arXiv subjects

Risa H. Wechsler

Publications and source records attributed to Risa H. Wechsler.

At least 19 recordsLinked to original sources

arXiv2026

The Cocytos Stream: A Disrupted Globular Cluster from our Last Major Merger?

The census of stellar streams and dwarf galaxies in the Milky Way provides direct constraints on galaxy formation models and the nature of dark matter. The DESI Milky Way survey -- with a footprint of $14{,}000$ deg$^2$ and a depth of $r<19$ mag -- delivers the largest sample of distant metal-poor stars compared to previous optical fiber-fed spectroscopic surveys. This makes DESI an ideal survey to search for previously undetected streams and dwarf galaxies. We present a detailed characterization of the Cocytos stream, which was re-discovered using a clustering analysis with a catalog of giants in the DESI year 3 data, supplemented with Magellan/MagE spectroscopy. Our analysis reveals a relatively metal-rich ([Fe/H]$=-1.3$) and thick stream (width$=1.6^\circ$) at a heliocentric distance of $\approx 25$ kpc, with an internal velocity dispersion of $6$--$9$ km s$^{-1}$. The stream's metallicity, radial orbit, and proximity to the Virgo stellar overdensities suggest that it is most likely a disrupted globular cluster that came in with the Gaia-Enceladus merger. We also confirm its association with the Pyxis globular cluster. Our result showcases the ability of wide-field spectroscopic surveys to kinematically discover faint disrupted dwarfs and clusters, enabling constraints on the dark matter distribution in the Milky Way.

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arXiv2026

Warm, not Fuzzy: Generalized Ultralight Dark Matter Limits from Milky Way Satellites

We generalize lower limits on the dark matter (DM) particle mass $m$ derived from Milky Way (MW) satellite galaxy abundances to scenarios in which DM is an ultralight scalar field produced with a field power spectrum peaked at a subhorizon wavenumber $k_*$. In these models, the DM field free-streams similarly to warm DM while also exhibiting significant small-scale wave interference effects. The resulting dimensionless density power spectrum shows two effects: (i) free-streaming suppression at $k_{\rm fs}\sim k_{\rm eq}/[(k_*/a_{\rm eq}m)\ln(a_{\rm eq}m/k_*)]$; (ii) Poisson-like enhancement related to wave interference at $k\gtrsim10^{-2}k_*$, which saturates near the Jeans scale $k_{\rm J}\sim k_{\rm eq}/(k_*/a_{\rm eq}m)$. Comparing these predictions with established constraints on a free-streaming cutoff in the linear matter power spectrum from the MW satellite population and assuming that warm ultralight DM does not change the form of the galaxy--halo connection relative to cold DM, we obtain $m>6\times10^{-18}\,{\rm eV}\,(k_*/10^4\,{\rm Mpc}^{-1})$ for $k_*>10^4\,{\rm Mpc}^{-1}$ at 95% confidence. For smaller $k_*$, Poisson-noise enhancement on MW satellite scales weakens the constraint, yielding $m>6\times10^{-18}\,{\rm eV}\,(k_*/10^4\,{\rm Mpc}^{-1})^2$ for $k_*<10^4\,{\rm Mpc}^{-1}$ at 95% confidence.

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arXiv2026

The Azahar Project: Non-Thermal Physics Drives Star Formation Burstiness and the Evolution of the UV Luminosity Density at Cosmic Dawn

JWST observations, which have pushed the discovery and characterization of galaxies to cosmic dawn, have revealed significant deficiencies in state-of-the-art galaxy formation simulations, motivating the need for novel, physically grounded models. We present the first results from the Azahar suite of ten high-resolution ($\sim$20 pc), large-volume zoom-in cosmological simulations, which follow the formation of thousands of galaxies and progressively incorporate radiative transfer (RT), cosmic rays (CRs), and magnetohydrodynamics in addition to the 'standard' baryonic physics. Our Non-Thermal model, which simultaneously includes RT and CRs on the fly, reproduces the observed UV luminosity function from high redshifts ($z \sim 14$) to cosmic noon ($z \sim 3$), as well as the evolution of the stellar mass function, the galaxy main sequence, and observed gas metallicities. It does so through a combination of burstier star formation, with enhanced variability on both long ($\gtrsim 50$ Myr) and short ($\lesssim 10$ Myr) timescales, and a transition from high to low outflow mass-loading factors over cosmic time as CR pressure builds up in the ISM. We find that simple (boosted) thermal SN feedback models fail to capture the cosmic evolution of the very first galaxies because they lack realistic feedback channels that operate on different timescales and have distinct thermodynamical properties, thereby either overproducing stellar mass or driving overly explosive outflows. Our results indicate that non-thermal galaxy formation physics is crucial for providing a robust theoretical framework with which to interpret the high-redshift galaxy populations now being uncovered by JWST.

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arXiv2026

Non-Thermal Physics Drives Compact, Self-Regulated Galaxy Morphologies at Cosmic Dawn

James Webb Space Telescope (JWST) has discovered unexpectedly bright, rapidly growing galaxies in the early universe, which were not predicted by most previously existing galaxy formation models. Using synthetic JWST observations of the Azahar simulation suite, we show that comprehensive non-thermal physics ("Full-Physics") produces compact, self-regulated galaxies that match observations from $z = 12$ to $z = 3$. This model also produces broad surface brightness distributions, where the bright end is dominated by compact sizes and bursty star formation. This compact starburst scenario naturally explains the detection of bright $z > 10$ galaxies in flux-limited surveys. By contrast, a model with standard hydrodynamics yields systems that are smaller and more concentrated than current data, while a model with calibrated supernova feedback produces unphysically large systems nearly twice the size of those observed. At lower masses ($\mathrm{M}_{*} < 10^{8}\,\mathrm{M}_{\odot}$), the Full-Physics model predicts sizes that are smaller than can be resolved with JWST, consistent with extrapolations from observations of higher-mass systems. Future observations with higher resolution could resolve this population and elucidate the physics driving the formation of the first galaxies.

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arXiv2026

Symfind: Addressing the Fragility of Subhalo Finders and Revealing the Durability of Subhalos

A major question in $Λ$CDM is what this theory actually predicts for the properties of subhalo populations. Subhalos are difficult to simulate and to find within simulations, and this propagates into uncertainty in theoretical predictions for satellite galaxies. We present Symfind, a new particle-tracking-based subhalo finder, and demonstrate that it can track subhalos to orders-of-magnitude lower masses than commonly used halo-finding tools, with a focus on Rockstar and consistent-trees. These longer survival mean that at a fixed peak subhalo mass, we find $\approx 15\%{-}40\%$ more subhalos within the virial radius, $R_\textrm{vir}$, and $\approx 35\%-120\%$ more subhalos within $R_\textrm{vir}/4$ in the Symphony dark-matter-only simulation suite. More subhalos are found as resolution is increased. We perform extensive numerical testing. In agreement with idealized simulations, we show that the $v_{\rm max}$ of subhalos is only resolved at high resolutions ($n_\textrm{peak}\gtrsim3\times 10^4$), but that mass loss itself can be resolved at much more modest particle counts ($n_\textrm{peak}\gtrsim4\times 10^3$). We show that Rockstar converges to false solutions for the mass function, radial distribution, and disruption masses of subhalos. We argue that our new method can trace resolved subhalos until the point of typical galaxy disruption without invoking ``orphan'' modeling. We outline a concrete set of steps for determining whether other subhalo finders meet the same criteria. We publicly release Symfind catalogs and particle data for the Symphony simulation suite at \url{web.stanford.edu/group/gfc/gfcsims/}.

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arXiv2026

Exploring gas thermodynamics around galaxies from the Sunyaev-Zel'dovich effects: impact of galaxy-halo connection, 2D projection and velocity field

A complete picture of the gas thermodynamics around galaxies is imprinted on the cosmic microwave background (CMB). Indeed, the thermal, kinematic, and relativistic Sunyaev-Zel'dovich effects (tSZ, kSZ, rSZ) measure the gas density, temperature, pressure of baryonic feedback and bulk velocity around galaxies, along with the gravitational potential it sits in. This full thermodynamic picture promises to constrain galaxy formation models and gas related uncertainties in the impact on galaxy lensing. Recent kSZ measurements around galaxies suggest that the gas may be more extended than anticipated, pointing to powerful feedback processes and large baryonic corrections to lensing. How robust are these conclusions about the galaxy-halo connection, including satellite fraction and high-mass outliers, or to 2D projection effects and large-scale velocity modes? In this paper, we give estimates for these effects using a simulated sample of DESI-like luminous red galaxies within the IllustrisTNG hydrodynamical simulation and the Abacus N-body simulation. We show that analyzing projected 2D profiles can lead to biases when computing quantities like the gas fraction. We also find that in the absence of spatial filtering, the 2-halo term is non-negligible for kSZ even at the smaller radii where the 1-halo term dominates. We show that a 1% uncertainty in the satellite fraction of galaxies can propagate into uncertainties of $\pm$1%, $\pm$3% and $\pm$5% in the 1-halo terms of the kSZ, tSZ, and rSZ signals, respectively. We show that masking the 2% most massive objects in the sample reduces the profile amplitudes by up to 10%, 40%, and 75% for the kSZ, tSZ, and rSZ signals, respectively. Finally, we show that naive simulations of the kSZ effect can be biased by an artificial Doppler term, which is automatically removed when high-pass or compensated aperture filtering is applied.

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arXiv2026

Joint analysis of small-scale galaxy clustering and galaxy--galaxy lensing from BOSS galaxies

We present a joint analysis of galaxy clustering and galaxy--galaxy lensing measurements from BOSS galaxies using a simulation-based emulation method combined with a halo occupation distribution model. Our emulators are constructed with the Aemulus $ν$ simulations, a suite of $wν$CDM $N$-body simulations with massive neutrinos as independent particle species. We combine small-scale analysis of clustering from $0.1h^{-1}$Mpc to $60.2~h^{-1}$Mpc and lensing from $1.7h^{-1}$Mpc to $60.2~h^{-1}$Mpc to perform cosmological constraints. We split the BOSS galaxies into three redshift bins to measure their clustering and employ galaxies from Dark Energy Camera Legacy Survey and Hyper Suprime-Cam as source galaxies to measure lensing separately. We find that the addition of lensing significantly improves the constraining power on $S_{8}=σ_8(Ω_m/0.3)^{0.5}$, with a weak improvement for $fσ_{8}$. Our results of $fσ_{8}$ indicate tensions of around $1\sim4σ$ below the results of CMB observations of Planck. For $S_{8}$, our results are also lower than Planck, and the tension can be mitigated when considering possible systematics in lensing measurement. As a byproduct, our analysis prefers a non-zero neutrino mass but without strong significance, with the constraining power dominated by the clustering. Given the accuracy and precision of our model and the observational data, it is anticipated that larger and higher-quality spectroscopic datasets will improve the constraints on this fundamental property in the near future.

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arXiv2025

The Pandora project. II: how non-thermal physics drives bursty star formation and temperate mass-loaded outflows in dwarf galaxies

Dwarf galaxies provide powerful laboratories for studying galaxy formation physics. Their early assembly, shallow gravitational potentials, and bursty, clustered star formation histories make them especially sensitive to the processes that regulate baryons through multi-phase outflows. Using high-resolution, cosmological zoom-in simulations of a dwarf galaxy from \textit{the Pandora suite}, we explore the impact of stellar radiation, magnetic fields, and cosmic ray feedback on star formation, outflows, and metal retention. We find that our purely hydrodynamical model without non-thermal physics - in which supernova feedback is boosted to reproduce realistic stellar mass assembly - drives violent, overly enriched outflows that suppress the metal content of the host galaxy. Including radiation reduces the clustering of star formation and weakens feedback. However, the additional incorporation of cosmic rays produces fast, mass-loaded, multi-phase outflows consisting of both ionized and neutral gas components, in better agreement with observations. These outflows, which entrain a denser, more temperate ISM, exhibit broad metallicity distributions while preserving metals within the galaxy. Furthermore, the star formation history becomes more bursty, in agreement with recent JWST findings. These results highlight the essential role of non-thermal physics in galaxy evolution and the need to incorporate it in future galaxy formation models.

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arXiv2025

Quasars acting as Strong Lenses Found in DESI DR1

Quasars acting as strong gravitational lenses offer a rare opportunity to probe the redshift evolution of scaling relations between supermassive black holes and their host galaxies, particularly the $M_{\mathrm{BH}}$--$M_{\mathrm{host}}$ relation. Using these powerful probes, the mass of the host galaxy can be precisely inferred from the Einstein radius $θ_{\mathrm{E}}$. Using 812{,}118 quasars from DESI DR1 ($0.03 \leq z \leq 1.8$), we searched for quasars lensing higher-redshift galaxies by identifying background emission-line features in their spectra. To detect these rare systems, we trained a convolutional neural network (CNN) on mock lenses constructed from real DESI spectra of quasars and emission-line galaxies (ELGs), achieving a high classification performance (AUC = 0.99). We also trained a regression network to estimate the redshift of the background ELG. Applying this pipeline, we identified seven high-quality (Grade~A) lens candidates, each exhibiting a strong [O\,\textsc{ii}] doublet at a higher redshift than the foreground quasar; four candidates additionally show H$β$ and [O\,\textsc{iii}] emission. These results significantly expand the sample of quasar lens candidates beyond the twelve identified and three confirmed in previous work, and demonstrate the potential for scalable, data-driven discovery of quasars as strong lenses in upcoming spectroscopic surveys.

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arXiv2025

Shaping the Milky Way. II. The dark matter halo's response to the LMC's passage in a cosmological context

The distribution of dark matter in the Milky Way (MW) is expected to exhibit a large-scale dynamical response to the recent infall of the LMC. This event produces a dynamical friction wake and shifts the MW's halo density center. The structure of this response encodes information about the LMC- MW mass ratio, the LMC's orbit, the MW halo's pre-infall structure and could provide constraints on dark matter physics. To extract this information, a method to separate these effects and recover the initial shape of the MW's halo is required. Here, we use basis function expansions to analyze the halo response in eighteen simulations of MW-LMC-like interactions from the MWest cosmological, dark-matter-only zoom-in simulations. The results show that mergers similar to the LMC consistently generate a significant dipole and a secondary quadrupole response in the halo. The dipole arises from the host density center displacement and halo distortions, and its amplitude scales as the square of the MW-LMC mass ratio, peaking 0.2-0.7 Gyr after the LMC's pericenter. The quadrupole's strength depends primarily on the original axis ratios of the host halo, though contributions from the dynamical friction wake cause it to peak less than 0.3 Gyr before pericenter. Future measurements of both the dipole and quadrupole imprints of the LMC's passage in the density of the MW's stellar halo should be able to disentangle these effects and provide insight into the initial structure of the MW's halo, the MW's response, and the mass of the LMC.

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arXiv2025

Dwarf galaxy halo masses from spectroscopic and photometric lensing in DESI and DES

We present the most precise and lowest-mass weak lensing measurements of dwarf galaxies to date, enabled by spectroscopic lenses from the Dark Energy Spectroscopic Instrument (DESI) and photometric lenses from the Dark Energy Survey (DES) calibrated with DESI redshifts. Using DESI spectroscopy from the first data release, we construct clean samples of galaxies with median stellar masses $\log_{10}(M_*/M_{\odot})=8.3-10.1$ and measure their weak lensing signals with sources from DES, KiDS, and SDSS, achieving detections with $S/N$ up to 14 for dwarf galaxies ($\log_{10}(M_*/M_{\odot})<$9.25) -- opening up a new regime for lensing measurements of low-mass systems. Leveraging DES photometry calibrated with DESI, we extend to a photometric dwarf sample of over 700,000 galaxies, enabling robust lensing detections of dwarf galaxies with combined $S/N=38$ and a significant measurement down to $\log_{10}(M_*/M_{\odot})=8.0$. We show that the one-halo regime (scales $\lesssim 0.15h^{-1}\rm Mpc$) is insensitive to various systematic and sample selection effects, providing robust halo mass estimates, while the signal in the two-halo regime depends on galaxy color and environment. These results demonstrate that DESI already enables precise dwarf lensing measurements, and that calibrated photometric samples extend this capability. Together, they pave the way for novel constraints on dwarf galaxy formation and dark matter physics with upcoming surveys like the Vera C. Rubin Observatory's LSST.

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arXiv2025

How do uncertainties in galaxy formation physics impact field-level galaxy bias?

Our ability to extract cosmological information from galaxy surveys is limited by uncertainties in the galaxy-dark matter halo relationship for a given galaxy population, which are governed by the intricacies of galaxy formation. To quantify these uncertainties, we examine quenched and star-forming galaxies using two distinct approaches to modeling galaxy formation: UniverseMachine, an empirical semianalytic model, and the IllustrisTNG hydrodynamical simulation. We apply a second-order hybrid N-body perturbative bias expansion to each galaxy sample, enabling direct comparison of modeling approaches and revealing how uncertainties in the galaxy-halo connection affect bias parameters and non-Poisson noise across number densities and redshifts. Notably, we find that quenched and star-forming galaxies occupy distinct parts of the bias parameter space, and that the scatter induced from these different galaxy formation models is small when conditioned on similar selections of galaxies. We also detect the signature of assembly bias in our samples; this leads to small but significant deviations from analytic bias predictions, while assembly bias-removed samples match these predictions well. This work indicates that galaxy samples from a spectrum of reasonable, physically motivated models for galaxy formation give a relatively small range of field-level galaxy bias parameters. We estimate a set of priors from these models that should be useful in extracting cosmological constraints from luminous red galaxy- and emission line galaxy-like samples. Looking forward, careful estimates of the range of impacts of galaxy formation, for a given sample and cosmological analysis, will be an essential ingredient for extracting the most precise cosmological information from current and future large galaxy surveys.

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arXiv2025

SAGAbg III: Environmental Stellar Mass Functions, Self-Quenching, and the Stellar-to-Halo Mass Relation in the Dwarf Galaxy Regime

Recent efforts have extended our view of the number and properties of satellite galaxies beyond the Local Group firmly down to $\rm M_\star\sim 10^6 M_\odot$. A similarly complete view of the field dwarf population has lagged behind. Using the background galaxies sample from the Satellites Around Galactic Analogs (SAGA) Survey at $z<0.05$, we take inventory of the dwarf population down to $\rm M_\star \sim 5\times10^6 M_\odot$ using three metrics: the stellar mass function (SMF) as function of environment, the stellar-to-halo mass relation (SHMR) of dwarf galaxies inferred via abundance matching, and the quenched fraction of highly isolated dwarfs. We find that the low-mass SMF shape shows minimal environmental dependence, with the field dwarf SMF described by a low-mass power-law index of $α_1=-1.44\pm0.09$ down to $\rm M_\star \sim 5\times10^6 M_\odot$, and that the quenched fraction of isolated dwarfs drops monotonically to $f_{q} \sim 10^{-3}$ at $\rm M_\star \sim \rm 10^{8.5} M_\odot$. Though slightly steeper than estimates from \HI{} kinematic measures, our inferred SHMR agrees with literature measurements of satellite systems, consistent with minimal environmental dependence of the SHMR in the probed mass range. Finally, although most contemporary cosmological simulations against which we compare accurately predict the \sagalocal{} SHMR, we find that big-box cosmological simulations largely over-predict isolated galaxy quenched fractions via a turnaround in $f_q(\rm M_\star)$ at $\rm 10^8\lesssim M_\star/M_\odot\lesssim 10^9$, underscoring the complexities in disentangling the drivers of galaxy formation and the need for systematic multidimensional observations of the dwarf population across environments.

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arXiv2025

The SAGA Survey. VI. The Size-Mass Relation for Low-Mass Galaxies Across Environments

We investigate how Milky Way-like environments influence the sizes and structural properties of low-mass galaxies by comparing satellites of Milky Way analogs from the Satellites Around Galactic Analogs (SAGA) Survey with two control samples: an environmentally agnostic population from the SAGA background (SAGAbg) sample and isolated galaxies from the SDSS NASA-Sloan Atlas. All sizes and structural parameters are measured uniformly using pysersic to ensure consistency across samples. We find the half-light sizes of SAGA satellites are systematically larger than those of isolated galaxies, with the magnitude of the offset ranging from 0.05 to 0.12 dex (10-24%) depending on the comparison sample and completeness cuts. This corresponds to physical size differences between 85-200 pc at 10^7.5 solar masses and 220-960 pc at 10^10 solar masses. This offset persists among star-forming galaxies, suggesting that environment can influence the structure of low-mass galaxies even before it impacts quenching. The intrinsic scatter in the size-mass relation is lower for SAGA satellites than isolated galaxies, and the Sérsic index distributions of satellites and isolated galaxies are similar. In comparison to star-forming satellites, quenched SAGA satellites have a slightly shallower size-mass relation and rounder morphologies at low-mass, suggesting that quenching is accompanied by structural transformation and that the processes responsible differ between low- and high-mass satellites. Our results show that environmental processes can imprint measurable structural differences on satellites in Milky Way-mass halos.

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arXiv2025

A DECADE of dwarfs: first detection of weak lensing around spectroscopically confirmed low-mass galaxies

We present the first detection of weak gravitational lensing around spectroscopically confirmed dwarf galaxies, using the large overlap between DESI DR1 spectroscopic data and DECADE/DES weak lensing catalogs. A clean dwarf galaxy sample with well-defined redshift and stellar mass cuts enables excess surface mass density measurements in two stellar mass bins ($\log \rm{M}_*=[8.2, 9.2]~M_\odot$ and $\log \rm{M}_*=[9.2, 10.2]~M_\odot$), with signal-to-noise ratios of $5.6$ and $12.4$ respectively. This signal-to-noise drops to $4.5$ and $9.2$ respectively for measurements without applying individual inverse probability (IIP) weights, which mitigates fiber incompleteness from DESI's targeting. The measurements are robust against variations in stellar mass estimates, photometric shredding, and lensing calibration systematics. Using a simulation-based modeling framework with stellar mass function priors, we constrain the stellar mass-halo mass relation and find a satellite fraction of $\simeq 0.3$, which is higher than previous photometric studies but $1.5σ$ lower than $Λ$CDM predictions. We find that IIP weights have a significant impact on lensing measurements and can change the inferred $f_{\rm{sat}}$ by a factor of two, highlighting the need for accurate fiber incompleteness corrections for dwarf galaxy samples. Our results open a new observational window into the galaxy-halo connection at low masses, showing that future massively multiplexed spectroscopic observations and weak lensing data will enable stringent tests of galaxy formation models and $Λ$CDM predictions.

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arXiv2025

Dwarf Galaxies at Cosmic Noon: New JWST Constraints on Satellite Models and Subhalo Tidal Evolution

The advent of JWST has revolutionized the study of faint satellite galaxies at $z \gtrsim 1$, enabling statistical constraints on galaxy evolution and the galaxy$-$halo connection in a previously unexplored mass and redshift regime. We compare satellite abundances at $1 < z < 3.5$ from recent JWST observations with predictions from cosmological dark matter-only zoom-in simulations. We identify and quantify several sources of biases that can impact theoretical satellite counts, finding that assumptions about subhalo tidal evolution introduce the largest uncertainty in predictions for the satellite mass function. Using a flexible galaxy disruption model, we explore a range of disruption scenarios, spanning hydrodynamically motivated and idealized prescriptions, to bracket plausible physical outcomes. We show that varying galaxy durability can change the predicted satellite mass functions by a factor of $\sim3.5$. The JWST data and our fiducial model are consistent within $1-2σ$ across the full redshift ($1 < z < 3.5$) and stellar mass ($M_\star> 10^7~\mathrm{M}_\odot$) range probed. We find evidence that subhalos are at least as long-lived as predicted by hydrodynamic simulations. Our framework will enable robust constraints on the tidal evolution of subhalos with future observations. This work presents the first direct comparison between cosmological models and observations of the high-redshift satellite population in this low-mass regime. These results showcase JWST's emerging power to test structure formation in the first half of the Universe in a new domain and to constrain the physical processes driving the evolution of low-mass galaxies across cosmic time.

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arXiv2025

The Roman View of Strong Gravitational Lenses

Galaxy-galaxy strong gravitational lenses can constrain dark matter models and the Lambda Cold Dark Matter cosmological paradigm at sub-galactic scales. Currently, there is a dearth of images of these rare systems with high signal-to-noise and angular resolution. The Nancy Grace Roman Space Telescope (hereafter, Roman), scheduled for launch in late 2026, will play a transformative role in strong lensing science with its planned wide-field surveys. With its remarkable 0.281 square degree field of view and diffraction-limited angular resolution of ~0.1 arcsec, Roman is uniquely suited to characterizing dark matter substructure from a robust population of strong lenses. We present a yield simulation of detectable strong lenses in Roman's planned High Latitude Wide Area Survey (HLWAS). We simulate a population of galaxy-galaxy strong lenses across cosmic time with Cold Dark Matter subhalo populations, select those detectable in the HLWAS, and generate simulated images accounting for realistic Wide Field Instrument detector effects. For a fiducial case of single 146-second exposures, we predict around 160,000 detectable strong lenses in the HLWAS, of which about 500 will have sufficient signal-to-noise to be amenable to detailed substructure characterization. We investigate the effect of the variation of the point-spread function across Roman's field of view on detecting individual subhalos and the suppression of the subhalo mass function at low masses. Our simulation products are available to support strong lens science with Roman, such as training neural networks and validating dark matter substructure analysis pipelines.

astro-ph.CO↗

arXiv2025

The SAGA Survey. V. Modeling Satellite Systems around Milky Way-mass Galaxies with Updated UniverseMachine

Environment plays a critical role in shaping the assembly of low-mass galaxies. Here, we use the UniverseMachine (UM) galaxy-halo connection framework and the Data Release 3 of the Satellites Around Galactic Analogs (SAGA) Survey to place dwarf galaxy star formation and quenching into a cosmological context. UM is a data-driven forward model that flexibly parameterizes galaxy star formation rates (SFR) using only halo mass and assembly history. We add a new quenching model to UM, tailored for galaxies with stellar masses $\lesssim 10^9$ solar masses, and constrain the model down to a stellar mass $\gtrsim 10^7$ solar masses using new SAGA observations of 101 satellite systems around Milky Way (MW)-mass hosts and a sample of isolated field galaxies in a similar mass range from the Sloan Digital Sky Survey (SDSS). The new best-fit model, 'UM-SAGA,' reproduces the satellite stellar mass functions, average SFRs, and quenched fractions in SAGA satellites while keeping isolated dwarfs mostly star forming. The enhanced quenching in satellites relative to isolated field galaxies leads the model to maximally rely on halo assembly to explain the observed environmental quenching. Extrapolating the model down to a stellar mass $\sim 10^{6.5}$ solar masses yields a quenched fraction of $\gtrsim$ 30% for isolated field galaxies and $\gtrsim$ 80% for satellites of MW-mass hosts at this stellar mass. This specific prediction can soon be tested by spectroscopic surveys to reveal the relative importance of internal feedback, cessation of mass and gas accretion, satellite-specific gas processes, and reionization for the evolution of faint low-mass galaxies.

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