J. Leja

Abstract: The recent observation of the compact star XTE J1814-338 with a mass of $M=1.2^{+0.05}_{-0.05}~{\rm M_{\odot}}$ and a radius of $R=7^{+0.4}_{-0.4}$ km, together with the HESS J1731-347, which has a mass of $M=0.77^{+0.20}_{-0.17}~{\rm M_{\odot}}$ and a radius of $R=10.4^{+0.86}_{-0.78}$ km, shows they provide evidence for the possible presence of exotic matter in the core of neutron stars and sign… ▽ More The recent observation of the compact star XTE J1814-338 with a mass of $M=1.2^{+0.05}_{-0.05}~{\rm M_{\odot}}$ and a radius of $R=7^{+0.4}_{-0.4}$ km, together with the HESS J1731-347, which has a mass of $M=0.77^{+0.20}_{-0.17}~{\rm M_{\odot}}$ and a radius of $R=10.4^{+0.86}_{-0.78}$ km, shows they provide evidence for the possible presence of exotic matter in the core of neutron stars and significantly enhance our understanding of the equation of state for the dense nuclear matter. In the present study, we investigate the possible existence of negative charged kaons and neutral anti-kaons in neutron stars by employing the relativistic mean field model with first order kaonic (${K^{-}}$ and ${\bar{K}^{0}}$) condensates. To the best of our knowledge, this represents a first alternative attempt aimed to explain the bulk properties of the XTE J1814-338 object and at the same time the HESS J1731-347 object, using a mixture of kaons condensation in dense nuclear matter. In addition, we compare our analysis approach with the recent observation of PSR J0437-4715 and PSR J1231-1411 pulsars, proposing that to simultaneously explain the current variety of astrophysical objects, it is essential to resurrect a scenario of two distinct branches, each corresponding to a different composition of nuclear matter. △ Less

Submitted 5 September, 2025; v1 submitted 2 December, 2024; originally announced December 2024.

Comments: v1: 4 pages, 3 figures. v2: 4 pages, 3 figures, updated text and references; accepted for publication in Physical Review D

Journal ref: Phys. Rev. D 111, L061308 (2025)

Little Red Dots at an Inflection Point: Ubiquitous "V-Shaped" Turnover Consistently Occurs at the Balmer Limit

Authors: David J. Setton, Jenny E. Greene, Anna de Graaff, Yilun Ma, Joel Leja, Jorryt Matthee, Rachel Bezanson, Leindert A. Boogaard, Nikko J. Cleri, Harley Katz, Ivo Labbe, Michael V. Maseda, Ian McConachie, Tim B. Miller, Sedona H. Price, Katherine A. Suess, Pieter van Dokkum, Bingjie Wang, Andrea Weibel, Katherine E. Whitaker, Christina C. Williams

Abstract: Among the most puzzling early discoveries of JWST are "Little Red Dots" -- compact red sources that host broad Balmer emission lines and, in many cases, exhibit a "V shaped" change in slope in the rest-optical. The physical properties of Little Red Dots currently have order-of-magnitude uncertainties, because models to explain the continuum of these sources differ immensely. Here, we leverage the… ▽ More Among the most puzzling early discoveries of JWST are "Little Red Dots" -- compact red sources that host broad Balmer emission lines and, in many cases, exhibit a "V shaped" change in slope in the rest-optical. The physical properties of Little Red Dots currently have order-of-magnitude uncertainties, because models to explain the continuum of these sources differ immensely. Here, we leverage the complete selection of red sources in the RUBIES program, supplemented with public PRISM spectra, to study the origin of this "V shape". By fitting a broken power law with a flexible inflection point, we find that a large fraction (20/44, nearly all spatially unresolved) of extremely red H$α$ emitters at $2<z<6$ exhibit a strong change in slope, and that all strong inflections appear associated with the Balmer limit ($0.3645$ $μ$m). Using a simple model of a reddened AGN with an unobscured scattered light component, we demonstrate that the observed "V shape" in Little Red Dots is unlikely to occur at any specific wavelength if the entire continuum is dominated by light from a power law AGN continuum. In contrast, models with an intrinsic feature at the Balmer limit, such as those that are dominated by evolved stellar populations in the rest-UV-to-optical, can produce the observed spectral shapes, provided that a reddened component picks up sufficiently redward of the break. While no model can comfortably explain the full Little Red Dot spectral energy distribution, the common inflection location suggests that it is most likely a single component that consistently dominates the rest-UV-to-optical in Little Red Dots, and that this component is associated with $T\sim10^4$ K hydrogen due to the clear preference for a break at H$_\infty$. △ Less

Submitted 5 November, 2024; originally announced November 2024.

Comments: 14 pages, 5 figures. Submitted to ApJ, comments welcome!

On the Significance of Covariance for Constraining Theoretical Models From Galaxy Observables

Authors: Yongseok Jo, Shy Genel, Joel Leja, Benjamin Wandelt

Abstract: In this study, we investigate the impact of covariance within uncertainties on the inference of cosmological and astrophysical parameters, specifically focusing on galaxy stellar mass functions derived from the CAMELS simulation suite. Utilizing both Fisher analysis and Implicit Likelihood Inference (ILI), we explore how different covariance structures, including simple toy models and physics-moti… ▽ More In this study, we investigate the impact of covariance within uncertainties on the inference of cosmological and astrophysical parameters, specifically focusing on galaxy stellar mass functions derived from the CAMELS simulation suite. Utilizing both Fisher analysis and Implicit Likelihood Inference (ILI), we explore how different covariance structures, including simple toy models and physics-motivated uncertainties, affect posterior distributions and parameter variances. Our methodology utilizes forward modeling via emulators that are trained on CAMELS simulations to produce stellar mass functions based on input parameters, subsequently incorporating Gaussian noise as defined by covariance matrices. We examine both toy model covariance matrices and physically motivated covariance matrices derived from observational factors like the stellar Initial Mass Function (IMF) and photometric aperture size. Our results demonstrate that covariance terms significantly influence parameter inference, often leading to tighter constraints or revealing complex, multimodal posterior distributions. These findings underscore the necessity of accounting for covariance when interpreting astrophysical observations, especially in fields where accurate parameter estimation is critical for model validation and hypothesis testing. △ Less

Submitted 29 October, 2024; originally announced October 2024.

Comments: 19 pages, 6 figures, submitted to ApJ

$α$-MC: Self-consistent $α$-enhanced stellar population models covering a wide range of age, metallicity, and wavelength

Authors: Minjung Park, Charlie Conroy, Benjamin D. Johnson, Joel Leja, Aaron Dotter, Phillip A. Cargile

Abstract: We present new stellar population models, $α$-MC, self-consistently taking into account non-solar $\rm [α/Fe]$ abundances for both isochrones and stellar spectra. The $α$-MC models are based on $α$-enhanced MIST isochrones and C3K spectral libraries, which are publicly available in FSPS. Our new models cover a wide range of ages ($\rm \log (age/yr) = 5.0 - 10.3$), metallicities (… ▽ More We present new stellar population models, $α$-MC, self-consistently taking into account non-solar $\rm [α/Fe]$ abundances for both isochrones and stellar spectra. The $α$-MC models are based on $α$-enhanced MIST isochrones and C3K spectral libraries, which are publicly available in FSPS. Our new models cover a wide range of ages ($\rm \log (age/yr) = 5.0 - 10.3$), metallicities ($\rm [Fe/H]=[-2.5,+0.5]$ in steps of 0.25, $\rm [α/Fe]=-0.2,+0.0,+0.2,+0.4,+0.6$), and wavelengths ($0.1-2.5\,\rm μm$). We investigate the separate and combined effects of $α$-enhanced isochrones and stellar spectral libraries on simple stellar populations (SSPs), including their broadband colors, spectral indices, and full spectra. We find that the primary effect of $α$-enhancement in isochrones is to lower the overall continuum levels and redden the continuum shapes, while $α$-enhancement in stellar spectra mainly affects individual spectral lines. At constant $\rm [Fe/H]$, $α$-enhancement has significant impacts on the broadband colors by $\rm \sim 0.1-0.4\,mag$ across all ages ($\rm 0.01 - 10\,Gyr$). The effects of $α$-enhancement on colors at fixed $\rm [Z/H]$ are smaller, by $\rm \sim 0.1-0.2\,mag$. The spectral indices involving $α$-elements, Ca4227 and Mg b, increase with $\rm [α/Fe]$ (both at fixed $\rm [Fe/H]$ and fixed $\rm [Z/H]$) due to enhanced $α$-abundances. At constant $\rm [Fe/H]$, $α$-enhancement weakens most Fe-sensitive and Hydrogen Balmer lines. Our new self-consistent $α$-enhanced models will be essential in deriving accurate physical properties of high-redshift galaxies, where $α$-enhancement is expected to be common. △ Less

Submitted 28 October, 2024; originally announced October 2024.

Comments: Submitted to ApJ. Comments welcome

RUBIES: JWST/NIRSpec resolves evolutionary phases of dusty star-forming galaxies at $z\sim2$

Authors: Olivia R. Cooper, Gabriel Brammer, Kasper E. Heintz, Sune Toft, Caitlin M. Casey, David J. Setton, Anna de Graaff, Leindert Boogaard, Nikko J. Cleri, Steven Gillman, Rashmi Gottumukkala, Jenny E. Greene, Bitten Gullberg, Michaela Hirschmann, Raphael E. Hviding, Erini Lambrides, Joel Leja, Arianna S. Long, Sinclaire M. Manning, Michael V. Maseda, Ian McConachie, Jed McKinney, Desika Narayanan, Sedona H. Price, Victoria Strait , et al. (2 additional authors not shown)

Abstract: The dearth of high quality spectroscopy of dusty star-forming galaxies (DSFGs) -- the main drivers of the assembly of dust and stellar mass at the peak of activity in the Universe -- greatly hinders our ability to interpret their physical processes and evolutionary pathways. We present JWST/NIRSpec observations from RUBIES of four submillimeter-selected, ALMA-detected DSFGs at cosmic noon,… ▽ More The dearth of high quality spectroscopy of dusty star-forming galaxies (DSFGs) -- the main drivers of the assembly of dust and stellar mass at the peak of activity in the Universe -- greatly hinders our ability to interpret their physical processes and evolutionary pathways. We present JWST/NIRSpec observations from RUBIES of four submillimeter-selected, ALMA-detected DSFGs at cosmic noon, $z\sim2.3-2.7$. While photometry uniformly suggests vigorous ongoing star formation for the entire sample in line with canonical DSFGs, the spectra differ: one source has spectroscopic evidence of an evolved stellar population, indicating a recent transition to a post-starburst phase, while the remainder show strong spectroscopic signatures of ongoing starbursts. All four galaxies are infrared-luminous (log$_{10}$$L_{\rm{IR}}$/L$_{\rm \odot}$ $>12.4$), massive (log$_{10}\,M_\star$/M$_{\rm \odot}$ $>11$), and very dust-obscured ($A_V\sim3-4$ ABmag). Leveraging detections of multiple Balmer and Paschen lines, we derive an optical attenuation curve consistent with Calzetti overall, yet an optical extinction ratio $R_V\sim2.5$, potentially indicating smaller dust grains or differences in star-dust geometry. This case study provides some of the first detailed spectroscopic evidence that the DSFGs encompass a heterogeneous sample spanning a range of star formation properties and evolutionary stages, and illustrates the advantages of synergistic JWST and ALMA analysis of DSFGs. △ Less

Submitted 10 October, 2024; originally announced October 2024.

Comments: 20 pages, 4 figures; submitted to ApJ

UNCOVER: 404 Error -- Models Not Found for the Triply Imaged Little Red Dot A2744-QSO1

Authors: Yilun Ma, Jenny E. Greene, David J. Setton, Marta Volonteri, Joel Leja, Bingjie Wang, Rachel Bezanson, Gabriel Brammer, Sam E. Cutler, Pratika Dayal, Pieter van Dokkum, Lukas J. Furtak, Karl Glazebrook, Andy D. Goulding, Anna de Graaff, Vasily Kokorev, Ivo Labbe, Richard Pan, Sedona H. Price, John R. Weaver, Christina C. Williams, Katherine E. Whitaker, Adi Zitrin

Abstract: JWST has revealed an abundance of compact, red objects at $z\approx5-8$ dubbed "little red dots" (LRDs), whose SEDs display a faint blue UV continuum followed by a steep rise in the optical. Despite extensive study of their characteristic V-shaped SEDs, the nature of LRDs remains unknown. We present a new analysis of the NIRSpec/PRISM spectrum of A2744-QSO1, a triply imaged LRD at $z=7.04$ from th… ▽ More JWST has revealed an abundance of compact, red objects at $z\approx5-8$ dubbed "little red dots" (LRDs), whose SEDs display a faint blue UV continuum followed by a steep rise in the optical. Despite extensive study of their characteristic V-shaped SEDs, the nature of LRDs remains unknown. We present a new analysis of the NIRSpec/PRISM spectrum of A2744-QSO1, a triply imaged LRD at $z=7.04$ from the UNCOVER survey. The spectrum shows a strong Balmer break and broad Balmer emission lines, both of which are difficult to explain with models invoking exclusively AGN or stellar contributions. Our fiducial model decomposes the spectrum into a post-starburst galaxy dominating the UV-optical continuum and a reddened AGN being sub-dominant at all wavelength and contributing at $\sim20\%$ level. However, our most credible model infers a stellar mass of $M_\star\approx 4\times10^9\,\mathrm{M_\odot}$ within a radius of $r_\mathrm{e}<30\,$pc, driving its central density to the highest among observations to date. This high central density could be explained if A2744-QSO-1 is the early-forming core of a modern-day massive elliptical galaxy that later puffed up via the inside-out growth channel. The models also necessitate an unusually steep dust law to preserve the strong break strength, though this steepness may be explained by a deficit of large dust grains. It is also probable that these challenges reflect our ignorance of A2744-QSO1's true nature. Future variability and reverberation mapping studies could help disentangle the galaxy and AGN contribution to the continuum, and deeper redder observations could also unveil the dust properties in LRDs. △ Less

Submitted 16 March, 2025; v1 submitted 8 October, 2024; originally announced October 2024.

Comments: Accepted by ApJ

How the HESS J1731-347 event could be explained using $\bf{K^{-}}$ condensation

Authors: M. Veselsky, P. S. Koliogiannis, V. Petousis, J. Leja, Ch. C. Moustakidis

Abstract: The recent observation of a compact star with a mass of $M=0.77^{+0.20}_{-0.17}~{\rm M_{\odot}}$ and a radius of $R=10.4^{+0.86}_{-0.78}$ km, located within the supernova remnant HESS J1731-347, has substantially reinforced the evidence for the presence of exotic matter in neutron stars core. This finding has markedly enhanced our comprehension of the equation of state for dense nuclear matter. In… ▽ More The recent observation of a compact star with a mass of $M=0.77^{+0.20}_{-0.17}~{\rm M_{\odot}}$ and a radius of $R=10.4^{+0.86}_{-0.78}$ km, located within the supernova remnant HESS J1731-347, has substantially reinforced the evidence for the presence of exotic matter in neutron stars core. This finding has markedly enhanced our comprehension of the equation of state for dense nuclear matter. In the present work, we investigate the possible existence of a kaon condensation in hadronic neutron stars by employing and comparing two theoretical frameworks: the Relativistic Mean Field model with first order kaon condensate and the Momentum-Dependent Interaction model complemented by chiral effective theory. To the best of our knowledge, this represents a first alternative attempt aimed to explain the bulk properties of the specific object with the inclusion of a kaon condensation in dense nuclear matter. The application of two different models enriches the research, providing insights from the aspect of different theoretical frameworks that accurately predict the existence of HESS J1731-347. In both cases significant insights are extracted for the parameter space of models, emphasizing to those concerning the nucleon-kaon potential, the threshold density for the appearance of a kaon condensation, as well as the parameter $a_{3}m_{s}$ which is related to the strangeness content of the proton. Concluding, the present research indicates that a more systematic investigation of similar objects could offer valuable constraints on the properties of dense nuclear matter. △ Less

Submitted 14 December, 2024; v1 submitted 7 October, 2024; originally announced October 2024.

Comments: v1: 9 pages, 7 figures, 1 table. v2: 9 pages, 7 figures, 1 table, updated text; accepted for publication in Physics Letters B

Journal ref: Phys. Lett. B 860, 139185 (2025)

All the Little Things in Abell 2744: $>$1000 Gravitationally Lensed Dwarf Galaxies at $z=0-9$ from JWST NIRCam Grism Spectroscopy

Authors: Rohan P. Naidu, Jorryt Matthee, Ivan Kramarenko, Andrea Weibel, Gabriel Brammer, Pascal A. Oesch, Peter Lechner, Lukas J. Furtak, Claudia Di Cesare, Alberto Torralba, Gauri Kotiwale, Rachel Bezanson, Rychard J. Bouwens, Vedant Chandra, Adélaïde Claeyssens, A. Lola Danhaive, Anna Frebel, Anna de Graaff, Jenny E. Greene, Kasper E. Heintz, Alexander P. Ji, Daichi Kashino, Harley Katz, Ivo Labbe, Joel Leja , et al. (9 additional authors not shown)

Abstract: Dwarf galaxies hold the key to crucial frontiers of astrophysics, however, their faintness renders spectroscopy challenging. Here we present the JWST Cycle 2 survey, All the Little Things (ALT, PID 3516), which is designed to seek late-forming Pop III stars and the drivers of reionization at $z\sim6-7$. ALT has acquired the deepest NIRCam grism spectroscopy yet (7-27 hr), at JWST's most sensitive… ▽ More Dwarf galaxies hold the key to crucial frontiers of astrophysics, however, their faintness renders spectroscopy challenging. Here we present the JWST Cycle 2 survey, All the Little Things (ALT, PID 3516), which is designed to seek late-forming Pop III stars and the drivers of reionization at $z\sim6-7$. ALT has acquired the deepest NIRCam grism spectroscopy yet (7-27 hr), at JWST's most sensitive wavelengths (3-4 $μ$m), covering the powerful lensing cluster Abell 2744. Over the same 30 arcmin$^2$, ALT's ultra-deep F070W+F090W imaging ($\sim$30 mag) enables selection of very faint sources at $z>6$. We demonstrate the success of ALT's novel ``butterfly" mosaic to solve spectral confusion and contamination, and introduce the ``Allegro" method for emission line identification. By collecting spectra for every source in the field of view, ALT has measured precise ($R\sim1600$) redshifts for 1630 sources at $z=0.2-8.5$. This includes one of the largest samples of distant dwarf galaxies: [1015, 475, 50] sources less massive than the SMC, Fornax, and Sculptor with $\log(M_{*}/M_{\odot})<$[8.5, 7.5, 6.5]. We showcase ALT's discovery space with: (i) spatially resolved spectra of lensed clumps in galaxies as faint as $M_{\rm{UV}}\sim-15$; (ii) large-scale clustering -- overdensities at $z$=[2.50, 2.58, 3.97, 4.30, 5.66, 5.77, 6.33] hosting massive galaxies with striking Balmer breaks; (iii) small-scale clustering -- a system of satellites around a Milky Way analog at $z\sim6$; (iv) spectroscopically confirmed multiple images that help constrain the lensing model underlying all science in this legacy field; (v) sensitive star-formation maps based on dust-insensitive tracers such as Pa$α$; (vi) direct spectroscopic discovery of rare sources such as AGN with ionized outflows. These results provide a powerful proof of concept for how grism surveys maximize the potential of strong lensing fields. △ Less

Submitted 2 October, 2024; originally announced October 2024.

Comments: Submitted to the Open Journal of Astrophysics. Figs. 6 and 10 illustrate the quality of the spectra and imaging, while Fig. 12 summarizes the yield of the survey. Comments warmly welcomed and greatly appreciated. The ALT DR1 catalog is available at https://zenodo.org/records/13871850