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RUBIES: The Evolution of the Ionization Parameter from 0 < z < 9
Authors:
Nikko J. Cleri,
Zach J. Lewis,
Joel Leja,
Jakob M. Helton,
Emilie Burnham,
Olivia Curtis,
Anna de Graaff,
Michaela Hirschmann,
Harley Katz,
Michael V. Maseda,
Ian McConachie,
Adele Plat,
Lucie Scharre
Abstract:
The dimensionless ionization parameter, U=q/c, where q is the ratio of the local ionizing photon flux to the local hydrogen density, is a key metric to parameterize nebular conditions. Prior to JWST, the rest-frame optical emission lines and their ratios which trace the ionization parameter (e.g., O32=[OIII]/[OII]) were inaccessible at high redshifts. Here we quantify, for the first time, the evol…
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The dimensionless ionization parameter, U=q/c, where q is the ratio of the local ionizing photon flux to the local hydrogen density, is a key metric to parameterize nebular conditions. Prior to JWST, the rest-frame optical emission lines and their ratios which trace the ionization parameter (e.g., O32=[OIII]/[OII]) were inaccessible at high redshifts. Here we quantify, for the first time, the evolution of the ionization parameter in galaxies across the last 13 billion years of cosmic time by comparing JWST/NIRSpec PRISM and G395M spectroscopy of 434 galaxies at 3<z<9 from the RUBIES survey with z<3 samples from SDSS, LEGA-C, and KBSS. We leverage a large suite of photoionization models to infer U from [OIII] and [OII]. We find that U increases with redshift and specific star formation rate (sSFR), and decreases with stellar mass. Crucially, and in contrast to previous linear best-fit calibrations, our inference results in a systematic uncertainty in logU of ~0.3 dex at zero measurement uncertainty due to the wide range of models that predict the same O32 ratio without informative priors. We compare to SPHINX20 and LUMEN simulations and find that the simulated galaxies exhibit higher O32 ratios at fixed redshift and stellar mass compared to RUBIES observations. Finally, we combine the predictive power of observed and inferred quantities with multivariate relations to estimate U from redshift, stellar mass, and sSFR for use where O32 is not available. We find that U increases at fixed stellar mass and sSFR by a factor of ~4 from z=2 to z=6, demonstrating that the redshift evolution encapsulates physics beyond that traced by stellar mass and sSFR alone. Finally, we show that a toy model with the first order assumption that HII region volume is proportional to galaxy volume can explain the excess redshift dependence of logU as being consistent with observed evolution in galaxy sizes.
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Submitted 23 July, 2026; v1 submitted 28 May, 2026;
originally announced May 2026.
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It's More Complicated Than You Think: A Forward Model to Infer the Recent Star Formation History, Bursty or Not, of Galaxy Populations
Authors:
Emilie Burnham,
Bingjie Wang,
Joel Leja,
Owen Gonzales,
Jenny E. Greene,
Kartheik G. Iyer,
Abby Mintz,
David J. Setton,
Sarah Wellons,
Rachel Bezanson,
Olivia Curtis,
Robert Feldmann,
Tim B. Miller,
Themiya Nanayakkara,
Joshua S. Speagle,
Katherine A. Suess,
Guochao Sun
Abstract:
Observations of the early Universe (z > 4) with the James Webb Space Telescope reveal galaxy populations with a wide range of intrinsic luminosities and colors. Bursty star formation histories (SFHs), characterized by short-term fluctuations in the star formation rate (SFR), may explain this diversity, but constraining burst timescales and amplitudes in individual galaxies is challenging due to de…
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Observations of the early Universe (z > 4) with the James Webb Space Telescope reveal galaxy populations with a wide range of intrinsic luminosities and colors. Bursty star formation histories (SFHs), characterized by short-term fluctuations in the star formation rate (SFR), may explain this diversity, but constraining burst timescales and amplitudes in individual galaxies is challenging due to degeneracies and sensitivity limits. We introduce a population-level simulation-based inference framework that recovers the power and timescales of SFR fluctuations by forward-modeling galaxy populations and distributions of rest-UV to rest-optical spectral features sensitive to star formation timescales. We adopt a stochastic SFH model based on a power spectral density formalism spanning 1 Myr-10 Gyr. Using simulated samples of N=500 galaxies at z~4 with typical JWST/NIRSpec uncertainties, we demonstrate that: (i) the power of SFR fluctuations can be measured with sufficient precision to distinguish between simulations (e.g., FIRE-2-like vs. Illustris-like populations at >99% confidence for timescales < 100 Myr); (ii) simultaneously modeling stochastic fluctuations and the recent (t_L < 500 Myr) average SFH slope is essential, as secular trends otherwise mimic burstiness in common diagnostics; (iii) frequent, intense bursts impose an outshining limit, and bias inference toward underestimating burstiness due to the obscuration of long-timescale power; and (iv) the power of SFR fluctuations can be inferred to 95% confidence across all timescales in both smooth and bursty populations. This framework establishes a novel and robust method for placing quantitative constraints on the feedback physics regulating star formation using large, uniformly selected spectroscopic samples.
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Submitted 28 January, 2026;
originally announced January 2026.
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UNCOVER/MegaScience Finds Uniform and Highly Bursty Star Formation at 3 < z < 9, consistent with the High-Redshift UV Luminosity Function
Authors:
Ikki Mitsuhashi,
Katherine A. Suess,
Joel Leja,
Rachel Bezanson,
Jenny E. Greene,
Emilie Burnham,
Gourav Khullar,
Abby Mintz,
Themiya Nanayakkara,
Karl Glazebrook,
Sedona H. Price,
David J. Setton,
Bingjie Wang,
John R. Weaver,
Hakim Atek,
Pratika Dayal,
Robert Feldmann,
Seiji Fujimoto,
Lukas J. Furtak,
Brian Lorenz,
Natalia Porraz Barrera,
Ivo Labbe,
Gabriel Brammer,
Sam E. Cutler,
Richard Pan
, et al. (2 additional authors not shown)
Abstract:
Star formation timescales are key to understanding fundamental physics like feedback mechanisms, as well as the abundance of bright galaxies at $z>10$. We investigate galaxy star formation histories (SFHs) and their evolution across $z\sim3$--9 by measuring the line-to-UV ratio (\rline) and line equivalent width (EW) of \hanii\ and \oiiihb\ directly from UNCOVER/MegaScience spectro-photometry with…
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Star formation timescales are key to understanding fundamental physics like feedback mechanisms, as well as the abundance of bright galaxies at $z>10$. We investigate galaxy star formation histories (SFHs) and their evolution across $z\sim3$--9 by measuring the line-to-UV ratio (\rline) and line equivalent width (EW) of \hanii\ and \oiiihb\ directly from UNCOVER/MegaScience spectro-photometry without relying on a specific SFH or nebular line modeling. Our photometric measurements recover \rline\ and EW to $<10\%$ systematic accuracy compared to spectroscopy. This allows us to construct a large mass- (and flux-) complete sample and quantitatively examine how \rline\ evolves with redshift and stellar mass. We find that the intrinsic scatter in \rline\ does not significantly evolve with redshift across $3<z<7$, though it may increase at $z\gtrsim8$. We build population-level toy models using \texttt{fsps} to help interpret our observations, and find that scatter in \rline\ primarily reflects the amplitude of SFH fluctuations; this implies that our observed lack of evolution in the scatter of \rline\ is due to similar star formation burstiness from $z\sim3$ to $z\sim7$. Our observations are best reproduced by a set of SFHs with rising, long-duration, and large-amplitude bursts. Finally, we demonstrate that the toy model that best describes our $z\sim6$ data can boost UV brightness by up to $ΔM_{\rm UV}\sim-2.0\,{\rm mag}$ compared with a 200\,Myr constant SFH, and naturally produces a large number of galaxies at $z>10$. This suggests that no significant evolution in star formation burstiness is required to explain the abundance of UV-bright galaxies at high redshift.
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Submitted 22 January, 2026;
originally announced January 2026.
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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…
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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.
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Submitted 26 June, 2025;
originally announced June 2025.
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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,
Jorryt Matthee,
Michael V. Maseda,
Tim B. Miller,
Ikki Mitsuhashi,
Themiya Nanayakkara,
Richard Pan,
Sedona H. Price
, et al. (3 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 43 JWST PRISM spectra of low-mass galaxies ($M_\star < 10^{9.5}\,M_\odot$) at cosmic noon ($1<z<3$), uniformly selected on F200W…
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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 43 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.
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Submitted 10 March, 2026; v1 submitted 19 June, 2025;
originally announced June 2025.
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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…
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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.
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Submitted 26 May, 2025; v1 submitted 21 April, 2025;
originally announced April 2025.