Showing 1–50 of 248 results for author: McConnachie, A

Characterizing the disruption of Böotes III: a missing link in the Galactic halo?

Authors: Jaclyn Jensen, Daniel A. Boyea, Alan W. McConnachie, Raphaël Errani, Akshara Viswanathan, Nicolas Martin, Zhen Yuan, Guillaume F. Thomas, Julio F. Navarro, Kim A. Venn, Pascale Jablonka, Khyati Malhan, Anya Dovgal, Simon E. T. Smith, Gustavo E. Medina, Thomas de Boer, Gregory S. H. Paek

Abstract: The Böotes III (Boo3) dwarf galaxy has long been suspected of being the progenitor of Styx, a ~50°-long stellar stream that was simultaneously discovered in the same region of sky. Boo3's diffuse morphology, large velocity dispersion, small pericenter, and excess of candidate stars at large radii suggest it is undergoing active tidal disruption. A link to Styx is therefore logical; however, a clea… ▽ More The Böotes III (Boo3) dwarf galaxy has long been suspected of being the progenitor of Styx, a ~50°-long stellar stream that was simultaneously discovered in the same region of sky. Boo3's diffuse morphology, large velocity dispersion, small pericenter, and excess of candidate stars at large radii suggest it is undergoing active tidal disruption. A link to Styx is therefore logical; however, a clear connection between these structures has not yet been clearly demonstrated. Here, we re-examine the Boo3-Styx association by searching for Boo3's tidal debris using a combination of Gaia-selected members, new CaHK narrow-band imaging with CFHT/MegaCam, and stellar tracer catalogues of blue horizontal branch and red giant branch stars. We also conduct a broad search for a putative stream using matched filter techniques applied to SDSS DR17 and DELVE DR2. Despite our extensive search, we find no observational evidence directly linking Boo3 to Styx. Furthermore, our results suggest that either Boo3's extended substructure is too diffuse to be detected with current data, or that its particular orbit may have erased a coherent tidal signature. Boo3 thus remains an enigmatic system, and exemplifies the need for spectroscopic follow-up to properly disentangle the nature between this faint Milky Way satellite and nearby stream. △ Less

Submitted 30 July, 2026; originally announced July 2026.

Comments: 27 pages, 16 figures; Submitted to ApJ

The Next Generation Virgo Cluster Survey (NGVS). II. A Catalog of Galaxies in the Virgo Cluster

Authors: Laura Ferrarese, Patrick Cote, Lauren A. MacArthur, Joel C. Roediger, John P. Blakeslee, Michele Cantiello, Jean-Charles Cuillandre, Puragra Guhathakurta, Stephen Gwyn, Max M. Kurzner, Eric W. Peng, Matthew Santos, Eleanore B. Todd, Elisa Toloba, Pierre-Alain Duc, Patrick R. Durrell, Nicholas Fantin, Yuting Feng, Ariane Lancon, Sungsoon Lim, Chengze Liu, Deborah Lokhorst, Alessia Longobardi, Simona Mei, J. Christopher Mihos , et al. (29 additional authors not shown)

Abstract: The Next Generation Virgo Cluster Survey (NGVS) is a deep, high resolution imaging campaign that used the 1 deg$^2$ MegaCam instrument on the Canada-France-Hawaii Telescope to carry out a comprehensive optical survey of the Virgo cluster, from its core to its virial radius. The NGVS covers a contiguous area of 104 deg$^2$ (8.63 Mpc$^2$ at the 16.5 Mpc distance of Virgo) in the $u^*$-,$g$-,$i$-, an… ▽ More The Next Generation Virgo Cluster Survey (NGVS) is a deep, high resolution imaging campaign that used the 1 deg$^2$ MegaCam instrument on the Canada-France-Hawaii Telescope to carry out a comprehensive optical survey of the Virgo cluster, from its core to its virial radius. The NGVS covers a contiguous area of 104 deg$^2$ (8.63 Mpc$^2$ at the 16.5 Mpc distance of Virgo) in the $u^*$-,$g$-,$i$-, and $z$-band, with additional limited coverage in $r$. In this paper, we present the final catalog of Virgo galaxies across the entire NGVS area. The catalog includes 3680 galaxies considered to be $bona~fide$ members of the cluster, spanning a factor of 2.5 million in luminosity, from $g = 8.42$ mag to $g = 24.41$ mag ($M_g = -22.67$ mag to $M_g = -6.68$ mag). With 2100 previously uncataloged galaxies, the NGVS catalog augments the number of known Virgo members by a factor 2.3. The catalog is complete down to $g = 18.6$ mag ($M_g=-12.5$ mag, corresponding to a stellar mass $M_* \sim 1.6\times10^7~M_{\odot}$ for an old stellar population) and 50% complete at $g = 22.0$ mag ($M_g=-9.1$ mag, $M_* \sim 6.2\times10^5~M_{\odot}$), three magnitudes deeper than the venerable Virgo Cluster Catalog (VCC), which for over 40 years has served as the reference standard for Virgo. Photometric and structural parameters are derived for all NGVS galaxies and presented in a series of tables, alongside nuclear and morphological classification, as well as stellar masses and, when available, radial velocities. △ Less

Submitted 20 July, 2026; originally announced July 2026.

Comments: The Astrophysical Journal Supplement Series, accepted

Assessing the large-scale angular clustering of UNIONS Lyman Break Galaxies via cross-correlations

Authors: Constantin Payerne, Christophe Yèche, William d'Assignies Doumerg, Hendrik Hildebrandt, Martin Kilbinger, Calum Murray, Thomas de Boer, Kenneth C. Chambers, Scott Chapman, Alan W. McConnachie

Abstract: Lyman-break galaxies (LBGs), selected via the strong spectral break blueward of the Lyman limit, are powerful tracers of large-scale structure at redshifts $z>2$. In this work, we assess the feasibility of using LBGs selected from the Ultraviolet Near Infrared Optical Northern Survey (UNIONS) multi-band photometric catalog as cosmological probes of the high-redshift Universe using two-point statis… ▽ More Lyman-break galaxies (LBGs), selected via the strong spectral break blueward of the Lyman limit, are powerful tracers of large-scale structure at redshifts $z>2$. In this work, we assess the feasibility of using LBGs selected from the Ultraviolet Near Infrared Optical Northern Survey (UNIONS) multi-band photometric catalog as cosmological probes of the high-redshift Universe using two-point statistics. We demonstrate that spatially varying imaging systematics, driven by variations in PSF depth, seeing across the UNIONS footprint, limit robust measurements of the LBG auto-angular power spectrum on large scales, even after correcting the LBG field with linear or non-linear mitigation techniques. This study shows that clustering analyses of faint galaxy samples close to survey depth are challenging. We therefore turn to cross-correlation measurements with external tracers, in particular the \textit{Planck} CMB lensing convergence and quasars from DESI DR1 and \textit{Quaia}, which are less sensitive to the angular imaging systematics. Using both data and mock catalogues, we demonstrate that the LBG--CMB lensing cross-power spectrum can be measured more robustly than the auto-spectrum, with an amplitude consistent with theoretical predictions. Residual systematics primarily manifest as excess variance at large angular scales, without introducing a significant bias in the recovered signal. Taken together, these results establish UNIONS-selected LBGs as reliable tracers for cross-correlation cosmology at $z\sim 2.5$, and highlight cross-correlation techniques as a powerful and robust avenue for extracting cosmological information from photometric high-redshift galaxy samples in the presence of complex imaging systematics. △ Less

Submitted 15 July, 2026; v1 submitted 10 July, 2026; originally announced July 2026.

Comments: 20 pages, 19 figures, to be submitted to The Open Journal of Astrophysics

Revisiting the 'Lensing is Low' Problem with UNIONS

Authors: Martine C. T. Campbell, Jack Elvin-Poole, Michael J. Hudson, Thomas de Boer, Sacha Guerrini, Fabian Hervas-Peters, Hendrik Hildebrandt, Martin Kilbinger, Eugene A. Magnier, Alan W. McConnachie, Charlie T. Mpetha, Romain Paviot, Ludovic Van Waerbeke, Anna Wittje

Abstract: We present new measurements of the galaxy-galaxy lensing (GGL) signal around Baryon Oscillation Spectroscopic Survey (BOSS) CMASS galaxies using background sources from the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS). With high-quality imaging of background sources and a survey overlap of approximately 2650 square degrees, we obtain precise large-scale GGL measurements. Building on… ▽ More We present new measurements of the galaxy-galaxy lensing (GGL) signal around Baryon Oscillation Spectroscopic Survey (BOSS) CMASS galaxies using background sources from the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS). With high-quality imaging of background sources and a survey overlap of approximately 2650 square degrees, we obtain precise large-scale GGL measurements. Building on these new measurements, we revisit the so-called 'lensing is low' problem, wherein galaxy-halo connection models calibrated on galaxy clustering (GC) data over-predict the GGL signal by 20-40% assuming CMB-based cosmological parameters. We model the galaxy-halo connection using a halo occupation distribution (HOD), and perform joint fits to both GGL and GC signals across a wide range of scales, as well as a GC-only fit. In contrast to previous work, we do not find a significant 'lensing is low' effect in the CMASS sample, although the best joint fits are achieved by decreasing the amplitude of the matter power spectrum slightly relative to the Planck cosmological parameters. Overall, we find that two models describe our observables similarly well: one where HOD and cosmological parameters are free, and one where HOD, cosmological, and feedback parameters are free. Importantly, we emphasise the role of large scales in constraining the lensing is low effect, shifting the narrative away from an exclusively small-scale issue. △ Less

Submitted 22 June, 2026; originally announced June 2026.

Comments: 21 article pages, 19 figures, submitted to MNRAS

It's Not Just Star Formation: A trend of low dark matter densities in the Andromeda dwarf galaxy system

Authors: Connor S. Pickett, Michelle L. M. Collins, Justin I. Read, R. Michael Rich, Emily J. E. Charles, Erik Tollerud, Nicolas Martin, Scott Chapman, Alan McConnachie, Alessandro Savino, Daniel R. Weisz

Abstract: Dynamical mass modeling of Andromeda (M31) dwarf spheroidal (dSph) galaxies has revealed a growing trend of lower central dark matter (DM) densities than predicted by pure DM structure formation in Lambda Cold Dark Matter ($Λ$CDM) cosmology simulations and lower than most Milky Way (MW) satellites. So far, however, only four of the 35 confirmed M31 dSphs have been successfully mass modeled. In thi… ▽ More Dynamical mass modeling of Andromeda (M31) dwarf spheroidal (dSph) galaxies has revealed a growing trend of lower central dark matter (DM) densities than predicted by pure DM structure formation in Lambda Cold Dark Matter ($Λ$CDM) cosmology simulations and lower than most Milky Way (MW) satellites. So far, however, only four of the 35 confirmed M31 dSphs have been successfully mass modeled. In this second paper of a series, we aim to better understand growing Local Group (LG) dSph patterns by mass modeling seven more M31 dSphs: Andromeda I, III, V, VII, IX, XXXI, and XXXII. We update the kinematics of each dwarf and estimate their central dark matter densities at 150 pc using the dynamical Jeans modeling tool, GravSphere. We also update their DM halo mass, $M_{\rm{200}}$, via abundance matching. We find Andromeda III and V to have central DM densities in line with $Λ$CDM expectations, resembling dSphs around the Milky Way. The remaining five dwarfs have anomalously low central densities, continuing a growing trend seen for M31 satellites. We investigate each dwarf's star formation history and find that star formation-induced `DM heating' is disfavored as the sole explanation of these lower central densities. We consider the effect of tides and halo concentration scatter on these systems and predict that they should be on more plunging orbits than their denser counterparts. If this prediction is misaligned with the data, it could necessitate new physics beyond the Standard Cosmological Model. △ Less

Submitted 29 May, 2026; originally announced June 2026.

Comments: 21 pages, 14 Figures; Submitted to MNRAS

UNIONS-3500 Weak Lensing: I. A Galaxy Shape Catalogue in the Northern Sky

Authors: F. Hervas-Peters, S. Guerrini, M. Kilbinger, L. Baumont, A. Guinot, C. Daley, C. Bonini, A. Wittje, C. Murray, L. W. K. Goh, A. Paradis, A. Tersenov, M. J. Hudson, L. Van Waerbeke, H. Hildebrandt, S. Fabbro, J. -C. Cuillandre, A. W. McConnachie

Abstract: Weak gravitational lensing has become a widely used effect to characterise the dark-matter distribution on large scales in the Universe by measuring galaxy ellipticities and their statistical correlations. We present the first weak gravitational lensing catalogue for cosmic-shear cosmology of the Ultraviolet Near Infrared Optical Northern Survey (UNIONS). We analyse approximately $3\,500$ square d… ▽ More Weak gravitational lensing has become a widely used effect to characterise the dark-matter distribution on large scales in the Universe by measuring galaxy ellipticities and their statistical correlations. We present the first weak gravitational lensing catalogue for cosmic-shear cosmology of the Ultraviolet Near Infrared Optical Northern Survey (UNIONS). We analyse approximately $3\,500$ square degrees of sky area in the Northern Hemisphere, observed in the $r$-band by MegaCam on the Canada-France Hawai'i Telescope, achieving a median seeing of 0.7 arcsec. Starting from images calibrated for astrometry and photometry, we describe the steps from image processing to catalogue creation. These steps include masking, source detection and selection, star selection, point spread function (PSF) modelling, shape measurement, and calibration. We conduct extensive validation tests, particularly to assess and mitigate the leakage of PSF ellipticity into galaxy shapes. We demonstrate the robustness of the catalogue by investigating correlations between ellipticity and other observational variables as well as structural elements, such as observer-frame image positions and proximity to bright stars. The final galaxy catalogue contains $62$ million galaxies, corresponding to an effective source density of $4.96$ arcmin$^{-2}$. The ellipticity dispersion, commonly referred to as shape noise, is $σ_ε= 0.27$. Initiating the first major cosmological analysis by the UNIONS collaboration, this is the first in a series of five papers which cover the various aspects of a robust cosmic shear analysis. Two companion papers discuss the robustness of the catalogue, one through the level of $B$-mode contamination and another by producing and analysing dedicated image simulations for shear calibration, while the other two present cosmological results in real and harmonic space. △ Less

Submitted 13 May, 2026; originally announced May 2026.

Comments: 13 article pages, 7 appendix, submitted to A&A

UNIONS-3500 Weak Lensing: III. 2D Cosmological Constraints in Configuration Space

Authors: L. W. K. Goh, S. Guerrini, C. Daley, F. Hervas-Peters, M. Kilbinger, A. Wittje, C. Murray, S. Fabbro, H. Hildebrandt, M. J. Hudson, L. van Waerbeke, A. H. Wright, T. de Boer, J. -C. Cuillandre, E. Magnier, A. W. McConnachie

Abstract: We present the first cosmological constraints from the cosmic shear analysis of the UNIONS-3500 weak lensing galaxy catalogue in configuration space. The Ultraviolet Near Infrared Optical Northern Survey (UNIONS) is the largest and deepest photometric survey of the northern hemisphere to date, with the UNIONS-3500 catalogue using high-quality $r$-band imaging across 3500 deg2 of the sky. We perfor… ▽ More We present the first cosmological constraints from the cosmic shear analysis of the UNIONS-3500 weak lensing galaxy catalogue in configuration space. The Ultraviolet Near Infrared Optical Northern Survey (UNIONS) is the largest and deepest photometric survey of the northern hemisphere to date, with the UNIONS-3500 catalogue using high-quality $r$-band imaging across 3500 deg2 of the sky. We perform a 2D cosmic shear analysis with a single tomographic bin, using the two-point correlation function (2PCF) statistic. Assuming a flat LCDM model, we obtain constraints on the clustering amplitude of S_8 = 0.831^{+0.067}_{-0.078}, which is consistent with constraints from Planck CMB measurements and precedent cosmic shear results within 1sigma. We outline the construction of our cosmological inference pipeline, including the estimation of the source redshift distribution, shear calibration, and covariance matrix, and describe methodologies for the mitigation of systematic effects arising from PSF systematics and B-modes. We demonstrate that our results are robust to variations in analysis choices, including scale cuts, prior ranges, and nonlinear modelling. This paper is part of a coordinated release which collectively demonstrates the maturity and readiness of UNIONS to deliver competitive cosmological results, positioning it as a key stepping stone towards the forthcoming era of Stage IV weak lensing experiments. △ Less

Submitted 13 May, 2026; originally announced May 2026.

UNIONS-3500 Weak Lensing: IV. 2D cosmological constraints in harmonic space

Authors: S. Guerrini, L. W. K. Goh, F. Hervas-Peters, C. Daley, M. Kilbinger, A. Wittje, C. Murray, L. Baumont, S. Fabbro, H. Hildebrandt, M. J. Hudson, L. van Waerbeke, A. H. Wright, T. de Boer, J. -C. Cuillandre, E. Magnier, A. W. McConnachie

Abstract: The Ultraviolet Near Infrared Optical Northern Survey (UNIONS) is a photometric survey in the northern sky. The quality of the data in the $r$ band provides precise shape measurements to measure the growth of structures using cosmic shear. This work aims to constrain cosmological parameters using a harmonic-space estimator of the cosmic shear signal, known as pseudo-$C_\ell$, in a non-tomographic… ▽ More The Ultraviolet Near Infrared Optical Northern Survey (UNIONS) is a photometric survey in the northern sky. The quality of the data in the $r$ band provides precise shape measurements to measure the growth of structures using cosmic shear. This work aims to constrain cosmological parameters using a harmonic-space estimator of the cosmic shear signal, known as pseudo-$C_\ell$, in a non-tomographic analysis. We perform our analysis in the context of the standard $Λ$CDM cosmology. We model astrophysical systematic effects such as baryonic feedback and intrinsic alignments of galaxies. We verify that the point spread function systematic contribution does not affect our results. We assess the impact of different scale cuts and modelling choices on the constraints. We find $S_8 \equiv σ_8 \sqrt{Ω_{\rm m}/0.3} = 0.891^{+0.057}_{-0.084}$, consistent at the $0.79 \, σ$ level with \emph{Planck} and between $0.87$ to $1.51 \, σ$ with other weak lensing surveys. Our results are robust to analysis choices, and we use lognormal simulations to assess the consistency between configuration and harmonic space results, finding a $2.18 \, σ$ agreement between the two statistics. The degeneracy between $S_8$ and the amplitude of the intrinsic alignment, $A_{\rm IA}$, sampled from a prior obtained from direct measurements, is one of the largest sources of uncertainty. This work is part of the first cosmological analysis of the UNIONS survey using cosmic shear and paves the way for future tomographic and $3 \times 2$ point cross-correlation analyses, exploiting the unique overlap of UNIONS with deep spectroscopic surveys in the northern hemisphere. △ Less

Submitted 2 June, 2026; v1 submitted 13 May, 2026; originally announced May 2026.

Comments: 18 pages, 17 figures, to be submitted to A&A

UNIONS-3500 Weak Lensing: II. B-mode validation for cosmic shear

Authors: C. Daley, A. Guinot, S. Guerrini, F. Hervas-Peters, L. W. K. Goh, C. Murray, M. Kilbinger, A. Wittje, M. J. Hudson, H. Hildebrandt, L. van Waerbeke, A. W. McConnachie

Abstract: At Stage-III sensitivities, cosmic shear $B$ modes unambiguously indicate systematic contamination and are often used to inform data selection and scale cuts for cosmological inference. We validate $B$ modes for the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS)-3500 (2894 deg$^2$, $n_\mathrm{eff} \approx 5.0$ arcmin$^{-2}$) using three $E$/$B$-separable statistics: pure-mode correlati… ▽ More At Stage-III sensitivities, cosmic shear $B$ modes unambiguously indicate systematic contamination and are often used to inform data selection and scale cuts for cosmological inference. We validate $B$ modes for the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS)-3500 (2894 deg$^2$, $n_\mathrm{eff} \approx 5.0$ arcmin$^{-2}$) using three $E$/$B$-separable statistics: pure-mode correlation functions $ξ_\pm^{\mathrm{B}}(θ)$, Complete Orthogonal Sets of $E$/$B$-mode Integrals (COSEBI) $B$-mode amplitudes $B_n$, and harmonic-space power spectra $C_\ell^{BB}$. For each statistic, we compute probability-to-exceed (PTE) values over a two-dimensional grid of scale-cut boundaries; our adopted cuts lie in broad stable regions of acceptable PTE. $B$-mode detections and PTE failures on initial catalog versions led us to investigate galaxy size cuts and stellar halo masking. After cuts, all three statistics pass the null test (minimum PTE $= 0.18$). Before scale cuts, we measure an oscillatory COSEBI $B$-mode pattern consistent with repeating additive shear bias, a detector-level effect seen across multiple Stage-III surveys including CFHTLenS, which used the same MegaCam camera; scale cuts that exclude the charge-coupled device (CCD) angular scale suppress it. Although these statistics probe the same two-point shear field, scale cuts in one do not map exactly onto cuts in another, because their respective filter functions weight angular scales differently. The most conservative validation therefore requires scale and sample selections that pass null tests across all frameworks simultaneously, an approach that applies directly to Stage-IV surveys where systematic errors dominate. △ Less

Submitted 3 June, 2026; v1 submitted 3 April, 2026; originally announced April 2026.

Comments: 11 pages, 8 figures, submitted to A&A

The primordial nature of the C-19 stellar stream

Authors: Kim A. Venn, Zhen Yuan, Nicolas F. Martin, Anya Dovgal, Daria Zaremba, Else Starkenburg, Felipe Gran, Christian R. Hayes, Vanessa Hill, Chiaki Kobayashi, Carmela Lardo, Alan W. McConnachie, Tadafumi Matsuno, Martin Montelius, Vinicius Placco, Federico Sestito, Anke Ardern-Arentsen, Guiseppina Battaglia, Piercarlo Bonifacio, Raymond Carlberg, Sebastien Fabbro, Morgan Fouesneau, Rodrigo Ibata, Pascale Jablonka, Jaclyn Jensen , et al. (10 additional authors not shown)

Abstract: Stellar streams, remnants of compact star systems stretched out by the tidal forces of the Milky Way, offer a unique way to study stellar populations that formed billions of years ago. A particularly unique stream is C-19, the most metal-poor stellar stream known at less than a thousandth of the Sun's metallicity. The nature of C-19 is not yet clear, with properties that resemble both star cluster… ▽ More Stellar streams, remnants of compact star systems stretched out by the tidal forces of the Milky Way, offer a unique way to study stellar populations that formed billions of years ago. A particularly unique stream is C-19, the most metal-poor stellar stream known at less than a thousandth of the Sun's metallicity. The nature of C-19 is not yet clear, with properties that resemble both star clusters and ultra faint dwarf galaxies, yet in either case its extremely low metallicity indicates very early star formation, <1 Gyr after the Big Bang. Here, we present the first detailed study on the nature of C-19 based on the chemical abundances of 14 member stars from high-resolution spectroscopy. These reveal that C-19 formed stars in an early, rapid, and prolific star formation event, with mild inhomogeneous mixing of elements produced in massive stars. There is otherwise no evidence for subsequent star formation, multiple stellar populations, nor chemical evolution. Although C-19 is currently disrupted in the Milky Way halo, it offers a rare and complementary window into the details of star formation and chemical evolution in the early universe, ideal for comparisons with current studies of primordial star formation in the high-redshift universe. △ Less

Submitted 2 March, 2026; originally announced March 2026.

Comments: 13 pages, 6 figures, under review - comments welcome

Bright Region Reset: an on-detector strategy for minimizing the impacts of atmospheric emission lines on spectral observations

Authors: Theodore A. Grosson, Edward L. Chapin, Tim Hardy, Masen Lamb, Jordan Lothrop, Alan W. McConnachie, Richard Murowinski

Abstract: Observations in the near-infrared using large ground-based telescopes are adversely impacted by bright atmospheric emission lines, particularly the OH Meinel bands. These lines can saturate a moderate-resolution spectrograph on the order of minutes, resulting in information loss at the wavelengths of the lines. OH lines also vary on similar timescales, requiring frequent sky exposures to be able t… ▽ More Observations in the near-infrared using large ground-based telescopes are adversely impacted by bright atmospheric emission lines, particularly the OH Meinel bands. These lines can saturate a moderate-resolution spectrograph on the order of minutes, resulting in information loss at the wavelengths of the lines. OH lines also vary on similar timescales, requiring frequent sky exposures to be able to subtract the sky spectrum from that of the target. In this paper we present a new method, which we call bright region reset (BRR), to prevent the saturation of these lines in near-infrared spectra while simultaneously improving information about their variability. This is accomplished by periodically resetting pixels that contain bright lines on a detector capable of sub-window readout while the rest of the detector continues integrating. This method is demonstrated on the McKellar Spectrograph in the 1.2 m telescope at the Dominion Astrophysical Observatory in Victoria, Canada. Using a Teledyne H2RG detector, we reset the emission lines produced by an arc lamp while still recording their flux. We show no degradation in the resulting spectrum compared to a conventional observing mode. Unlike other OH line mitigation strategies, the BRR method not only avoids loss of information at wavelengths containing the lines, but also provides higher-cadence information on sky line variability, making it a promising technique for implementation at observatories. We advocate demonstrating this method on sky at existing 8--10 m class facilities with near-infrared spectrographs equipped with HxRG detectors in order to test its feasibility for use in sky subtraction schemes for premier modern spectrographs, including the upcoming generation of instruments for the Extremely Large Telescopes. △ Less

Submitted 2 December, 2025; originally announced December 2025.

Comments: 22 pages, 12 figures. Accepted for publication in JATIS

Accreted Globular Clusters and Horizontal Branch Morphology in the Outer Halo of M31

Authors: Gracie McGill, Annette M. N. Ferguson, Dougal Mackey, Avon P. Huxor, Geraint F. Lewis, Nicolas F. Martin, Alan W. McConnachie, Charli M. Sakari, Nial R. Tanvir, Kim A. Venn

Abstract: M31 hosts a rich population of outer halo ($R_{\rm{proj}} > 25$ kpc) globular clusters (GCs), many of which show strong evidence for spatial and/or kinematical associations with large-scale tidal debris features. We present deep Hubble Space Telescope photometry of 48 halo GCs, including 18 with clear ties to stellar streams and 13 with potential associations. Using the colour-magnitude diagrams (… ▽ More M31 hosts a rich population of outer halo ($R_{\rm{proj}} > 25$ kpc) globular clusters (GCs), many of which show strong evidence for spatial and/or kinematical associations with large-scale tidal debris features. We present deep Hubble Space Telescope photometry of 48 halo GCs, including 18 with clear ties to stellar streams and 13 with potential associations. Using the colour-magnitude diagrams (CMDs), we quantify the horizontal branch (HB) morphologies and employ new empirical relationships, calibrated on Milky Way (MW) GCs, to consistently derive metallicities and line-of-sight extinctions. We find a remarkable correlation between HB morphology and the presence of substructure: GCs with very red HBs are almost exclusively associated with substructure, while non-substructure GCs have extended blue HBs. This provides the first direct evidence that red HB halo clusters originate from satellite accretion, a notion introduced nearly 50 years ago from MW studies which has remained unconfirmed until now. In addition to a more metal-rich tail, the substructure GC sample also contains a few clusters with very low metallicities and red HBs, unlike any objects known in the MW. We suggest these are recently accreted young clusters, supporting the growing evidence that M31 has experienced a more prolonged accretion history than the MW. △ Less

Submitted 26 June, 2025; originally announced June 2025.

Comments: 7 pages, 4 figures. Accepted for publication in MNRAS

Galaxies OBserved as Low-luminosity Identified Nebulae (GOBLIN): a catalog of 43,000 high-probability dwarf galaxy candidates in the UNIONS survey

Authors: Nick Heesters, David Chemaly, Oliver Müller, Elisabeth Sola, Sébastien Fabbro, Ashley Ferreira, Alan W. McConnachie, Eugene Magnier, Michael J. Hudson, Kenneth Chambers, François Hammer, Ruben Sanchez-Janssen

Abstract: The detection of low surface brightness galaxies beyond the Local Group poses significant observational challenges, yet these faint systems are fundamental to our understanding of dark matter, hierarchical galaxy formation, and cosmic structure. Their abundance and distribution provide crucial tests for cosmological models, particularly regarding the small-scale predictions of $Λ$CDM. We present a… ▽ More The detection of low surface brightness galaxies beyond the Local Group poses significant observational challenges, yet these faint systems are fundamental to our understanding of dark matter, hierarchical galaxy formation, and cosmic structure. Their abundance and distribution provide crucial tests for cosmological models, particularly regarding the small-scale predictions of $Λ$CDM. We present a systematic detection framework for dwarf galaxy candidates in Ultraviolet Near Infrared Optical Northern Survey (UNIONS) data covering 4,861 deg$^{2}$. Our pipeline preprocesses UNIONS gri-band data through binning, artifact removal, and stellar masking, then employs MTObjects (MTO) for low surface brightness detection. After parameter cuts and cross-matching, we obtain $\sim$360 candidates per deg$^{2}$, totaling $\sim$1.5 million candidates forming our GOBLIN (Galaxies OBserved as Low-luminosity Identified Nebulae) catalog. We fine-tuned the deep learning model Zoobot, pre-trained on Galaxy Zoo labels, for classification. Training data came from visual inspection of literature candidates with probability labels from expert assessments, capturing consensus and uncertainty. Applied to all MTO objects, our method identifies 42,965 dwarf candidates with probability $>$ 0.8, including 23,072 with probability $>$ 0.9. High-probability candidates correlate spatially with massive galaxies (log$(M_{*}/M_{\odot}) \geq$ 10) within 120 Mpc. While some of these objects may have been previously identified in other surveys, we present this extensive catalog of candidates, including their positions, structural parameter estimates, and classification probabilities, as a resource for the community to enable studies of galaxy formation, evolution, and the distribution of dwarf galaxies in different environments. △ Less

Submitted 23 May, 2025; originally announced May 2025.

Comments: 22 pages, 14 figures. Accepted for publication in A&A. Abstract abridged

Journal ref: A&A 699, A232 (2025)

Mass Modeling the Andromeda Dwarf Galaxies: Andromeda VI and Andromeda XXIII

Authors: Connor S. Pickett, Michelle L. M. Collins, R. Michael Rich, Justin I. Read, Emily J. E. Charles, Nicolas Martin, Scott Chapman, Alan McConnachie, Alessandro Savino, Daniel R. Weisz

Abstract: Accurately mapping the mass profiles of low mass dwarf spheroidal (dSph) galaxies allows us to test predictions made by dark matter (DM) models. To date, such analyses have primarily been performed on Milky Way (MW) satellites. Meanwhile, the Andromeda Galaxy (M31) is home to 35 known dwarf galaxies, yet only two have been successfully mass-modeled so far. A more comprehensive study of Local Group… ▽ More Accurately mapping the mass profiles of low mass dwarf spheroidal (dSph) galaxies allows us to test predictions made by dark matter (DM) models. To date, such analyses have primarily been performed on Milky Way (MW) satellites. Meanwhile, the Andromeda Galaxy (M31) is home to 35 known dwarf galaxies, yet only two have been successfully mass-modeled so far. A more comprehensive study of Local Group dwarfs is necessary to better understand the nature of dark matter. In this study, we have undertaken a dynamical study of two higher-luminosity Andromeda dwarf galaxies: Andromeda VI (And VI) and Andromeda XXIII (And XXIII). We infer an enclosed mass for And VI of M(r $<$ r$_{h}$) = (4.9 $\pm$ 1.5) $\times$ 10$^{7}$ M$_{\odot}$, corresponding to a mass-to-light ratio of $[M/L]_{r_{\rm{h}}}$ = (27.1 $\pm$ 8.2) M$_{\odot}$/L$_{\odot}$. We infer an enclosed mass for And XXIII of M(r $<$ r$_{h}$) = (3.1 $\pm$ 1.9) $\times$ 10$^{7}$ M$_{\odot}$, corresponding to a mass-to-light ratio of $[M/L]_{r_{\rm{h}}}$ = (90.2 $\pm$ 53.9) M$_{\odot}$/L$_{\odot}$. Using the dynamical Jeans modeling tool, \gravsphere, we determine And VI and And XXIII's dark matter density at 150 pc, finding $ρ_{\rm{DM,VI}}$(150 pc) = (1.4 $\pm$ 0.5) $\times 10^{8}$ M$_{\odot}$ kpc$^{-3}$ and $ρ_{\rm{DM,XXIII}}$(150 pc) = 0.5$\substack{+0.4 \\ -0.3} \times 10^{8}$ M$_{\odot}$ kpc$^{-3}$. Our results make And VI the first mass-modeled M31 satellite to fall into the cuspy regime. And XXIII has a lower density, implying either a more cored central dark matter density, or a lowering of the density through tides. This adds And XXIII to a growing list of M31 dwarfs with a central density lower than most MW dwarfs and lower than expected for isolated dwarfs in the Standard Cosmology. This could be explained by the M31 dwarfs having experienced stronger tides than their MW counterparts. △ Less

Submitted 19 May, 2025; v1 submitted 7 May, 2025; originally announced May 2025.

Comments: 19 pages, 16 figures. Accepted to MNRAS

Low Surface Brightness structures from annotated deep CFHT images: effects of the host galaxy's properties and environment

Authors: Elisabeth Sola, Pierre-Alain Duc, Mathias Urbano, Felix Richards, Adeline Paiement, Michal Bílek, Mustafa K. Yıldız, Alessandro Boselli, Patrick Côté, Jean-Charles Cuillandre, Laura Ferrarese, Stephen Gwyn, Olivier Marchal, Alan W. McConnachie, Matthieu Baumann, Thomas Boch, Florence Durret, Matteo Fossati, Rebecca Habas, Francine Marleau, Oliver Müller, Mélina Poulain, Vasily Belokurov

Abstract: Hierarchical galactic evolution models predict that mergers drive galaxy growth, producing low surface brightness (LSB) tidal features that trace galaxies' late assembly. These faint structures encode information about past mergers and are sensitive to the properties and environment of the host galaxy. We investigated the relationships between LSB features and their hosts in a sample of 475 nearby… ▽ More Hierarchical galactic evolution models predict that mergers drive galaxy growth, producing low surface brightness (LSB) tidal features that trace galaxies' late assembly. These faint structures encode information about past mergers and are sensitive to the properties and environment of the host galaxy. We investigated the relationships between LSB features and their hosts in a sample of 475 nearby massive galaxies spanning diverse environments (field, groups, Virgo cluster) using deep optical imaging from the Canada-France-Hawaii Telescope (MATLAS, UNIONS/CFIS, VESTIGE, NGVS). Using Jafar, an online annotation tool, we manually annotated tidal features and extended stellar haloes, including 199 tidal tails and 100 streams. Geometric and photometric measurements were extracted to analyse their dependence on galaxy mass, environment, and internal kinematics. At our surface brightness limit of 29 mag$\,$arcsec$^{-2}$, tidal features and stellar haloes contribute 2% and 10% of total galaxy luminosity, respectively. Tidal features are detected in 36% of galaxies, with none fainter than 27.8 mag$\,$arcsec$^{-2}$. The most massive galaxies are twice as likely to host tidal debris, and for early-type galaxies their halos are twice as luminous as those in lower-mass systems, a trend not observed in late-type galaxies. Although small-scale interactions increase the frequency of tidal features, the large-scale environment does not influence it. An anticorrelation between this frequency and rotational support is found, but may reflect the mass-driven effect. We release our database of annotated features for deep learning applications. Our findings confirm that galaxy mass is the dominant factor influencing tidal feature prevalence, consistent with hierarchical formation models. △ Less

Submitted 8 July, 2025; v1 submitted 24 March, 2025; originally announced March 2025.

Comments: Accepted for publication in MNRAS on 2025 July 07

Journal ref: Mon Not R Astron Soc (2025) 3015-3042

UNIONS: The Ultraviolet Near-Infrared Optical Northern Survey

Authors: Stephen Gwyn, Alan W. McConnachie, Jean-Charles Cuillandre, Ken C. Chambers, Eugene A. Magnier, Michael J. Hudson, Masamune Oguri, Hisanori Furusawa, Hendrik Hildebrandt, Raymond Carlberg, Sara L. Ellison, Junko Furusawa, Raphaël Gavazzi, Rodrigo Ibata, Yannick Mellier, Ken Osato, H. Aussel, Lucie Baumont, Manuel Bayer, Olivier Boulade, Patrick Côté, David Chemaly, Cail Daley, Pierre-Alain Duc, A. Ellien , et al. (64 additional authors not shown)

Abstract: The Ultraviolet Near-Infrared Optical Northern Survey (UNIONS) is a "collaboration of collaborations" that is using the Canada-France-Hawai'i Telescope, the Pan-STARRS telescopes, and the Subaru Observatory to obtain $ugriz$ images of a core survey region of 6250 deg$^2$ of the northern sky. The $10σ$ point source depth of the data, as measured within a 2-arcsecond diameter aperture, are… ▽ More The Ultraviolet Near-Infrared Optical Northern Survey (UNIONS) is a "collaboration of collaborations" that is using the Canada-France-Hawai'i Telescope, the Pan-STARRS telescopes, and the Subaru Observatory to obtain $ugriz$ images of a core survey region of 6250 deg$^2$ of the northern sky. The $10σ$ point source depth of the data, as measured within a 2-arcsecond diameter aperture, are $[u,g,r,i,z] = [23.7, 24.5, 24.2, 23.8, 23.3]$\ in AB magnitudes. UNIONS is addressing some of the most fundamental questions in astronomy, including the properties of dark matter, the growth of structure in the Universe from the very smallest galaxies to large-scale structure, and the assembly of the Milky Way. It is set to become the major ground-based legacy survey for the northern hemisphere for the next decade and provides an essential northern complement to the static-sky science of the Vera C. Rubin Observatory's Legacy Survey of Space and Time. UNIONS supports the core science mission of the {\it Euclid} space mission by providing the data necessary in the northern hemisphere for the calibration of the wavelength dependence of the {\it Euclid} point-spread function and derivation of photometric redshifts in the North Galactic Cap. This region contains the highest quality sky for {\it Euclid}, with low backgrounds from the zodiacal light, stellar density, extinction, and emission from Galactic cirrus. Here, we describe the UNIONS survey components, science goals, data products, and the current status of the overall program. △ Less

Submitted 17 March, 2025; originally announced March 2025.

Comments: submitted to AJ

GHOST commissioning science results -- IV: Chemodynamical analyses of Milky Way satellites Sagittarius II and Aquarius II

Authors: Daria Zaremba, Kim Venn, Christian R. Hayes, Raphaël Errani, Triana Cornejo, Jennifer Glover, Jaclyn Jensen, Alan W. McConnachie, Julio F. Navarro, John Pazder, Federico Sestito, André Anthony, Dave Andersen, Gabriella Baker, Timothy Chin, Vladimir Churilov, Ruben Diaz, Tony Farrell, Veronica Firpo, Manuel Gomez-Jimenez, David Henderson, Venu M. Kalari, Jon Lawrence, Steve Margheim, Bryan Miller , et al. (6 additional authors not shown)

Abstract: We present Gemini/GHOST high-resolution spectra of five stars observed in two low surface brightness Milky Way satellites, Sagittarius II (Sgr2) and Aquarius II (Aqu2). For Aqu2, the velocities and metallicities of the two stars are consistent with membership in a dark matter-dominated ultra faint dwarf galaxy (UFD). The chemical abundance ratios suggest inefficient star formation from only one or… ▽ More We present Gemini/GHOST high-resolution spectra of five stars observed in two low surface brightness Milky Way satellites, Sagittarius II (Sgr2) and Aquarius II (Aqu2). For Aqu2, the velocities and metallicities of the two stars are consistent with membership in a dark matter-dominated ultra faint dwarf galaxy (UFD). The chemical abundance ratios suggest inefficient star formation from only one or a few supernovae (e.g., low Na, Sr, Ba), and enriched potassium (K) from super-AGB stars. For Sgr2, the velocity and metallicity dispersions of its members are not clearly resolved and our detailed chemical abundances show typical ratios for metal-poor stars, with low dispersions. There is only one exception - we report the discovery of an r-process enhanced star (Sgr2584, [Eu/Fe]= $+0.7 \pm 0.2 $; thus, an r-I star). As r-I stars are found in both UFDs (Tuc III, Tuc IV, Grus II) and globular clusters (M15 and M92), then this does not help to further classify the nature of Sgr2. Our exploration of Sgr2 demonstrates the difficulty in classifying some of the faintest (ambiguous) satellites. We advocate for additional diagnostics in analysing the ambiguous systems, such as exploring radial segregation (by mass and/or chemistry), N-body simulations, and the need for dark matter to survive Galactic tidal effects. The spectra analysed in this paper were taken as part of the GHOST commissioning observations, testing faint observation limits (G < 18.8) and the single and double IFU observing modes. △ Less

Submitted 4 June, 2025; v1 submitted 7 March, 2025; originally announced March 2025.

Comments: 32 pages, 25 figures, 6 tables

A Pristine-UNIONS view on the Galaxy: Kinematics of the distant spur feature of the Sagittarius stream traced by Blue Horizontal Branch stars

Authors: M. Bayer, E. Starkenburg, G. F. Thomas, N. Martin, A. Helmi, A. Byström, T. de Boer, E. Fernández Alvar, S. Gwyn, R. Ibata, P. Jablonka, G. Kordopatis, T. Matsuno, A. W. McConnachie, G. E. Medina, R. Sánchez-Janssen, F. Sestito

Abstract: Providing a detailed picture of the Sagittarius (Sgr) stream offers important constraints on the build-up of the Galactic halo as well as its gravitational potential at large radii. While several attempts have been made to model the structure of the Sgr stream, no model has yet been able to match all the features observed for the stream. Moreover, for several of these features, observational chara… ▽ More Providing a detailed picture of the Sagittarius (Sgr) stream offers important constraints on the build-up of the Galactic halo as well as its gravitational potential at large radii. While several attempts have been made to model the structure of the Sgr stream, no model has yet been able to match all the features observed for the stream. Moreover, for several of these features, observational characterisation of their properties is rather limited, particularly at large distances. The aim of this work is to investigate the kinematics of the Sgr stream outermost spur feature using blue horizontal branch (BHB) stars. Candidate BHB stars were selected by combining two approaches; one capitalising on Pan-STARRS1 3$Π$ griz and u photometry taken as part of UNIONS, the other using Pristine Survey CaHK and SDSS ugr photometry. Follow-up optical spectra are obtained using ESO/VLT/FORS2 to confirm their BHB nature and obtain line-of-sight (LOS) velocities. Of our 25 candidates, 20 stars can be confirmed as bona fide BHB stars. Their LOS velocities, together with the 3D positions of these stars qualitatively match well with Sgr model predictions and trace the outer apocentre of the trailing arm and its spur feature very nicely. The quantitative offsets that are found between our data and the different models can be used to provide information about the Galactic gravitational potential at large distances. We present a first, tentative, analysis in this direction, showing that the model of Vasiliev et al. (2021) would provide better agreement with our observations if the enclosed mass of the Milky Way within 100 kpc were lowered to $(5.3\!\pm\!0.4)\!\times\!10^{11}$ M$_\odot$ (versus $(5.6\!\pm\!0.4)\!\times\!10^{11}$ M$_\odot$). Our selection of BHB stars provides a new view on the outermost structure in 3D positions and LOS velocities of the Sgr debris. △ Less

Submitted 24 February, 2025; originally announced February 2025.

Comments: 21 pages, 12 figures, submitted to A&A, abridged abstract

Journal ref: A&A 701, A117 (2025)

No Evidence of Asymmetrically Enhanced Star Formation in Infalling Galaxies in UNIONS

Authors: Lauren M. Foster, Laura C. Parker, Stephen Gwyn, Ian D. Roberts, James E. Taylor, Michael J. Hudson, Alan W. McConnachie, Thomas de Boer

Abstract: Ram pressure stripping is a well-known environmental quenching mechanism that removes gas from galaxies infalling into groups and clusters. In some extreme examples of ram pressure stripping, galaxies with extended gas tails show evidence of enhanced star formation prior to quenching. In this work we use a sample of 5277 local satellite galaxies in which a stripped tail of gas has not necessarily… ▽ More Ram pressure stripping is a well-known environmental quenching mechanism that removes gas from galaxies infalling into groups and clusters. In some extreme examples of ram pressure stripping, galaxies with extended gas tails show evidence of enhanced star formation prior to quenching. In this work we use a sample of 5277 local satellite galaxies in which a stripped tail of gas has not necessarily been observed, to quantify the strength of ram pressure-enhanced star formation and compare these results to a control sample of 8360 field galaxies. We use u-band imaging from the Ultraviolet-Near Infrared Northern Survey (UNIONS) as a star formation tracer and several metrics to quantify star formation asymmetry. We compare these results to environmental properties of the galaxy, such as their time since infall and host halo mass, to constrain the degree of ram pressure enhanced star formation as a function of environment. We find no significant differences between the satellite and the field samples. We further restrict our sample to galaxies which we most expect to be experiencing significant ram pressure but find no strong evidence of these galaxies having systematically enhanced star formation. Finally, we investigate the properties of the most asymmetric galaxies in our sample and again find no strong evidence of ram pressure-induced star formation enhancement. We conclude that any star formation enhancement must be small for infalling galaxies, suggesting that this effect is either uncommon or short-lived. △ Less

Submitted 18 February, 2025; originally announced February 2025.

Comments: 19 pages, 10 figures, accepted by ApJ

Deep in the Fields of the Andromeda Halo: Discovery of the Pegasus VII dwarf galaxy in UNIONS

Authors: Simon E. T. Smith, Alan W. McConnachie, Stephen Gwyn, Christian R. Hayes, Massimiliano Gatto, Ken Chambers, Jean-Charles Cuillandre, Michael J. Hudson, Eugene Magnier, Nicolas Martin, Julio Navarro

Abstract: We present the newly discovered dwarf galaxy Pegasus VII (Peg VII), a member of the M31 sub-group which has been uncovered in the $ri$ photometric catalogs from the Ultraviolet Near-Infrared Optical Northern Survey and confirmed with follow-up imaging from both the Canada-France-Hawaii Telescope and the Gemini-North Telescope. This system has an absolute $V$-band magnitude of $-5.7 \pm 0.2$ mag an… ▽ More We present the newly discovered dwarf galaxy Pegasus VII (Peg VII), a member of the M31 sub-group which has been uncovered in the $ri$ photometric catalogs from the Ultraviolet Near-Infrared Optical Northern Survey and confirmed with follow-up imaging from both the Canada-France-Hawaii Telescope and the Gemini-North Telescope. This system has an absolute $V$-band magnitude of $-5.7 \pm 0.2$ mag and a physical half-light radius of $177^{+36}_{-34}$ pc, which is characteristic of dynamically-confirmed Milky Way satellite dwarf galaxies and about 5 times more extended than the most extended M31 globular clusters. Peg VII lies at a three-dimensional separation from M31 of $331^{+15}_{-4}$ kpc and a significant elongation ($ε\sim 0.5$) towards the projected direction of M31 could be indicative of a past tidal interaction, but additional investigation into the orbit, star formation history, and whether any gas remains in the galaxy is needed to better understand the evolution of Peg VII. △ Less

Submitted 13 February, 2025; originally announced February 2025.

Comments: 13 pages, 5 figures, accepted for publication in ApJ

Unions with UNIONS: Using galaxy-galaxy lensing to probe galaxy mergers

Authors: Isaac Cheng, Jack Elvin-Poole, Michael J. Hudson, Ruxin Barré, Sara L. Ellison, Robert W. Bickley, Thomas J. L. de Boer, Sébastien Fabbro, Leonardo Ferreira, Sacha Guerrini, Hendrik Hildebrandt, Martin Kilbinger, Alan W. McConnachie, Ludovic van Waerbeke, Anna Wittje

Abstract: We use galaxy-galaxy lensing to investigate how the dark matter (DM) haloes and stellar content of galaxies with $0.012 \leq z \leq 0.32$ and $10 \leq \log_{10}(M_\star/\mathrm{M}_\odot) \leq 12$ change as a result of the merger process. To this end, we construct two samples of galaxies obtained from the Ultraviolet Near Infrared Optical Northern Survey (UNIONS), comprising 1 623 post-mergers and… ▽ More We use galaxy-galaxy lensing to investigate how the dark matter (DM) haloes and stellar content of galaxies with $0.012 \leq z \leq 0.32$ and $10 \leq \log_{10}(M_\star/\mathrm{M}_\odot) \leq 12$ change as a result of the merger process. To this end, we construct two samples of galaxies obtained from the Ultraviolet Near Infrared Optical Northern Survey (UNIONS), comprising 1 623 post-mergers and $\sim$30 000 non-merging controls, that live in low-density environments to use as our lenses. These samples are weighted to share the same distributions of stellar mass, redshift, and geometric mean distance to a galaxy's three nearest neighbours to ensure differences in the lensing signal are due to the merger process itself. We do not detect a statistically significant difference in the excess surface density profile of post-mergers and non-merging controls with current data. Fitting haloes composed of a point-like stellar mass component and an extended DM structure described by a Navarro-Frenk-White profile to the lensing measurements yields, for both samples, halo masses of $M_\text{halo} \sim 4\times10^{12}\,\mathrm{M}_\odot$ and a moderately negative correlation between $M_\text{halo}$ and concentration $c$. This allows us to rule out, at the 95% confidence level, merger-induced starbursts in which more than 60% of the stellar mass is formed in the burst. The application of our methods to upcoming surveys that are able to provide samples $\sim$10$\times$ larger than our current catalogue are expected to detect the weak-lensing signatures of mergers and further constrain their properties. △ Less

Submitted 3 September, 2025; v1 submitted 1 February, 2025; originally announced February 2025.

Comments: 12 pages, 7 figures, 1 table. Accepted for publication in ApJ

The Hubble Space Telescope Survey of M31 Satellite Galaxies IV. Survey Overview and Lifetime Star Formation Histories

Authors: A. Savino, D. R. Weisz, A. E. Dolphin, M. J. Durbin, N. Kallivayalil, A. Wetzel, J. Anderson, G. Besla, M. Boylan-Kolchin, T. M. Brown, J. S. Bullock, A. A. Cole, M. L. M. Collins, M. C. Cooper, A. J. Deason, A. L. Dotter, M. Fardal, A. M. N. Ferguson, T. K. Fritz, M. C. Geha, K. M. Gilbert, P. Guhathakurta, R. Ibata, M. J. Irwin, M. Jeon , et al. (13 additional authors not shown)

Abstract: From $>1000$ orbits of HST imaging, we present deep homogeneous resolved star color-magnitude diagrams that reach the oldest main sequence turnoff and uniformly measured star formation histories (SFHs) of 36 dwarf galaxies ($-6 \ge M_V \ge -17$) associated with the M31 halo, and for 10 additional fields in M31, M33, and the Giant Stellar Stream. From our SFHs we find: i) the median stellar age and… ▽ More From $>1000$ orbits of HST imaging, we present deep homogeneous resolved star color-magnitude diagrams that reach the oldest main sequence turnoff and uniformly measured star formation histories (SFHs) of 36 dwarf galaxies ($-6 \ge M_V \ge -17$) associated with the M31 halo, and for 10 additional fields in M31, M33, and the Giant Stellar Stream. From our SFHs we find: i) the median stellar age and quenching epoch of M31 satellites correlate with galaxy luminosity and galactocentric distance. Satellite luminosity and present-day distance from M31 predict the satellite quenching epoch to within $1.8$ Gyr at all epochs. This tight relationship highlights the fundamental connection between satellite halo mass, environmental history, and star formation duration. ii) There is no difference between the median SFH of galaxies on and off the great plane of Andromeda satellites. iii) $\sim50$\% of our M31 satellites show prominent ancient star formation ($>12$ Gyr ago) followed by delayed quenching ($8-10$ Gyr ago), which is not commonly observed among the MW satellites. iv) A comparison with TNG50 and FIRE-2 simulated satellite dwarfs around M31-like hosts show that some of these trends (dependence of SFH on satellite luminosity) are reproduced in the simulations while others (dependence of SFH on galactocentric distance, presence of the delayed-quenching population) are weaker or absent. We provide all photometric catalogs and SFHs as High-Level Science Products on MAST. △ Less

Submitted 22 January, 2025; originally announced January 2025.

Comments: Accepted for publication on ApJ. 47 pages, 24 figures, 12 tables. Corresponding HLSP data can be retrieved at: https://archive.stsci.edu/hlsp/m31-satellites

Cosmology from UNIONS weak lensing profiles of galaxy clusters

Authors: Charlie T. Mpetha, James E. Taylor, Yuba Amoura, Roan Haggar, Thomas de Boer, Sacha Guerrini, Axel Guinot, Fabian Hervas Peters, Hendrik Hildebrandt, Michael J. Hudson, Martin Kilbinger, Tobias Liaudat, Alan McConnachie, Ludovic Van Waerbeke, Anna Wittje

Abstract: Cosmological information is encoded in the structure of galaxy clusters. In Universes with less matter and larger initial density perturbations, clusters form earlier and have more time to accrete material, leading to a more extended infall region. Thus, measuring the mean mass distribution in the infall region provides a novel cosmological test. The infall region is largely insensitive to baryoni… ▽ More Cosmological information is encoded in the structure of galaxy clusters. In Universes with less matter and larger initial density perturbations, clusters form earlier and have more time to accrete material, leading to a more extended infall region. Thus, measuring the mean mass distribution in the infall region provides a novel cosmological test. The infall region is largely insensitive to baryonic physics, and provides a cleaner structural test than other measures of cluster assembly time such as concentration. We consider cluster samples from three publicly available galaxy cluster catalogues: the Spectroscopic Identification of eROSITA Sources (SPIDERS) catalogue, the X-ray and Sunyaev-Zeldovich effect selected clusters in the meta-catalogue M2C, and clusters identified in the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Survey. Using a preliminary shape catalogue from the Ultraviolet Near Infrared Optical Northern Survey (UNIONS), we derive excess surface mass density profiles for each sample. We then compare the mean profile for the DESI Legacy sample, which is the most complete, to predictions from a suite of simulations covering a range of $Ω_{\rm m}$ and $σ_8$, obtaining constraints of $Ω_{\rm m}=0.34\pm 0.06$ and $σ_8=0.77 \pm 0.04$. We also measure mean (comoving) splashback radii for SPIDERS, M2C and DESI Legacy Imaging Survey clusters of $1.39^{+0.21}_{-0.18} {\rm cMpc}$, $1.77^{+0.20}_{-0.18} {\rm cMpc}/h$ and $1.42^{+0.11}_{-0.12} {\rm cMpc}/h$ respectively. Performing this analysis with the final UNIONS shape catalogue and the full sample of spectroscopically observed clusters in DESI, we can expect to improve on the best current constraints from cluster abundance studies by a factor of 2 or more. △ Less

Submitted 10 October, 2025; v1 submitted 15 January, 2025; originally announced January 2025.

Comments: 18 pages, 12 figures. Updated to match MNRAS version

Journal ref: Mon Not R Astron Soc (2025) 1393-1409

Galaxy-Point Spread Function correlations as a probe of weak-lensing systematics with UNIONS data

Authors: Sacha Guerrini, Martin Kilbinger, Hubert Leterme, Axel Guinot, Jingwei Wang, Fabian Hervas Peters, Hendrik Hildebrandt, Michael J. Hudson, Alan McConnachie

Abstract: Weak gravitational lensing requires precise measurements of galaxy shapes and therefore an accurate knowledge of the PSF model. The latter can be a source of systematics that affect the shear two-point correlation function. A key stake of weak lensing analysis is to forecast the systematics due to the PSF. Correlation functions of galaxies and the PSF, the so-called $ρ$- and $τ$-statistics, are us… ▽ More Weak gravitational lensing requires precise measurements of galaxy shapes and therefore an accurate knowledge of the PSF model. The latter can be a source of systematics that affect the shear two-point correlation function. A key stake of weak lensing analysis is to forecast the systematics due to the PSF. Correlation functions of galaxies and the PSF, the so-called $ρ$- and $τ$-statistics, are used to evaluate the level of systematics coming from the PSF model and PSF corrections, and contributing to the two-point correlation function used to perform cosmological inference. Our goal is to introduce a fast and simple method to estimate this level of systematics and assess its agreement with state-of-the-art approaches. We introduce a new way to estimate the covariance matrix of the $τ$-statistics using analytical expressions. The covariance allows us to estimate parameters directly related to the level of systematics associated with the PSF and provides us with a tool to validate the PSF model used in a weak-lensing analysis. We apply those methods to data from the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS). We show that the semi-analytical covariance yields comparable results than using covariances obtained from simulations or jackknife resampling. It requires less computation time and is therefore well suited for rapid comparison of the systematic level obtained from different catalogs. We also show how one can break degeneracies between parameters with a redefinition of the $τ$-statistics. The methods developed in this work will be useful tools in the analysis of current weak-lensing data but also of Stage IV surveys such as Euclid, LSST or Roman. They provide fast and accurate diagnostics on PSF systematics that are crucial to understand in the context of cosmic shear studies. △ Less

Submitted 19 December, 2024; originally announced December 2024.

Comments: 18 pages, 13 figures, submitted to A&A

Journal ref: A&A 700, A215 (2025)

Gemini High-resolution Optical SpecTrograph (GHOST) at Gemini-South: Instrument performance and integration, first science, and next steps

Authors: V. M. Kalari, R. J. Diaz, G. Robertson, A. McConnachie, M. Ireland, R. Salinas, P. Young, C. Simpson, C. Hayes, J. Nielsen, G. Burley, J. Pazder, M. Gomez-Jimenez, E. Martioli, S. B. Howell, M. Jeong, S. Juneau, R. Ruiz-Carmona, S. Margheim, A. Sheinis, A. Anthony, G. Baker, T. A. M. Berg, T. Cao, E. Chapin , et al. (35 additional authors not shown)

Abstract: The Gemini South telescope is now equipped with a new high-resolution spectrograph called GHOST (the Gemini High-resolution Optical SpecTrograph). This instrument provides high-efficiency, high-resolution spectra covering 347-1060 nm in a single exposure of either one or two targets simultaneously, along with precision radial velocity spectroscopy utilizing an internal calibration source. It can o… ▽ More The Gemini South telescope is now equipped with a new high-resolution spectrograph called GHOST (the Gemini High-resolution Optical SpecTrograph). This instrument provides high-efficiency, high-resolution spectra covering 347-1060 nm in a single exposure of either one or two targets simultaneously, along with precision radial velocity spectroscopy utilizing an internal calibration source. It can operate at a spectral element resolving power of either 76000 or 56000, and can reach a SNR$\sim$5 in a 1hr exposure on a V$\sim$20.8 mag target in median site seeing, and dark skies (per resolution element). GHOST was installed on-site in June 2022, and we report performance after full integration to queue operations in November 2023, in addition to scientific results enabled by the integration observing runs. These results demonstrate the ability to observe a wide variety of bright and faint targets with high efficiency and precision. With GHOST, new avenues to explore high-resolution spectroscopy have opened up to the astronomical community. These are described, along with the planned and potential upgrades to the instrument. △ Less

Submitted 12 November, 2024; v1 submitted 9 September, 2024; originally announced September 2024.

Comments: Accepted in the Astronomical Journal; 26 pages, 24 figures and 4 tables

Galaxy Mergers in UNIONS -- I: A Simulation-driven Hybrid Deep Learning Ensemble for Pure Galaxy Merger Classification

Authors: Leonardo Ferreira, Robert W. Bickley, Sara L. Ellison, David R. Patton, Shoshannah Byrne-Mamahit, Scott Wilkinson, Connor Bottrell, Sébastien Fabbro, Stephen D. J. Gwyn, Alan McConnachie

Abstract: Merging and interactions can radically transform galaxies. However, identifying these events based solely on structure is challenging as the status of observed mergers is not easily accessible. Fortunately, cosmological simulations are now able to produce more realistic galaxy morphologies, allowing us to directly trace galaxy transformation throughout the merger sequence. To advance the potential… ▽ More Merging and interactions can radically transform galaxies. However, identifying these events based solely on structure is challenging as the status of observed mergers is not easily accessible. Fortunately, cosmological simulations are now able to produce more realistic galaxy morphologies, allowing us to directly trace galaxy transformation throughout the merger sequence. To advance the potential of observational analysis closer to what is possible in simulations, we introduce a supervised deep learning Convolutional Neural Network (CNN) and Vision Transformer (ViT) hybrid framework, Mummi (MUlti Model Merger Identifier). Mummi is trained on realism-added synthetic data from IllustrisTNG100-1, and is comprised of a multi-step ensemble of models to identify mergers and non-mergers, and to subsequently classify the mergers as interacting pairs or post-mergers. To train this ensemble of models, we generate a large imaging dataset of 6.4 million images targeting UNIONS with RealSimCFIS. We show that Mummi offers a significant improvement over many previous machine learning classifiers, achieving 95% pure classifications even at Gyr long timescales when using a jury-based decision making process, mitigating class imbalance issues that arise when identifying real galaxy mergers from $z=0$ to $0.3$. Additionally, we can divide the identified mergers into pairs and post-mergers at 96% success rate. We drastically decrease the false positive rate in galaxy merger samples by 75%. By applying Mummi to the UNIONS DR5-SDSS DR7 overlap, we report a catalog of 13,448 high confidence galaxy merger candidates. Finally, we demonstrate that Mummi produces powerful representations solely using supervised learning, which can be used to bridge galaxy morphologies in simulations and observations. △ Less

Submitted 25 July, 2024; originally announced July 2024.

Comments: 23 pages, 16 figures. Accepted for publication in MNRAS

High-Resolution Dayside Spectroscopy of WASP-189b: Detection of Iron during the GHOST/Gemini South System Verification Run

Authors: Emily K. Deibert, Adam B. Langeveld, Mitchell E. Young, Laura Flagg, Jake D. Turner, Peter C. B. Smith, Ernst J. W. de Mooij, Ray Jayawardhana, Kristin Chiboucas, Roberto Gamen, Christian R. Hayes, Jeong-Eun Heo, Miji Jeong, Venu Kalari, Eder Martioli, Vinicius M. Placco, Siyi Xu, Ruben Diaz, Manuel Gomez-Jimenez, Carlos Quiroz, Roque Ruiz-Carmona, Chris Simpson, Alan W. McConnachie, John Pazder, Gregory Burley , et al. (8 additional authors not shown)

Abstract: With high equilibrium temperatures and tidally locked rotation, ultra-hot Jupiters (UHJs) are unique laboratories within which to probe extreme atmospheric physics and chemistry. In this paper, we present high-resolution dayside spectroscopy of the UHJ WASP-189b obtained with the new Gemini High-resolution Optical SpecTrograph (GHOST) at the Gemini South Observatory. The observations, which cover… ▽ More With high equilibrium temperatures and tidally locked rotation, ultra-hot Jupiters (UHJs) are unique laboratories within which to probe extreme atmospheric physics and chemistry. In this paper, we present high-resolution dayside spectroscopy of the UHJ WASP-189b obtained with the new Gemini High-resolution Optical SpecTrograph (GHOST) at the Gemini South Observatory. The observations, which cover three hours of post-eclipse orbital phases, were obtained during the instrument's System Verification run. We detect the planet's atmosphere via the Doppler cross-correlation technique, and recover a detection of neutral iron in the planet's dayside atmosphere at a significance of 7.5$σ$ in the red-arm of the data, verifying the presence of a thermal inversion. We also investigate the presence of other species in the atmosphere and discuss the implications of model injection/recovery tests. These results represent the first atmospheric characterization of an exoplanet with GHOST's high-resolution mode, and demonstrate the potential of this new instrument in detecting and studying ultra-hot exoplanet atmospheres. △ Less

Submitted 15 July, 2024; originally announced July 2024.

Comments: 21 pages, 12 figures, 3 tables. Accepted for publication in The Astronomical Journal

Euclid: Early Release Observations -- Deep anatomy of nearby galaxies

Authors: L. K. Hunt, F. Annibali, J. -C. Cuillandre, A. M. N. Ferguson, P. Jablonka, S. S. Larsen, F. R. Marleau, E. Schinnerer, M. Schirmer, C. Stone, C. Tortora, T. Saifollahi, A. Lançon, M. Bolzonella, S. Gwyn, M. Kluge, R. Laureijs, D. Carollo, M. L. M. Collins, P. Dimauro, P. -A. Duc, D. Erkal, J. M. Howell, C. Nally, E. Saremi , et al. (174 additional authors not shown)

Abstract: Euclid is poised to make significant advances in the study of nearby galaxies in the local Universe. Here we present a first look at 6 galaxies observed for the Nearby Galaxy Showcase as part of the Euclid Early Release Observations acquired between August and November, 2023. These targets, 3 dwarf galaxies (HolmbergII, IC10, NGC6822) and 3 spirals (IC342, NGC2403, NGC6744), range in distance from… ▽ More Euclid is poised to make significant advances in the study of nearby galaxies in the local Universe. Here we present a first look at 6 galaxies observed for the Nearby Galaxy Showcase as part of the Euclid Early Release Observations acquired between August and November, 2023. These targets, 3 dwarf galaxies (HolmbergII, IC10, NGC6822) and 3 spirals (IC342, NGC2403, NGC6744), range in distance from about 0.5 Mpc to 8.8 Mpc. Our assessment of the surface brightness depths in the stacked Euclid images confirms previous estimates in 100 arcsec^2 regions of 1sigma=30.5 mag/arcsec^2 for VIS, but slightly deeper than previous estimates for NISP with 1sigma=29.2-29.4 mag/arcsec^2. By combining Euclid HE, YE, and IE into RGB images, we illustrate the large field-of-view covered by a single Reference Observing Sequence, together with exquisite detail on parsec scales in these nearby galaxies. Radial surface brightness and color profiles demonstrate galaxy colors in agreement with stellar population synthesis models. Standard stellar photometry selection techniques find approximately 1.3 million stars across the 6 galaxy fields. Euclid's resolved stellar photometry allows us to constrain the star-formation histories of these galaxies, by disentangling the distributions of young stars, as well as asymptotic giant branch and red giant branch stellar populations. We finally examine 2 galaxies individually for surrounding satellite systems. Our analysis of the ensemble of dwarf satellites around NGC6744 reveals a new galaxy, EDwC1, a nucleated dwarf spheroidal at the end of a spiral arm. Our new census of the globular clusters around NGC2403 yields 9 new star-cluster candidates, 8 of which with colors indicative of evolved stellar populations. In summary, our investigation of the 6 Showcase galaxies demonstrates that Euclid is a powerful probe of the anatomy of nearby galaxies [abridged]. △ Less

Submitted 22 May, 2024; originally announced May 2024.

Comments: 36 pages; 20 figures in main text; 4 Appendices. Submitted to A&A, as part of the A&A special issue `Euclid on Sky', which contains Euclid key reference papers and first results from the Euclid Early Release Observations

Journal ref: A&A 697, A9 (2025)

Euclid: Early Release Observations -- Programme overview and pipeline for compact- and diffuse-emission photometry

Authors: J. -C. Cuillandre, E. Bertin, M. Bolzonella, H. Bouy, S. Gwyn, S. Isani, M. Kluge, O. Lai, A. Lançon, D. A. Lang, R. Laureijs, T. Saifollahi, M. Schirmer, C. Stone, Abdurro'uf, N. Aghanim, B. Altieri, F. Annibali, H. Atek, P. Awad, M. Baes, E. Bañados, D. Barrado, S. Belladitta, V. Belokurov , et al. (240 additional authors not shown)

Abstract: The Euclid ERO showcase Euclid's capabilities in advance of its main mission, targeting 17 astronomical objects, from galaxy clusters, nearby galaxies, globular clusters, to star-forming regions. A total of 24 hours observing time was allocated in the early months of operation, engaging the scientific community through an early public data release. We describe the development of the ERO pipeline t… ▽ More The Euclid ERO showcase Euclid's capabilities in advance of its main mission, targeting 17 astronomical objects, from galaxy clusters, nearby galaxies, globular clusters, to star-forming regions. A total of 24 hours observing time was allocated in the early months of operation, engaging the scientific community through an early public data release. We describe the development of the ERO pipeline to create visually compelling images while simultaneously meeting the scientific demands within months of launch, leveraging a pragmatic, data-driven development strategy. The pipeline's key requirements are to preserve the image quality and to provide flux calibration and photometry for compact and extended sources. The pipeline's five pillars are: removal of instrumental signatures; astrometric calibration; photometric calibration; image stacking; and the production of science-ready catalogues for both the VIS and NISP instruments. We report a PSF with a full width at half maximum of 0.16" in the optical and 0.49" in the three NIR bands. Our VIS mean absolute flux calibration is accurate to about 1%, and 10% for NISP due to a limited calibration set; both instruments have considerable colour terms. The median depth is 25.3 and 23.2 AB mag with a SNR of 10 for galaxies, and 27.1 and 24.5 AB mag at an SNR of 5 for point sources for VIS and NISP, respectively. Euclid's ability to observe diffuse emission is exceptional due to its extended PSF nearly matching a pure diffraction halo, the best ever achieved by a wide-field, high-resolution imaging telescope. Euclid offers unparalleled capabilities for exploring the LSB Universe across all scales, also opening a new observational window in the NIR. Median surface-brightness levels of 29.9 and 28.3 AB mag per square arcsec are achieved for VIS and NISP, respectively, for detecting a 10 arcsec x 10 arcsec extended feature at the 1 sigma level. △ Less

Submitted 22 May, 2024; originally announced May 2024.

Comments: Submitted to A&A, 44 pages, 36 figures - Part of the A&A special issue `Euclid on Sky', which contains Euclid key reference papers and first results from the Euclid Early Release Observations

Journal ref: A&A 697, A6 (2025)

Euclid. I. Overview of the Euclid mission

Authors: Euclid Collaboration, Y. Mellier, Abdurro'uf, J. A. Acevedo Barroso, A. Achúcarro, J. Adamek, R. Adam, G. E. Addison, N. Aghanim, M. Aguena, V. Ajani, Y. Akrami, A. Al-Bahlawan, A. Alavi, I. S. Albuquerque, G. Alestas, G. Alguero, A. Allaoui, S. W. Allen, V. Allevato, A. V. Alonso-Tetilla, B. Altieri, A. Alvarez-Candal, S. Alvi, A. Amara , et al. (1115 additional authors not shown)

Abstract: The current standard model of cosmology successfully describes a variety of measurements, but the nature of its main ingredients, dark matter and dark energy, remains unknown. Euclid is a medium-class mission in the Cosmic Vision 2015-2025 programme of the European Space Agency (ESA) that will provide high-resolution optical imaging, as well as near-infrared imaging and spectroscopy, over about 14… ▽ More The current standard model of cosmology successfully describes a variety of measurements, but the nature of its main ingredients, dark matter and dark energy, remains unknown. Euclid is a medium-class mission in the Cosmic Vision 2015-2025 programme of the European Space Agency (ESA) that will provide high-resolution optical imaging, as well as near-infrared imaging and spectroscopy, over about 14,000 deg^2 of extragalactic sky. In addition to accurate weak lensing and clustering measurements that probe structure formation over half of the age of the Universe, its primary probes for cosmology, these exquisite data will enable a wide range of science. This paper provides a high-level overview of the mission, summarising the survey characteristics, the various data-processing steps, and data products. We also highlight the main science objectives and expected performance. △ Less

Submitted 24 September, 2024; v1 submitted 22 May, 2024; originally announced May 2024.

Comments: Accepted for publication in the A&A special issue`Euclid on Sky'

Journal ref: A&A 697, A1 (2025)

Point-Spread Function errors for weak lensing - density cross-correlations. Application to UNIONS

Authors: Ziwen Zhang, Martin Kilbinger, Fabian Hervas Peters, Qinxun Li, Wentao Luo, Lucie Baumont, Jean-Charles Cuillandre, Sebastien Fabbro, Stephen Gwyn, Alan McConnachie, Anna Wittje

Abstract: Aims:Calibrating the point spread function (PSF) is a fundamental part of weak gravitational lensing analyses. Even with corrected galaxy images, imperfect calibrations can introduce biases. We propose an analytical framework for quantifying PSF-induced systematics as diagnostics for cross-correlation measurements of weak lensing with density tracers, e.g., galaxy-galaxy lensing. We show how those… ▽ More Aims:Calibrating the point spread function (PSF) is a fundamental part of weak gravitational lensing analyses. Even with corrected galaxy images, imperfect calibrations can introduce biases. We propose an analytical framework for quantifying PSF-induced systematics as diagnostics for cross-correlation measurements of weak lensing with density tracers, e.g., galaxy-galaxy lensing. We show how those systematics propagate to physical parameters of the density tracers. Those diagnostics only require a shape catalogue of PSF stars and foreground galaxy positions. Methods:We consider the PSF-induced multiplicative bias, and introduce three second-order statistics as additive biases. We compute both biases for the weak-lensing derived halo mass of spectroscopic foreground galaxy samples, in particular, their effect on the tangential shear and fitted halo mass as a function of stellar mass. In addition, we assess their impact on the recently published black-hole - halo-mass relation for type I Active Galactic Nuclei (AGNs). Results:Using weak-lensing catalogues from the Ultraviolet Near Infrared Optical Northern Survey (UNIONS) and Dark Energy Survey (DES), we find the multiplicative biases in the tangential shear to be less than $0.5\%$. No correlations between additive bias and galaxy properties of the foreground sample are detected. The combined PSF systematics affect low-mass galaxies and small angular scales; halo mass estimates can be biased by up to 18$\%$ for a sample of central galaxies in the stellar mass range 9.0 $\leq$ log $M_*/\rm M_{\odot}$ < 9.5. Conclusions:The PSF-induced multiplicative bias is a subdominant contribution to current studies of weak-lensing - density cross-correlations, but might become significant for upcoming Stage-VI surveys. For samples with a low tangential shear, additive PSF systematics can induce a significant bias on derived properties such as halo mass. △ Less

Submitted 6 May, 2024; originally announced May 2024.

Comments: 11 pages, 5 figures

Journal ref: A&A 691, A75 (2024)

GHOST Commissioning Science Results III: Characterizing an iron-poor damped Lyman $α$ system

Authors: Trystyn A. M. Berg, Christian R. Hayes, Stefano Cristiani, Alan McConnachie, J. Gordon Robertson, Federico Sestito, Chris Simpson, Fletcher Waller, Timothy Chin, Adam Densmore, Ruben J. Diaz, Michael L. Edgar, Javier Fuentes Lettura, Manuel Gómez-Jiménez, Venu M. Kalari, Jon Lawrence, Steven Margheim, John Pazder, Roque Ruiz-Carmona, Ricardo Salinas, Karleyne M. G. Silva, Katherine Silversides, Kim A. Venn

Abstract: The Gemini High-resolution Optical SpecTrograph (GHOST) is a new echelle spectrograph available on the Gemini-South telescope as of Semester 2024A. We present the first high resolution spectrum of the quasar J1449-1227 (redshift z_em=3.27) using data taken during the commissioning of GHOST. The observed quasar hosts an intervening iron-poor ([Fe/H] = -2.5) damped Lyman alpha (DLA) system at redshi… ▽ More The Gemini High-resolution Optical SpecTrograph (GHOST) is a new echelle spectrograph available on the Gemini-South telescope as of Semester 2024A. We present the first high resolution spectrum of the quasar J1449-1227 (redshift z_em=3.27) using data taken during the commissioning of GHOST. The observed quasar hosts an intervening iron-poor ([Fe/H] = -2.5) damped Lyman alpha (DLA) system at redshift z=2.904. Taking advantage of the high spectral resolving power of GHOST (R~55000), we are able to accurately model the metal absorption lines of the metal-poor DLA and find a supersolar [Si/Fe], suggesting the DLA gas is in an early stage of chemical enrichment. Using simple ionization models, we find that the large range in the C IV/Si IV column density ratio of individual components within the DLA's high ionization absorption profile can be reproduced by several metal-poor Lyman limit systems surrounding the low-ionization gas of the DLA. It is possible that this metal-poor DLA resides within a complex system of metal-poor galaxies or filaments with inflowing gas. The high spectral resolution, wavelength coverage and sensitivity of GHOST makes it an ideal spectrograph for characterizing the chemistry and kinematics of quasar absorption lines. △ Less

Submitted 18 April, 2024; v1 submitted 15 April, 2024; originally announced April 2024.

Comments: Accepted for publication in MNRAS. 8 Pages, 5 figures

The Science Performance of the Gemini High Resolution Optical Spectrograph

Authors: Alan W. McConnachie, Christian R. Hayes, J. Gordon Robertson, John Pazder, Michael Ireland, Greg Burley, Vladimir Churilov, Jordan Lothrop, Ross Zhelem, Venu Kalari, André Anthony, Gabriella Baker, Trystyn Berg, Edward L. Chapin, Timothy Chin, Adam Densmore, Ruben Diaz, Jennifer Dunn, Michael L. Edgar, Tony Farrell, Veronica Firpo, Javier Fuentes, Manuel Gomez-Jimenez, Tim Hardy, David Henderson , et al. (24 additional authors not shown)

Abstract: The Gemini High Resolution Optical Spectrograph (GHOST) is a fiber-fed spectrograph system on the Gemini South telescope that provides simultaneous wavelength coverage from 348 - 1061nm, and designed for optimal performance between 363 - 950nm. It can observe up to two objects simultaneously in a 7.5 arcmin diameter field of regard at R = 56,000 or a single object at R = 75,000. The spectral resol… ▽ More The Gemini High Resolution Optical Spectrograph (GHOST) is a fiber-fed spectrograph system on the Gemini South telescope that provides simultaneous wavelength coverage from 348 - 1061nm, and designed for optimal performance between 363 - 950nm. It can observe up to two objects simultaneously in a 7.5 arcmin diameter field of regard at R = 56,000 or a single object at R = 75,000. The spectral resolution modes are obtained by using integral field units to image slice a 1.2" aperture by a factor of five in width using 19 fibers in the high resolution mode and by a factor of three in width using 7 fibers in the standard resolution mode. GHOST is equipped with hardware to allow for precision radial velocity measurements, expected to approach meters per second precision. Here, we describe the basic design and operational capabilities of GHOST, and proceed to derive and quantify the key aspects of its on-sky performance that are of most relevance to its science users. △ Less

Submitted 14 January, 2024; originally announced January 2024.

Comments: 37 pages, 27 figures. Accepted for publication in Publications of the Astronomical Society of the Pacific

The discovery of the faintest known Milky Way satellite using UNIONS

Authors: Simon E. T. Smith, William Cerny, Christian R. Hayes, Federico Sestito, Jaclyn Jensen, Alan W. McConnachie, Marla Geha, Julio Navarro, Ting S. Li, Jean-Charles Cuillandre, Raphaël Errani, Ken Chambers, Stephen Gwyn, Francois Hammer, Michael J. Hudson, Eugene Magnier, Nicolas Martin

Abstract: We present the discovery of Ursa Major III/UNIONS 1, the least luminous known satellite of the Milky Way, which is estimated to have an absolute V-band magnitude of $+2.2^{+0.4}_{-0.3}$ mag, equivalent to a total stellar mass of 16$^{+6}_{-5}$ M$_{\odot}$. Ursa Major III/UNIONS 1 was uncovered in the deep, wide-field Ultraviolet Near Infrared Optical Northern Survey (UNIONS) and is consistent with… ▽ More We present the discovery of Ursa Major III/UNIONS 1, the least luminous known satellite of the Milky Way, which is estimated to have an absolute V-band magnitude of $+2.2^{+0.4}_{-0.3}$ mag, equivalent to a total stellar mass of 16$^{+6}_{-5}$ M$_{\odot}$. Ursa Major III/UNIONS 1 was uncovered in the deep, wide-field Ultraviolet Near Infrared Optical Northern Survey (UNIONS) and is consistent with an old ($τ> 11$ Gyr), metal-poor ([Fe/H] $\sim -2.2$) stellar population at a heliocentric distance of $\sim$ 10 kpc. Despite being compact ($r_{\text{h}} = 3\pm1$ pc) and composed of so few stars, we confirm the reality of Ursa Major III/UNIONS 1 with Keck II/DEIMOS follow-up spectroscopy and identify 11 radial velocity members, 8 of which have full astrometric data from $Gaia$ and are co-moving based on their proper motions. Based on these 11 radial velocity members, we derive an intrinsic velocity dispersion of $3.7^{+1.4}_{-1.0}$ km s$^{-1}$ but some caveats preclude this value from being interpreted as a direct indicator of the underlying gravitational potential at this time. Primarily, the exclusion of the largest velocity outlier from the member list drops the velocity dispersion to $1.9^{+1.4}_{-1.1}$ km s$^{-1}$, and the subsequent removal of an additional outlier star produces an unresolved velocity dispersion. While the presence of binary stars may be inflating the measurement, the possibility of a significant velocity dispersion makes Ursa Major III/UNIONS 1 a high priority candidate for multi-epoch spectroscopic follow-ups to deduce to true nature of this incredibly faint satellite. △ Less

Submitted 16 November, 2023; originally announced November 2023.

Comments: 21 pages, 9 figures, 3 tables; Accepted for publication in ApJ

Ursa Major III/UNIONS 1: the darkest galaxy ever discovered?

Authors: Raphaël Errani, Julio F. Navarro, Simon E. T. Smith, Alan W. McConnachie

Abstract: The recently discovered stellar system Ursa Major III/UNIONS 1 (UMa3/U1) is the faintest known Milky Way satellite to date. With a stellar mass of $16^{+6}_{-5}\,\rm M_\odot$ and a half-light radius of $3\pm1$pc, it is either the darkest galaxy ever discovered or the faintest self-gravitating star cluster known to orbit the Galaxy. Its line-of-sight velocity dispersion suggests the presence of dar… ▽ More The recently discovered stellar system Ursa Major III/UNIONS 1 (UMa3/U1) is the faintest known Milky Way satellite to date. With a stellar mass of $16^{+6}_{-5}\,\rm M_\odot$ and a half-light radius of $3\pm1$pc, it is either the darkest galaxy ever discovered or the faintest self-gravitating star cluster known to orbit the Galaxy. Its line-of-sight velocity dispersion suggests the presence of dark matter, although current measurements are inconclusive because of the unknown contribution to the dispersion of potential binary stars. We use $N$-body simulations to show that, if self-gravitating, the system could not survive in the Milky Way tidal field for much longer than a single orbit (roughly 0.4Gyr), which strongly suggests that the system is stabilized by the presence of large amounts of dark matter. If UMa3/U1 formed at the center of a ~$10^9\rm M_\odot$ cuspy LCDM halo, its velocity dispersion would be predicted to be of order ~1km/s. This is roughly consistent with the current estimate, which, neglecting binaries, places $σ_{\rm los}$ in the range 1 to 4km/s. Because of its dense cusp, such a halo should be able to survive the Milky Way tidal field, keeping UMa3/U1 relatively unscathed until the present time. This implies that UMa3/U1 is plausibly the faintest and densest dwarf galaxy satellite of the Milky Way, with important implications for alternative dark matter models and for the minimum halo mass threshold for luminous galaxy formation in the LCDM cosmology. Our results call for multi-epoch high-resolution spectroscopic follow-up to confirm the dark matter content of this extraordinary system. △ Less

Submitted 24 February, 2024; v1 submitted 16 November, 2023; originally announced November 2023.

Comments: updated to match accepted version

Journal ref: ApJ 965 (2024) 20

SPLUS J142445.34-254247.1: An R-Process Enhanced, Actinide-Boost, Extremely Metal-Poor star observed with GHOST

Authors: Vinicius M. Placco, Felipe Almeida-Fernandes, Erika M. Holmbeck, Ian U. Roederer, Mohammad K. Mardini, Christian R. Hayes, Kim Venn, Kristin Chiboucas, Emily Deibert, Roberto Gamen, Jeong-Eun Heo, Miji Jeong, Venu Kalari, Eder Martioli, Siyi Xu, Ruben Diaz, Manuel Gomez-Jimenez, David Henderson, Pablo Prado, Carlos Quiroz, Roque Ruiz-Carmona, Chris Simpson, Cristian Urrutia, Alan W. McConnachie, John Pazder , et al. (11 additional authors not shown)

Abstract: We report on the chemo-dynamical analysis of SPLUS J142445.34-254247.1, an extremely metal-poor halo star enhanced in elements formed by the rapid neutron-capture process. This star was first selected as a metal-poor candidate from its narrow-band S-PLUS photometry and followed up spectroscopically in medium-resolution with Gemini South/GMOS, which confirmed its low-metallicity status. High-resolu… ▽ More We report on the chemo-dynamical analysis of SPLUS J142445.34-254247.1, an extremely metal-poor halo star enhanced in elements formed by the rapid neutron-capture process. This star was first selected as a metal-poor candidate from its narrow-band S-PLUS photometry and followed up spectroscopically in medium-resolution with Gemini South/GMOS, which confirmed its low-metallicity status. High-resolution spectroscopy was gathered with GHOST at Gemini South, allowing for the determination of chemical abundances for 36 elements, from carbon to thorium. At [Fe/H]=-3.39, SPLUS J1424-2542 is one of the lowest metallicity stars with measured Th and has the highest logeps(Th/Eu) observed to date, making it part of the "actinide-boost" category of r-process enhanced stars. The analysis presented here suggests that the gas cloud from which SPLUS J1424-2542 was formed must have been enriched by at least two progenitor populations. The light-element (Z<=30) abundance pattern is consistent with the yields from a supernova explosion of metal-free stars with 11.3-13.4 Msun, and the heavy-element (Z>=38) abundance pattern can be reproduced by the yields from a neutron star merger (1.66Msun and 1.27Msun) event. A kinematical analysis also reveals that SPLUS J1424-2542 is a low-mass, old halo star with a likely in-situ origin, not associated with any known early merger events in the Milky Way. △ Less

Submitted 25 October, 2023; originally announced October 2023.

Comments: 26 pages, 11 figures, accepted for publication on ApJ

Probing the early Milky Way with GHOST spectra of an extremely metal-poor star in the Galactic disk

Authors: Anya Dovgal, Kim A. Venn, Federico Sestito, Christian R. Hayes, Alan W. McConnachie, Julio F. Navarro, Vinicius M. Placco, Else Starkenburg, Nicolas F. Martin, John S. Pazder, Kristin Chiboucas, Emily Deibert, Roberto Gamen, Jeong-Eun Heo, Venu M. Kalari, Eder Martioli, Siyi Xu, Ruben Diaz, Manuel Gomez-Jiminez, David Henderson, Pablo Prado, Carlos Quiroz, J. Gordon Robertson, Roque Ruiz-Carmona, Chris Simpson , et al. (9 additional authors not shown)

Abstract: Pristine_183.6849+04.8619 (P1836849) is an extremely metal-poor ([Fe/H]$=-3.3\pm0.1$) star on a prograde orbit confined to the Galactic disk. Such stars are rare and may have their origins in protogalactic fragments that formed the early Milky Way, in low mass satellites accreted later, or forming in situ in the Galactic plane. Here we present a chemo-dynamical analysis of the spectral features be… ▽ More Pristine_183.6849+04.8619 (P1836849) is an extremely metal-poor ([Fe/H]$=-3.3\pm0.1$) star on a prograde orbit confined to the Galactic disk. Such stars are rare and may have their origins in protogalactic fragments that formed the early Milky Way, in low mass satellites accreted later, or forming in situ in the Galactic plane. Here we present a chemo-dynamical analysis of the spectral features between $3700-11000$Å from a high-resolution spectrum taken during Science Verification of the new Gemini High-resolution Optical SpecTrograph (GHOST). Spectral features for many chemical elements are analysed (Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Fe, Ni), and valuable upper limits are determined for others (C, Na, Sr, Ba). This main sequence star exhibits several rare chemical signatures, including (i) extremely low metallicity for a star in the Galactic disk, (ii) very low abundances of the light $α$-elements (Na, Mg, Si) compared to other metal-poor stars, and (iii) unusually large abundances of Cr and Mn, where [Cr, Mn/Fe]$_{\rm NLTE}>+0.5$. A comparison to theoretical yields from supernova models suggests that two low mass Population III objects (one 10 M$_\odot$ supernova and one 17 M$_\odot$ hypernova) can reproduce the abundance pattern well (reduced $χ^2<1$). When this star is compared to other extremely metal-poor stars on quasi-circular, prograde planar orbits, differences in both chemistry and kinematics imply there is little evidence for a common origin. The unique chemistry of P1836849 is discussed in terms of the earliest stages in the formation of the Milky Way. △ Less

Submitted 26 November, 2023; v1 submitted 4 October, 2023; originally announced October 2023.

Comments: 16 pages, 10 figures, 6 tables. Accepted by MNRAS November 22; Revisions include comparisons to more EMP stars, results unchanged

Journal ref: MNRAS 527 (2024) 7810-7824

Anisotropies in the spatial distribution and kinematics of dwarf galaxies in the Local Group and beyond

Authors: Isabel M. E. Santos-Santos, Julio F. Navarro, Alan McConnachie

Abstract: The Local Group (LG) of galaxies is dominated by the M31 and Milky Way (MW) pair, a configuration which suggests that the mass distribution in the LG and its surroundings should be highly anisotropic. We use the APOSTLE cosmological simulations to examine how this anisotropy manifests on the spatial distribution and kinematics of dwarf galaxies out to a distance of 3 Mpc from the MW. The simulatio… ▽ More The Local Group (LG) of galaxies is dominated by the M31 and Milky Way (MW) pair, a configuration which suggests that the mass distribution in the LG and its surroundings should be highly anisotropic. We use the APOSTLE cosmological simulations to examine how this anisotropy manifests on the spatial distribution and kinematics of dwarf galaxies out to a distance of 3 Mpc from the MW. The simulations indicate a clear preference for dwarfs to be located close to the axis defined by the MW-M31 direction, even for dwarfs in the LG periphery (LGP; i.e., those at distances 1.25<d/Mpc<3). The LGP "Hubble flow" is also affected; at fixed distance from the MW the mean recession speed, <V_{rad} >, varies with angular distance to M31, peaking in the anti-M31 direction and reaching a minimum behind M31. The combined M31-MW mass decelerates the local expansion; the LG "turnaround radius" (where <V_{rad}>=0) in APOSTLE is located at r ~ 1.25 Mpc from the LG barycentre and the pure Hubble flow (where <V_{rad}> ~ H_0*d) is not reached out to at least 3 Mpc. The predicted flow is very cold, with a barycentric dispersion of <40 km/s. A comparison of these predicted features with existing observations gives mixed results. There is clear observational evidence for an angular anisotropy in V_{rad} around the LGP, but little evidence for a preferred spatial distribution of LGP dwarfs along the MW-M31 axis. The observed local Hubble flow is also peculiar. Although the coldness of the flow is consistent with the simulations, it is significantly less decelerated: relative to the MW, on average, all galaxies beyond d~1.25 Mpc seem to be on a pure Hubble flow. We argue that these oddities may result from incompleteness and inhomogeneous sky coverage, but a full explanation may need to await the completion of deep all-sky surveys able to fill the gaps in our current inventory of nearby dwarfs. △ Less

Submitted 3 October, 2023; originally announced October 2023.

Comments: 10 pages, 5 figures, submitted to MNRAS. [Fig. 1 is a video that will play on compatible software (e.g. Okular and Adobe Acrobat, but not Preview or browser viewers).]

Small-scale stellar haloes: detecting low surface brightness features in the outskirts of Milky Way dwarf satellites

Authors: Jaclyn Jensen, Christian R. Hayes, Federico Sestito, Alan W. McConnachie, Fletcher Waller, Simon E. T. Smith, Julio Navarro, Kim A. Venn

Abstract: Dwarf galaxies are valuable laboratories for dynamical studies related to dark matter and galaxy evolution, yet it is currently unknown just how physically extended their stellar components are. Satellites orbiting the Galaxy's potential may undergo tidal stripping by the host, or alternatively, may themselves have accreted smaller systems whose debris populates the dwarf's own stellar halo. Evide… ▽ More Dwarf galaxies are valuable laboratories for dynamical studies related to dark matter and galaxy evolution, yet it is currently unknown just how physically extended their stellar components are. Satellites orbiting the Galaxy's potential may undergo tidal stripping by the host, or alternatively, may themselves have accreted smaller systems whose debris populates the dwarf's own stellar halo. Evidence of these past interactions, if present, is best searched for in the outskirts of the satellite. However, foreground contamination dominates the signal at these large radial distances, making observation of stars in these regions difficult. In this work, we introduce an updated algorithm for application to Gaia data that identifies candidate member stars of dwarf galaxies, based on spatial, color-magnitude and proper motion information, and which allows for an outer component to the stellar distribution. Our method shows excellent consistency with spectroscopically confirmed members from the literature despite having no requirement for radial velocity information. We apply the algorithm to all $\sim$60 Milky Way dwarf galaxy satellites, and we find 9 dwarfs (Boötes 1, Boötes 3, Draco 2, Grus 2, Segue 1, Sculptor, Tucana 2, Tucana 3, and Ursa Minor) that exhibit evidence for a secondary, low-density outer profile. We identify many member stars which are located beyond 5 half-light radii (and in some cases, beyond 10). We argue these distant stars are likely tracers of dwarf stellar haloes or tidal streams, though ongoing spectroscopic follow-up will be required to determine the origin of these extended stellar populations. △ Less

Submitted 4 December, 2023; v1 submitted 14 August, 2023; originally announced August 2023.

Comments: 25 pages, 13 figures, 4 tables, published in MNRAS

GHOST Commissioning Science Results II: a very metal-poor star witnessing the early Galactic assembly

Authors: Federico Sestito, Christian R. Hayes, Kim A. Venn, Jaclyn Jensen, Alan W. McConnachie, John Pazder, Fletcher Waller, Anke Arentsen, Pascale Jablonka, Nicolas F. Martin, Tadafumi Matsuno, Julio F. Navarro, Else Starkenburg, Sara Vitali, John Bassett, Trystyn A. M. Berg, Ruben Diaz, Michael L. Edgar, Veronica Firpo, Manuel Gomez-Jimenez, Venu Kalari, Sam Lambert, Jon Lawrence, Gordon Robertson, Roque Ruiz-Carmona , et al. (3 additional authors not shown)

Abstract: This study focuses on Pristine$\_180956.78$$-$$294759.8$ (hereafter P180956, $[Fe/H] =-1.95\pm0.02$), a star selected from the Pristine Inner Galaxy Survey (PIGS), and followed-up with the recently commissioned Gemini High-resolution Optical SpecTrograph (GHOST) at the Gemini South telescope. The GHOST spectrograph's high efficiency in the blue spectral region ($3700-4800$~Å) enables the detection… ▽ More This study focuses on Pristine$\_180956.78$$-$$294759.8$ (hereafter P180956, $[Fe/H] =-1.95\pm0.02$), a star selected from the Pristine Inner Galaxy Survey (PIGS), and followed-up with the recently commissioned Gemini High-resolution Optical SpecTrograph (GHOST) at the Gemini South telescope. The GHOST spectrograph's high efficiency in the blue spectral region ($3700-4800$~Å) enables the detection of elemental tracers of early supernovae (\eg Al, Mn, Sr, Eu). The star exhibits chemical signatures resembling those found in ultra-faint dwarf systems, characterised by very low abundances of neutron-capture elements (Sr, Ba, Eu), which are uncommon among stars in the Milky Way halo. Our analysis suggests that P180956 bears the chemical imprints of a small number (2 or 4) of low-mass hypernovae ($\sim10-15 M_{\odot}$), which are needed to mostly reproduce the abundance pattern of the light-elements (\eg [Si, Ti/Mg, Ca] $\sim0.6$), and one fast-rotating intermediate-mass supernova ($\sim300\kms$, $\sim80-120 M_{\odot}$), which is the main channel contributing to the high [Sr/Ba] ($\sim +1.2$). The small pericentric ($\sim0.7$ kpc) and apocentric ($\sim13$ kpc) distances and its orbit confined to the plane ($\lesssim 2$ kpc), indicate that this star was likely accreted during the early Galactic assembly phase. Its chemo-dynamical properties suggest that P180956 formed in a system similar to an ultra-faint dwarf galaxy accreted either alone, as one of the low-mass building blocks of the proto-Galaxy, or as a satellite of Gaia-Sausage-Enceladus. The combination of Gemini's large aperture with GHOST's high efficiency and broad spectral coverage makes this new spectrograph one of the leading instruments for near-field cosmology investigations. △ Less

Submitted 20 January, 2024; v1 submitted 14 August, 2023; originally announced August 2023.

Comments: Accepted version, minor editing. New figure showing Sr and Ba lines. Section 4.7 revised

The Pristine survey -- XXIII. Data Release 1 and an all-sky metallicity catalogue based on Gaia DR3 BP/RP spectro-photometry

Authors: Nicolas F. Martin, Else Starkenburg, Zhen Yuan, Morgan Fouesneau, Anke Ardern-Arentsen, Francesca De Angeli, Felipe Gran, Martin Montelius, Samuel Rusterucci, René Andrae, Michele Bellazzini, Paolo Montegriffo, Anna F. Esselink, Hanyuan Zhang, Kim A. Venn, Akshara Viswanathan, David S. Aguado, Giuseppina Battaglia, Manuel Bayer, Piercarlo Bonifacio, Elisabetta Caffau, Patrick Côté, Raymond Carlberg, Sébastien Fabbro, Emma Fernández Alvar , et al. (16 additional authors not shown)

Abstract: We used the spectro-photometric information of ~219 million stars from Gaia's DR3 to calculate synthetic, narrow-band, metallicity-sensitive CaHK magnitudes that mimic the observations of the Pristine survey, a survey of photometric metallicities of Milky Way (MW) stars that has been mapping >6,500 deg^2 of the northern sky with CFHT since 2015. These synthetic magnitudes were used for an absolute… ▽ More We used the spectro-photometric information of ~219 million stars from Gaia's DR3 to calculate synthetic, narrow-band, metallicity-sensitive CaHK magnitudes that mimic the observations of the Pristine survey, a survey of photometric metallicities of Milky Way (MW) stars that has been mapping >6,500 deg^2 of the northern sky with CFHT since 2015. These synthetic magnitudes were used for an absolute recalibration of the deeper Pristine photometry and, combined with broadband Gaia information, synthetic and Pristine CaHK magnitudes were used to estimate photometric metallicities over the whole sky. The resulting metallicity catalogue is accurate down to [Fe/H]~-3.5 and is particularly suited for the exploration of the metal-poor MW ([Fe/H]<-1.0). We make available here the catalogue of synthetic CaHK_syn magnitudes for all stars with BP/RP information in Gaia DR3, as well as an associated catalogue of more than ~30 million photometric metallicities for high S/N FGK stars. This paper further provides the first public data release of the Pristine catalogue in the form of higher quality recalibrated Pristine CaHK magnitudes and photometric metallicities for all stars in common with the BP/RP spectro-photometric information in Gaia DR3. When available, the much deeper Pristine data greatly enhance the quality of the derived metallicities, in particular at the faint end of the catalogue (G_BP > 16). Combined, both photometric metallicity catalogues include more than two million metal-poor star candidates ([Fe/H]_phot<-1.0) as well as more than 200,000 and ~8,000 very and extremely metal-poor candidates ([Fe/H]_phot<-2.0 and <-3.0, respectively). Finally, we show that these metallicity catalogues can be used efficiently, among other applications to hunt for the most metal-poor stars, and to study how the structure of the MW varies with metallicity. △ Less

Submitted 1 October, 2024; v1 submitted 2 August, 2023; originally announced August 2023.

Comments: 31 pages, 25 figures, accepted for publication in A&A. Minor changes in v2, including ~2.5% more coverage for Pristine DR1 from new observations. First two authors are co-first author. The CaHK photometry catalogue and the two photometric metallicity catalogues are available, before acceptance, as large compressed csv files at: https://seafile.unistra.fr/d/ee0c0f05719d4368bcbb/

Journal ref: A&A 692, A115 (2024)

Binary star population of the Sculptor dwarf galaxy

Authors: José María Arroyo-Polonio, Giuseppina Battaglia, Guillaume F. Thomas, Michael J. Irwin, Alan W. McConnachie, Eline Tolstoy

Abstract: Aims: We aim to compute the binary fraction of "classical" dwarf spheroidal galaxies (dSphs) that are satellites of the Milky Way (MW). This value can offer insights into the binary fraction in environments that are less dense and more metal-poor than our own galaxy. Additionally, knowledge of the binary fraction in dwarf galaxies is important with respect to avoiding overestimations of their dark… ▽ More Aims: We aim to compute the binary fraction of "classical" dwarf spheroidal galaxies (dSphs) that are satellites of the Milky Way (MW). This value can offer insights into the binary fraction in environments that are less dense and more metal-poor than our own galaxy. Additionally, knowledge of the binary fraction in dwarf galaxies is important with respect to avoiding overestimations of their dark matter content, inferred from stellar kinematics. Methods: We refined an existing method from the literature, placing an emphasis on providing robust uncertainties on the value of the binary fraction. We applied this modified method to a VLT/FLAMES dataset for Sculptor, specifically acquired for the purpose of velocity monitoring of individual stars, as well as to literature datasets for other six MW "classical" dSphs. In all cases, the targeted stars were mainly red giant branch stars (RGBs), with expected masses of around 0.8 M$_{\odot}$. The VLT/FLAMES dataset offers the most precise binary fractions compared to literature datasets, due to its time baseline of 12 years, along with at least nine repeated observations for each star. Results: We found that the binary fraction of Sculptor is 0.55$^{+0.17}_{-0.19}$. We find that it is important to take into account the Roche lobe overflow for constraining the period distribution of binary stars. In contrast to what has recently been proposed in the literature, our analysis indicates that there is no evidence to support varying the properties of the binary stellar population or their deviations from those established for the solar neighborhood, based on the sample of MW dSphs analyzed here. △ Less

Submitted 19 July, 2023; originally announced July 2023.

Comments: 17 pages, 14 figures, Accepted for publication in Astronomy & Astrophysics

GHOST Commissioning Science Results: Identifying a new chemically peculiar star in Reticulum II

Authors: Christian R. Hayes, Kim A. Venn, Fletcher Waller, Jaclyn Jensen, Alan W. McConnachie, John Pazder, Federico Sestito, Andre Anthony, Gabriella Baker, John Bassett, Joao Bento, Gregory Burley, Jurek Brzeski, Scott Case, Edward Chapin, Timothy Chin, Eric Chisholm, Vladimir Churilov, Adam Densmore, Ruben Diaz, Jennifer Dunn, Michael Edgar, Tony Farrell, Veronica Firpo, Joeleff Fitzsimmons , et al. (57 additional authors not shown)

Abstract: The Gemini High-resolution Optical SpecTrograph (GHOST) is the newest high resolution spectrograph to be developed for a large aperture telescope, recently deployed and commissioned at the Gemini-South telescope. In this paper, we present the first science results from the GHOST spectrograph taking during its commissioning runs. We have observed the bright metal-poor benchmark star HD 122563, alon… ▽ More The Gemini High-resolution Optical SpecTrograph (GHOST) is the newest high resolution spectrograph to be developed for a large aperture telescope, recently deployed and commissioned at the Gemini-South telescope. In this paper, we present the first science results from the GHOST spectrograph taking during its commissioning runs. We have observed the bright metal-poor benchmark star HD 122563, along with two stars in the ultra faint dwarf galaxy, Ret II, one of which was previously identified as a candidate member, but did not have a previous detailed chemical abundance analysis. This star (GDR3 0928) is found to be a bona fide member of Ret II, and from a spectral synthesis analysis, it is also revealed to be a CEMP-r star, with significant enhancements in the several light elements (C, N, O, Na, Mg, and Si), in addition to featuring an r-process enhancement like many other Ret II stars. The light-element enhancements in this star resemble the abundance patterns seen in the CEMP-no stars of other ultra faint dwarf galaxies, and are thought to have been produced by an independent source from the r-process. These unusual abundance patterns are thought to be produced by faint supernovae, which may be produced by some of the earliest generations of stars. △ Less

Submitted 7 June, 2023; originally announced June 2023.

Comments: 23 pages, 9 figures, 7 tables, submitted to the AAS Journals

The Hubble Space Telescope Survey of M31 Satellite Galaxies II. The Star Formation Histories of Ultra-Faint Dwarf Galaxies

Authors: A. Savino, D. R. Weisz, E. D. Skillman, A. Dolphin, A. A. Cole, N. Kallivayalil, A. Wetzel, J. Anderson, G. Besla, M. Boylan-Kolchin, T. M. Brown, J. S. Bullock, M. L. M. Collins, M. C. Cooper, A. J. Deason, A. L. Dotter, M. Fardal, A. M. N. Ferguson, T. K. Fritz, M. C. Geha, K. M. Gilbert, P. Guhathakurta, R. Ibata, M. J. Irwin, M. Jeon , et al. (12 additional authors not shown)

Abstract: We present the lifetime star formation histories (SFHs) for six ultra-faint dwarf (UFD; $M_V>-7.0$, $ 4.9<\log_{10}({M_*(z=0)}/{M_{\odot}})<5.5$) satellite galaxies of M31 based on deep color-magnitude diagrams constructed from \textit{Hubble Space Telescope} imaging. These are the first SFHs obtained from the oldest main sequence turn-off of UFDs outside the halo of the Milky Way (MW). We find th… ▽ More We present the lifetime star formation histories (SFHs) for six ultra-faint dwarf (UFD; $M_V>-7.0$, $ 4.9<\log_{10}({M_*(z=0)}/{M_{\odot}})<5.5$) satellite galaxies of M31 based on deep color-magnitude diagrams constructed from \textit{Hubble Space Telescope} imaging. These are the first SFHs obtained from the oldest main sequence turn-off of UFDs outside the halo of the Milky Way (MW). We find that five UFDs formed at least 50\% of their stellar mass by $z=5$ (12.6~Gyr ago), similar to known UFDs around the MW, but that 10-40\% of their stellar mass formed at later times. We uncover one remarkable UFD, \A{XIII}, which formed only 10\% of its stellar mass by $z=5$, and 75\% in a rapid burst at $z\sim2-3$, a result that is robust to choices of underlying stellar model and is consistent with its predominantly red horizontal branch. This "young" UFD is the first of its kind and indicates that not all UFDs are necessarily quenched by reionization, which is consistent with predictions from several cosmological simulations of faint dwarf galaxies. SFHs of the combined MW and M31 samples suggest reionization did not homogeneously quench UFDs. We find that the least massive MW UFDs ($M_*(z=5) \lesssim 5\times10^4 M_{\odot}$) are likely quenched by reionization, whereas more massive M31 UFDs ($M_*(z=5) \gtrsim 10^5 M_{\odot}$) may only have their star formation suppressed by reionization and quench at a later time. We discuss these findings in the context of the evolution and quenching of UFDs. △ Less

Submitted 13 September, 2023; v1 submitted 22 May, 2023; originally announced May 2023.

Comments: 18 pages, 14 figures, 5 appendices, accepted for publication in ApJ

The Andromeda Galaxy's Last Major Merger: Constraints from the survey of Planetary Nebulae

Authors: Souradeep Bhattacharya, Magda Arnaboldi, Ortwin Gerhard, Nelson Caldwell, Chiaki Kobayashi, Francois Hammer, Yanbin Yang, Kenneth C. Freeman, Johanna Hartke, Alan McConnachie

Abstract: The Andromeda galaxy (M 31) has experienced a tumultuous merger history as evidenced by the many substructures present in its inner halo. We use planetary nebulae (PNe) as chemodynamic tracers to shed light on the recent merger history of M 31. We identify the older dynamically hotter thicker disc in M 31 and a distinct younger dynamically colder thin disc. The two discs are also chemically distin… ▽ More The Andromeda galaxy (M 31) has experienced a tumultuous merger history as evidenced by the many substructures present in its inner halo. We use planetary nebulae (PNe) as chemodynamic tracers to shed light on the recent merger history of M 31. We identify the older dynamically hotter thicker disc in M 31 and a distinct younger dynamically colder thin disc. The two discs are also chemically distinct with the PN chemodynamics implying their formation in a `wet' major merger (mass ratio ~1:5) ~2.5-4 Gyr ago. From comparison of PN line-of-sight velocities in the inner halo substructures with predictions of a major-merger model in M 31, we find that the same merger event that formed the M 31 thick and thin disc is also responsible for forming these substructures. We thereby obtain constraints on the recent formation history of M 31 and the properties of its cannibalized satellite. △ Less

Submitted 8 May, 2023; v1 submitted 4 May, 2023; originally announced May 2023.

Comments: 5 pages, 1 figure, Proceedings of the IAUS 377: Early Disk-Galaxy Formation from JWST to the Milky Way. Kuala-Lumpur, February 6-10, 2023

Journal ref: Proceedings of the International Astronomical Union , Volume 18 , Symposium S377: Early Disk-Galaxy Formation from JWST to the Milky Way , December 2022 , pp. 123 - 126

Stars on the edge: Galactic tides and the outskirts of the Sculptor dwarf spheroidal

Authors: Federico Sestito, Joel Roediger, Julio F. Navarro, Jaclyn Jensen, Kim A. Venn, Simon E. T. Smith, Christian Hayes, Alan W. McConnachie

Abstract: The formation of "stellar halos" in dwarf galaxies have been discussed in terms of early mergers or Galactic tides, although fluctuations in the gravitational potential due to stellar feedback is also a possible candidate mechanism. A Bayesian algorithm is used to find new candidate members in the extreme outskirts of the Sculptor dwarf galaxy. Precise metallicities and radial velocities for two d… ▽ More The formation of "stellar halos" in dwarf galaxies have been discussed in terms of early mergers or Galactic tides, although fluctuations in the gravitational potential due to stellar feedback is also a possible candidate mechanism. A Bayesian algorithm is used to find new candidate members in the extreme outskirts of the Sculptor dwarf galaxy. Precise metallicities and radial velocities for two distant stars are measured from their spectra taken with the Gemini South GMOS spectrograph. The radial velocity, proper motion and metallicity of these targets are consistent with Sculptor membership. As a result, the known boundary of the Sculptor dwarf extends now out to an elliptical distance of $\sim10$ half-light radii, which corresponds to a projected physical distance of $\sim3$ kpc. As reported in earlier work, the overall distribution of radial velocities and metallicities indicate the presence of a more spatially and kinematically dispersed metal-poor population that surrounds the more concentrated and colder metal-rich stars. Sculptor's density profile shows a "kink" in its logarithmic slope at a projected distance of $\sim25$ arcmin (620 pc), which we interpret as evidence that Galactic tides have helped to populate the distant outskirts of the dwarf. We discuss further ways to test and validate this tidal interpretation for the origin of these distant stars. △ Less

Submitted 9 May, 2023; v1 submitted 25 April, 2023; originally announced April 2023.

Comments: Accepted for publication on MNRAS. Figure 1 now displays all the candidate members. arXiv admin note: text overlap with arXiv:2301.13214

A UNIONS view of the brightest central galaxies of candidate fossil groups

Authors: Aline Chu, F. Durret, A. Ellien, F. Sarron, C. Adami, I. Marquez, N. Martinet, T. de Boer, K. C. Chambers, J. -C. Cuillandre, S. Gwyn, E. A. Magnier, A. W. McConnachie

Abstract: The formation process of fossil groups (FGs) is still under debate, and large samples of such objects are still missing. The aim of this paper is to increase the sample of known FGs, and to analyse the properties of their brightest group galaxies (BGG) and compare them with a control sample of non-FG BGGs. Based on the Tinker spectroscopic catalogue of haloes and galaxies, we extract 87 FG and 100… ▽ More The formation process of fossil groups (FGs) is still under debate, and large samples of such objects are still missing. The aim of this paper is to increase the sample of known FGs, and to analyse the properties of their brightest group galaxies (BGG) and compare them with a control sample of non-FG BGGs. Based on the Tinker spectroscopic catalogue of haloes and galaxies, we extract 87 FG and 100 non-FG candidates. For all the objects with data available in UNIONS in the u and r bands, and/or in an extra r-band processed to preserve all low surface brightness features (rLSB), we made a 2D photometric fit of the BGG with GALFIT with one or two Sersic components and analysed how the subtraction of intracluster light contribution modifies the BGG properties. From the SDSS spectra available for the BGGs of 65 FGs and 82 non-FGs, we extracted the properties of their stellar populations with Firefly. We also investigated the origin of the emission lines in a nearby FG, NGC 4104, that has an AGN. A single Sersic profile can fit most objects in the u band, while two Sersics are needed in the r and rLSB bands, both for FGs and non-FGs. Non-FG BGGs cover a larger range of Sersic index. FG BGGs follow the Kormendy relation derived for almost one thousand brightest cluster galaxies (BCGs) by Chu et al. (2022) while non-FGs BGGs are mostly located below this relation, suggesting that FG BGGs have evolved similarly to BCGs, while non-FG BGGs have evolved differently. The above properties can be strongly modified by the subtraction of intracluster light contribution. The stellar populations of FG and non-FG BGGs do not differ significantly. Our results suggest FG and non-FG BGGs have had different formation histories, but it is not possible to trace differences in their stellar populations or large scale distributions. △ Less

Submitted 9 March, 2023; originally announced March 2023.

Comments: Accepted for publication in Astronomy & Astrophysics

Journal ref: A&A 673, A100 (2023)

The PAndAS View of the Andromeda Satellite System. IV Global properties

Authors: Amandine Doliva-Dolinsky, Nicolas F. Martin, Zhen Yuan, Alessandro Savino, Daniel R. Weisz, Annette M. N. Ferguson, Rodrigo A. Ibata, Stacy Y. Kim, Geraint F. Lewis, Alan W. McConnachie, Guillaume F. Thomas

Abstract: We build a statistical framework to infer the global properties of the satellite system of the Andromeda galaxy (M31) from the properties of individual dwarf galaxies located in the Pan-Andromeda Archaelogical Survey (PAndAS) and the previously determined completeness of the survey. Using forward modeling, we infer the slope of the luminosity function of the satellite system, the slope of its spat… ▽ More We build a statistical framework to infer the global properties of the satellite system of the Andromeda galaxy (M31) from the properties of individual dwarf galaxies located in the Pan-Andromeda Archaelogical Survey (PAndAS) and the previously determined completeness of the survey. Using forward modeling, we infer the slope of the luminosity function of the satellite system, the slope of its spatial density distribution, and the size-luminosity relation followed by the dwarf galaxies. We find that the slope of the luminosity function is $β=-1.5\pm0.1$. Combined with the spatial density profile, it implies that, when accounting for survey incompleteness, M31 hosts $92_{-26}^{+19}$ dwarf galaxies with $M_\textrm{V}<-5.5$ and a sky-projected distance from M31 between 30 and 300kpc. We conclude that many faint or distant dwarf galaxies remain to be discovered around Andromeda, especially outside the PAndAS footprint. Finally, we use our model to test if the higher number of satellites situated in the hemisphere facing the Milky Way could be explained simply by the detection limits of dwarf galaxy searches. We rule this out at $>99.9\%$ confidence and conclude that this anisotropy is an intrinsic feature of the M31 satellite system. The statistical framework we present here is a powerful tool to robustly constrain the properties of a satellite system and compare those across hosts, especially considering the upcoming start of the Euclid or Rubin large photometric surveys that are expected to uncover a large number of dwarf galaxies in the Local Volume. △ Less

Submitted 2 March, 2023; originally announced March 2023.

Comments: Submitted to ApJ - 12 pages, 6 figures, 2 tables

The extended "stellar halo" of the Ursa Minor dwarf galaxy

Authors: Federico Sestito, Daria Zaremba, Kim A. Venn, Lina D'Aoust, Christian Hayes, Jaclyn Jensen, Julio F. Navarro, Pascale Jablonka, Emma Fernández-Alvar, Jennifer Glover, Alan W. McConnachie, André-Nicolas Chené

Abstract: Stellar candidates in the Ursa Minor (UMi) dwarf galaxy have been found using a new Bayesian algorithm applied to \textit{Gaia} EDR3 data. Five of these targets are located in the extreme outskirts of UMi, from $\sim5$ to 12 elliptical half-light radii (r$_h$), where r$_h$(UMi) $= 17.32 \pm 0.11$ arcmin, and have been observed with the GRACES high resolution spectrograph at the Gemini-Northern tel… ▽ More Stellar candidates in the Ursa Minor (UMi) dwarf galaxy have been found using a new Bayesian algorithm applied to \textit{Gaia} EDR3 data. Five of these targets are located in the extreme outskirts of UMi, from $\sim5$ to 12 elliptical half-light radii (r$_h$), where r$_h$(UMi) $= 17.32 \pm 0.11$ arcmin, and have been observed with the GRACES high resolution spectrograph at the Gemini-Northern telescope. Precise radial velocities ($σ_{\rm{RV}} < 2$ km s$^{-1}$) and metallicities ($σ_{\rm{[Fe/H]}} < 0.2$ dex) confirm their memberships of UMi. Detailed analysis of the brightest and outermost star (Target~1, at $\sim12$ r$_h$), yields precision chemical abundances for the $α$- (Mg, Ca, Ti), odd-Z (Na, K, Sc), Fe-peak (Fe, Ni, Cr), and neutron-capture (Ba) elements. With data from the literature and APOGEE DR17, we find the chemical patterns in UMi are consistent with an outside-in star formation history that includes yields from core collapse supernovae, asymptotic giant branch stars, and supernovae Ia. Evidence for a knee in the [$α$/Fe] ratios near [Fe/H] $\sim-2.1$ indicates a low star formation efficiency similar to that in other dwarf galaxies. Detailed analysis of the surface number density profile shows evidence that UMi's outskirts have been populated by tidal effects, likely as a result of completing multiple orbits around the Galaxy. △ Less

Submitted 4 August, 2023; v1 submitted 30 January, 2023; originally announced January 2023.

Comments: NEW DISCUSSION ON THE PRESENCE OF TIDES. Final version

The shape of dark matter haloes: results from weak lensing in the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS)

Authors: Bailey Robison, Michael J. Hudson, Jean-Charles Cuillandre, Thomas Erben, Sébastien Fabbro, Raphaël Gavazzi, Axel Guinot, Stephen Gwyn, Hendrik Hildebrandt, Martin Kilbinger, Alan McConnachie, Lance Miller, Isaac Spitzer, Ludovic van Waerbeke

Abstract: Cold dark matter haloes are expected to be triaxial, and so appear elliptical in projection. We use weak gravitational lensing from the Canada-France Imaging Survey (CFIS) component of the Ultraviolet-Near Infrared Optical Northern Survey (UNIONS) to measure the ellipticity of the dark matter haloes around Luminous Red Galaxies (LRGs) from the Sloan Digital Sky Survey Data Release 7 (DR7) and from… ▽ More Cold dark matter haloes are expected to be triaxial, and so appear elliptical in projection. We use weak gravitational lensing from the Canada-France Imaging Survey (CFIS) component of the Ultraviolet-Near Infrared Optical Northern Survey (UNIONS) to measure the ellipticity of the dark matter haloes around Luminous Red Galaxies (LRGs) from the Sloan Digital Sky Survey Data Release 7 (DR7) and from the CMASS and LOWZ samples of the Baryon Oscillation Spectroscopic Survey (BOSS), assuming their major axes are aligned with the stellar light. We find that DR7 LRGs with masses $M \sim 2.7\times10^{13} \mathrm{M}_{\odot}/h$ have halo ellipticities $e=0.46\pm0.10$. Expressed as a fraction of the galaxy ellipticity, we find $f_h = 2.2\pm0.6$. For BOSS LRGs, the detection is of marginal significance: $e = 0.20\pm0.10$ and $f_h=0.7\pm0.7$. These results are in agreement with other measurements of halo ellipticity from weak lensing and, taken together with previous results, suggest an increase of halo ellipticity of $0.10\pm0.06$ per decade in halo mass. This trend agrees with the predictions from hydrodynamical simulations, which find that at higher halo masses, not only do dark matter haloes become more elliptical, but that the misalignment between major axis of the stellar light in the central galaxy and that of the dark matter decreases. △ Less

Submitted 19 May, 2023; v1 submitted 19 September, 2022; originally announced September 2022.

Comments: MNRAS, accepted