arXiv subjects

Duncan A. Forbes

Publications and source records attributed to Duncan A. Forbes.

At least 19 recordsLinked to original sources

arXiv2026

The Stellar Populations of Two Quiescent Low Surface Brightness Dwarf Galaxies in Low Density Environments

Dwarf galaxies located in low density environments that have quiescent stellar populations are extremely rare and the mechanism for quenching their stars is highly uncertain. Here we present two low surface brightness dwarf galaxies with large (>1.2 kpc) effective radii that were previously identified in the Dragonfly Ultrawide Survey for which we have obtained spectra using the KCWI instrument on the Keck II telescope. We derive their stellar populations and star formation histories in several radial bins out to 1.25 effective radii. Globally both galaxies are old (~9 Gyr) and metal-poor ([M/H] ~ -1) and reveal quenching on a relatively rapid timescale (~1 Gyr). We find flat to rising age and metallicity gradients in both galaxies. This is similar to those seen in ultra diffuse galaxies but is in contrast with typical classical dwarf galaxies and predictions from simulations. One galaxy, DFUWS 68, hosts several globular clusters which we find to share similar ages, metallicities and quenching timescales to that of the host galaxy stars.We briefly discuss possible quenching mechanisms and suggest that neither cosmic web stripping nor internal feedback processes alone can explain our results.

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arXiv2026

Phase-Space Diagnostics for Dwarf Galaxies in Cluster Environments

Ongoing effort is devoted to observing spectroscopic samples of dwarf galaxies in clusters, allowing the analysis of their distribution and associated trends in projected phase-space (PPS), i.e. line-of-sight velocity vs. projected clustercentric distance. By utilizing the resolved baryonic halos inside the galaxy clusters of a cosmological simulation from the Magneticum suite, we complement on prior studies with dedicated focus on the dwarf galaxy population ($M_\ast<10^9\,M_\odot$) and correlations between infall time and location in PPS. The inferred trend recovers the radial correlation reported by prior works, but we find a significant fraction ($\geq30\%$) of recently accreted galaxies at locations that were previously predicted to be dominated by ancient infallers. Splitting the diagram with an infall time threshold of 3 Gyr, we develop a detailed infall time template in PPS. We provide our data to allow observers to statistically infer the time of infall of their sample when placing them on the PPS. Additionally, we review the trajectories in PPS of different orbits and their dependence on the observer's orientation. Compared to massive galaxies, we find a much broader radial distribution for dwarfs in 3D PS. Utilizing a set of high-resolution idealized simulations, we predict strongly altered orbits for dark matter-deficient galaxies.

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arXiv2026

Diffuse Dwarf Galaxies in Galaxy Clusters: I. Stellar Populations and Radial Gradients

We use Keck/KCWI spectroscopy to study one ultra-diffuse galaxy (UDG) and five Nearly-UDGs (NUDGEs) in the Perseus cluster, together with an additional UDG in the Coma cluster. As the first paper in a series, we focus on the global and radial stellar population properties of our sample. We find that these galaxies host intermediate-to old stellar populations, with typical ages of ~7 Gyr, low metallicities ([M/H]$\simeq$ -0.9 dex), and enhanced [Mg/Fe] abundances (~0.3 dex), consistent with previous studies. Six galaxies lie within the scatter of the present-day mass-metallicity relation (MZR), whereas the Coma UDG (DF11) is more consistent with the MZR of high-z galaxies (z ~ 2). We find no strong correlation between global stellar population properties and cluster infall parameters, suggesting that any environmental impact is not easily traceable through integrated stellar populations. We go one step further and measure radial gradients for three galaxies. Two show flat age and mildly negative metallicity gradients, similar to classical dwarfs, while one shows a rising metallicity profile as recently found in other UDGs. Comparing with classical dwarfs, we find a continuous correlation between metallicity gradient and globular cluster (GC) richness, where more GC-rich systems tend to show rising profiles. We propose that preferential tidal disruption of GCs in the inner regions of galaxies naturally produces rising metallicity profiles, unlike GC-poor classical dwarfs. This mechanism, potentially coupled with strong stellar feedback from early concentrated star formation, may explain the unusual rising metallicity profiles observed in GC-rich UDGs/NUDGEs.

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arXiv2026

Formation of globular cluster-rich ultra-diffuse galaxies through mergers

We use high-resolution, idealized hydrodynamic simulations of gas-rich dwarf-galaxy mergers to test whether such encounters can form ultra-diffuse galaxies (UDGs) with globular cluster (GC) systems. We simulate 1:1 and 1:2 mergers alongside an isolated control model and identify stellar overdensities as GC candidates (GCCs). The remnants evolve into dispersion-supported, UDG-like systems with three-dimensional stellar half-mass radii $r^{3D} \sim 1.9-2.6$ kpc, while the isolated dwarf remains rotationally supported and forms no GCCs. Tidal heating and stellar feedback expel a large fraction of the gas beyond the dark matter (DM) halo, leaving stellar-dominated remnants whose DM haloes remain cuspy. Merger-driven star formation is highly clustered: the fraction of newly formed stellar mass bound in massive clusters exceeds 0.5 after the first pericentric passage and remains elevated thereafter. By the final snapshot, the remnants host GC populations numbering 20 (1:1) and 39 (1:2), more centrally concentrated than the field stars and consistent with the observed GC number-halo mass relation. The GCCs match observed star clusters in the planes of mass versus size, velocity dispersion, and density. More massive clusters exhibit stronger internal rotation and broader metallicity spreads. In one case, the merger produces a nucleated UDG via cluster inspiral followed by sustained in-situ star formation. These results demonstrate that gas-rich dwarf mergers are a viable pathway to GC-rich (and sometimes nucleated) UDGs, and predict correlated cluster mass, rotation, and metallicity-dispersion trends testable with observations.

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arXiv2026

Looking into the faintEst WIth MUSE (LEWIS): Exploring the nature of ultra-diffuse galaxies in the Hydra-I cluster. VI. A star-forming UDG in Hydra I: a rare UDG or a transition phase?

This paper presents a detailed analysis of a gas-rich star-forming ultra-diffuse galaxy (UDG) as part of the ESO Large Programme 'Looking into the faintEst WIth MUSE (LEWIS)'. Among the UDGs in the LEWIS sample, UDG 6 is the only galaxy that hosts a significant amount of ionised gas with evidence of emission lines, suggesting recent star-forming activity. The main goal of this work is to constrain the formation history of this UDG by comparing its properties with the main formation scenarios proposed for this extreme class of galaxies. We adopted integral field spectroscopy from MUSE to derive the morphology and the structural properties of the stellar and gas components of UDG 6. We applied spectral fitting and Voronoi tessellation algorithms to the MUSE data-cube to derive the kinematics and properties of the gas and stellar component. Moreover, we derived the GCs populations' properties by applying a multi-band spectrophotometric analysis. We confirmed that UDG 6 is a member of Hydra I cluster. It is characterised by a regular and elongated shape and contains a significant dust content, a metal-poor ionised gas fraction and an underlying old-to-intermediate stellar component. Evidence of local and clumpy star-forming activity has been revealed through the analysis of emission line, and an arc-like tidal feature was discovered from unsharp masking analysis. UDG 6 might originate from a 'puffed-up dwarf' whose stellar content has been stretched out to larger radii, passively evolving into a more diffuse galaxy. Being located in a dynamically active region of the cluster, characterised by tidal features and stripping phenomena, we suggest that the environmental processes have played a role in shaping the properties of UDG 6. A tidal interaction with a nearby galaxy might have triggered recent star-formation activity, without dramatically altering the coherent gas rotation in UDG 6.

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arXiv2026

Dark matter-deficient twins: FCC 224 and FCC 240 as possible analogues of NGC 1052-DF2 and DF4

The recent "bullet-dwarf" model proposes that high-velocity collisions between dwarf galaxies can produce stellar systems with overluminous globular clusters (GCs) and a deficiency of dark matter, as observed in the NGC 1052 group galaxies NGC 1052-DF2 and NGC 1052-DF4. We present a possible analogue system in the outskirts of the Fornax cluster: the ultra-diffuse galaxy FCC 224 and its close companion FCC 240. Using deep VLT/MUSE integral-field spectroscopy, we characterize their stellar populations, internal kinematics, and GC systems to test this formation scenario. Both galaxies exhibit low velocity dispersions. Interpreted with a standard mass estimator at the half light radius, and allowing for the known limitations associated with flattened systems, their inner dynamics are more naturally explained by stars alone than by either cuspy or cored dark matter halos. Both systems host unusually luminous GCs, closely resembling the top-heavy GCLF of the NGC 1052 pair. Moreover, FCC 224 and FCC 240 are coeval with each other, with mass-weighted stellar ages of ~10 Gyr, and their GC populations share similarly old ages, in agreement with predictions of the formation scenario. Despite these similarities, FCC 224 and FCC 240 form a much tighter system than DF2 and DF4, with a projected separation of 75 kpc (compared to 240 kpc) and a relative velocity of only 16 km/s (compared to 358 km/s). This distinct configuration may suggest a different present-day manifestation of the same general class of galaxies and provides additional observational constraints on models of their formation and evolution.

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arXiv2026

Merge and Strip II: Imprint of galaxy formation physics and viscosity on baryon-dominated dwarf galaxies

Motivated by the discovery of peculiar dwarf galaxies inside galaxy clusters such as blue candidates (BCs), dark galaxies and ultra-diffuse galaxies (UDGs), we present hydrodynamic simulations of galaxy mergers in cluster environments. We vary the viscosity and stellar feedback prescriptions, realistically modelling possible conditions for hydrodynamic drag and fluid instabilities, as well as internal destabilization through stellar feedback-driven heating and gas loss. We find that long-lived tidal dwarf galaxies (TDGs) can form throughout all viscosity values applicable to galaxy clusters if stellar feedback is moderate. Our results expand on studies of cloud crushing simulations, investigating the entrainment problem in intracluster medium ambience. The smallest clouds have gas masses on the order of $M_\text{gas} \sim 10^7 \text{ M}_\odot$ and reach relatively low final drift velocities of $\sim 100 \text{ km/s}$. The lowest possible Reynolds number acting on this class of clouds is $Re \sim 1$ for full Spitzer viscosity. Almost all TDGs display elevated star formation rates of $0.01-0.1 \text{ M}_\odot / \text{yr}$, which are stable across several Gyr. Based on their matching properties, we support that BCs observed in the Virgo cluster are likely stripped TDGs. Similar features are also found in comparison with dark galaxies and baryon-dominated UDGs, implying that a subsample of these objects are also long-lived TDGs. This work provides robust evidence that stripping from galaxy mergers is a viable channel for the formation of stable cold gas clouds and dark matter-deficient galaxies observed in galaxy clusters.

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arXiv2026

Revisiting the distance and the globular cluster system of the remarkable galaxy UDG1 in the NGC 5846 group

Two studies that utilised the same HST/WFC3 imaging of NGC5846_UDG1 have reported quite different total counts for its globular cluster (GC) system, i.e. 54 $\pm$ 9 vs 33 $\pm$ 3 GCs. In both cases they counted all GCs, that met their selection criteria, down to the faintest magnitudes. They also disagree as to whether NGC5846_UDG1 lies in the NGC 5846 group or well outside the group, in the field. As an ultra diffuse galaxy with one of the richest GC systems known, and therefore implications for its halo mass, it is important to understand which of these is closer to the truth. Here we present a new SBF-based distance to NGC5846_UDG1 from HST/ACS imaging of 26.5 $\pm$ 2.7 Mpc, which places it squarely within the NGC 5846 group. Using this distance we adopt the standard approach of only counting GCs brighter than the turnover magnitude. This has the advantage of considering only the brighter GCs which are resolved in HST imaging and largely confirmed by spectroscopy, while also avoiding the fainter candidates for which contamination is potentially an issue. With this robust approach we find that the two studies are entirely consistent with each other. Both imply a total GC system of around 50 GCs and by inference a massive galaxy halo of greater than 10$^{11}$ M$_{\odot}$. We also revisit the two previous photometric studies focusing on half a dozen intermediate magnitude objects that are selected by one study but excluded by the other. These objects have GC-like magnitudes, sizes and are nearly round with GC-like appearances. They are very unlikely to be background galaxies or interloper GCs and thus bona fide GCs associated with NGC5846_UDG1.

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arXiv2026

FAST and Dark: A catalogue of Dark Galaxy Candidates within 50 Mpc

Using the first data release of the Five-hundred-meter Aperture Spherical radio Telescope (FAST) All-Sky HI survey (FASHI), we compile a catalogue of 70 dark galaxy candidates (DGCs) within 50 Mpc. We select DGCs without an identified optical counterpart at a limiting g-band magnitude of ~ 28 mag arcsec^-2 in the DESI Legacy Survey, using both automatic cross-checking with optical catalogues and visual inspection of the colour images. After validating our DGCs, excluding potential spurious detections, issues in the registered position of the HI sources, and possible Radio Frequency Interferences (RFIs), we analyse their distribution over the surveyed sky, HI mass, linewidths, and inferred distance. They appear evenly distributed across the surveyed area, with no apparent bias to isolation. We did not find any DGC within the Local Volume (11 Mpc) in the sky surveyed by this first release of FASHI. We compare the observed properties of DGCs with those of galaxies with optical counterparts, finding that DGCs tend to have higher linewidths for a given HI mass. We discuss our DGCs in light of theoretical works, and compare them with other observational samples from previous HI surveys. This work presents a catalogue of dark galaxy candidates, which can serve as a basis for follow-up studies.

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arXiv2026

The Dawes Review 14: A Decade of Ultra-Diffuse Galaxies

It has been 10 years since the initial discovery of Ultra-Diffuse Galaxies (UDGs) in the Coma cluster and the revelation that large, low surface brightness galaxies may constitute a greater fraction of galaxies than first thought. This left an open question: Are UDGs something special, or just an extension of the previously known dwarf galaxy population? Seeking to answer this question, in the decade following, dedicated simulations have studied and proposed a myriad of formation pathways to create UDGs. Observations have then pushed the limits of world-class observatories to perform detailed studies of these galaxies in large numbers across the full range of environments in the local Universe. These observations stress test simulations and challenge previous galaxy formation wisdom, with UDGs posing many open puzzles beyond just their unknown formation mechanism. To provide a few pertinent examples: there is observational evidence that not all UDGs follow the standard stellar mass -- halo mass relationship; there is evidence for UDGs with extraordinarily high levels of alpha enhancement; and there is evidence that some UDGs are much more globular cluster rich than other dwarfs of similar stellar mass. In this Dawes review, we undertake the task of summarising the decade of science since the discovery of UDGs. We focus on the quiescent population of UDGs and review their general properties, their proposed formation scenarios, their internal properties and their globular cluster systems. We also provide a brief conjecture on some future directions for the next decade of UDG research.

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arXiv2026

MATLAS-42, A Globular Cluster-Rich Ultra-Diffuse Galaxy That Diverges from the "Failed Galaxy'' Formation Pathway

To date, there has been significant interest in globular cluster (GC)-rich ultra-diffuse galaxies (UDGs) and the evidence that they have formed via an unexpected, ``failed galaxy'' formation pathway. The majority of the evidence for ``failed galaxy'' UDGs originates from spectroscopic observations targeting passive GC-rich UDGs, with a focus on those residing in galaxy clusters. In this work, we study the gas-rich, GC-rich group UDG MATLAS-42 and derive its stellar population properties using the Keck Cosmic Web Imager. We measure a redshift for the galaxy ($V_{\rm R, \star}=2433\pm8$~km s$^{-1}$), confirming the previous assumptions that it is both part of the NGC~502 group and has an associated HI-reservoir ($V_{\rm R,HI}=2423\pm 15$~km s$^{-1}$). We measure integrated stellar populations and find the galaxy to be both young (mass-weighted age $=3.2^{+2.6}_{-1.5}$Gyr) and of average-to-low metallicity ($[M/H]=-1.19^{+0.42}_{-0.30}$ dex). When considering these properties in the context of the galaxy's formation, we note it likely does not follow the ``failed galaxy'' formation pathway commonly attributed to GC-rich, cluster UDGs, as it has experienced recent star formation. At most it started failed, however, it has recently rejuvenated its star formation. Finally, we build a toy model of the passive evolution of this galaxy, finding that its relative GC-richness (i.e., $M_{\rm GC}/M_\star$) will likely decrease with time as GCs slowly evaporate/disrupt to contribute to the stellar mass of the galaxy. Due to this, we hypothesise that it is likely not a low redshift analogue of the progenitor to a ``failed galaxy'' UDGs.

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arXiv2025

Looking into the faintEst WIth MUSE (LEWIS): on the nature of ultra-diffuse galaxies in the Hydra I cluster. V. Integrated stellar population properties

Context: This paper presents new results from the ESO Large Programme Looking into the faintEst WIth MUSE (LEWIS). The sample consists of low-surface brightness (LSB) and ultra-diffuse galaxies (UDGs) located inside 0.4 R$_{vir}$ of the Hydra I cluster. Integral field spectroscopy is acquired for 24 galaxies with the MUSE spectrograph mounted on the Very Large Telescope (VLT). Aims: Our main objective is to analyse possible correlations between the environment and the integrated stellar population properties. Methods: For each galaxy, we extract the 1D stacked spectrum in an aperture of one effective radius and adopt previously published stellar kinematics to derive age, metallicity and [Mg/Fe] through a full spectral fitting technique. Results: We find that the analysed LEWIS sample has a mean metallicity of [M/H] = -0.9 dex, a mean age of 10 Gyr, comparable to results of UDGs in other clusters. According to the projected phase-space, galaxies can be classified into two groups: early infallers, with slightly higher metallicities ([M/H]$_{early} = -0.8 \pm 0.1$ dex), and late infallers, with slightly lower values ([M/H]$_{late} = -1.0 \pm 0.1$ dex). Two types of galaxies are found in the early-infall region: half of them have metallicities consistent with the dwarf mass-metallicity relation, while the other half show higher values. Moreover, they display different timescales for stellar mass assembly. Conclusions: We identified different classes of UDGs in the Hydra I cluster, which suggest different formation mechanisms taking place. Almost all of the UDGs and LSBs in the cluster are consistent with the puffed-up dwarf formation scenario, having dwarf-like metallicities and being consistent with the dwarf mass-metallicity relation. In the innermost regions of the cluster, where metal-richer UDGs lie, tidal or environmental effects might have influenced their formation and evolution.

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arXiv2025

Connection Between Dwarf Galaxies and Globular Clusters: Insights from the Perseus Cluster Using Subaru Imaging and Keck Spectroscopy

We present a systematic study of 189 dwarf galaxies and their globular cluster (GC) systems in the Perseus cluster, based on deep Subaru Hyper Suprime-Cam imaging and Keck spectroscopy, supplemented by literature data. This constitutes the largest sample of dwarfs in a single galaxy cluster to date with simultaneous deep imaging, spectroscopic coverage, and GC measurements, while uniquely spanning a broad and continuous range of galaxy properties. We find an anti-correlation between GC specific mass and galaxy stellar mass for dwarfs in Perseus similar to observations in other clusters. At fixed stellar mass, dwarfs with lower surface brightness or larger effective radius tend to be more GC-rich -- suggesting either high GC formation efficiency in an earlier compact-galaxy phase, or less efficient GC disruption. The correlation between GC richness and axis ratio in Perseus is weaker than in other environments. We find some connection between GC richness and infall time, but not with the clear correlations found in Virgo, Coma, and cosmological simulations. More complete observations are needed to test for cluster-to-cluster variations in galaxy and GC evolutionary histories. This work demonstrates the potential of new wide-field imaging and spectroscopy surveys for understanding GCs and dwarf galaxies, and highlights the need for further work in theoretical modeling.

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arXiv2025

Ultra-Diffuse, Ultra-Different: Observed vs. Simulated Ultra-Diffuse Galaxies Live in Fundamentally Different Halos

In this work, we compare galaxies from the NIHAO and HESTIA simulation suites to ultra-diffuse galaxies (UDGs) with spectroscopically measured dynamical masses. For each observed UDG, we identify the simulated dark matter halo that best matches its dynamical mass. In general, observed UDGs are matched to simulated galaxies with lower stellar masses than they are observed to have. These simulated galaxies also have halo masses much less than would be expected given the observed UDG's stellar mass and the stellar mass -- halo mass relationship. We use the recently established relation between globular cluster (GC) number and halo mass, which has been shown to be applicable to UDGs, to better constrain their observed halo masses. This method indicates that observed UDGs reside in relatively massive dark matter halos. This creates a striking discrepancy: the simulated UDGs are matched to the dynamical masses of observed ones, but not their total halo masses. In other words, simulations can produce UDGs in halos with the correct inner dynamics, but not with the massive halos implied by GC counts. We explore several possible explanations for this tension, from both the observational and theoretical sides. We propose that the most likely resolution is that observed UDGs may have fundamentally different dark matter halo profiles than those produced in NIHAO and HESTIA. This highlights the need for a simulation that self-consistently produces galaxies of a stellar mass of $\sim 10^8 M_\odot$ in dark matter halos that exhibit the full range of large dark matter cores to cuspy NFW-like halos.

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arXiv2025

Analysis of Galaxies at the Extremes: Failed Galaxy Progenitors in the MAGNETICUM Simulations

There is increasing observational evidence for a failed galaxy formation pathway for some ultradiffuse galaxies (UDGs) at low redshift however they currently lack simulated counterparts. We attempt to identify dark matter halos at high redshift within the MAGNETICUM cosmological simulations that could plausibly be their progenitors. We build a toy model of passive galaxy evolution within the stellar mass-halo mass relation to trace z = 0 observations of UDGs back to their z = 2 locations. We identify a population of 443 galaxies that match these parameter space positions within the simulation. We build two comparison samples within the simulation that follow the stellar mass-halo mass relationship at z = 2, one of which is stellar mass matched (with varying smaller halo masses) and the other is halo mass matched (with varying larger stellar masses) to our sample. We identify that our failed galaxy progenitor candidates have 1) flatter, cored dark matter halos; 2) more extended stellar bodies; 3) a larger fraction of their gas in the outskirts of their halos; 4) lower metallicities and 5) higher star formation rates than the control samples. Findings 1) and 2) are similar to low redshift observations of UDGs. Finding 3) will aid the removal of gas and permanent quenching of star formation which is a requirement of the failed galaxy formation scenario. The low metallicities of finding 4) match those observed in low redshift failed galaxy UDGs. Comparing the high star formation rates of finding 5) to recent JWST observations suggests that a starburst would naturally explain the high globular cluster richness of the UDGs. Many of the properties we find for these failed galaxy progenitors can be explained by an assembly bias of their dark matter halo to later formation times. We conclude by proposing that the fraction of failed galaxy UDGs is expected to increase with environmental density.

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arXiv2025

Looking into the faintEst WIth MUSE (LEWIS): Exploring the nature of ultra-diffuse galaxies in the Hydra-I cluster IV. A study of the Globular Cluster population in four UDGs

As old stellar systems, globular clusters (GCs) are key fossil tracers of galaxy formation and interaction histories. This paper is part of the LEWIS project, an integral-field spectroscopic survey of ultra-diffuse galaxies (UDGs) in the Hydra I cluster. We use MUSE spectroscopy and new VIRCAM $H$-band imaging data to study the GC populations and dark matter content in four dwarf galaxies. We retrieved line-of-sight velocities for all sources in the observed MUSE fields. Since the spectroscopic measurements are limited to relatively bright sources, we developed a multi-band photometric procedure to identify additional GC candidates too faint for spectroscopic confirmation. GC candidates were selected using a combination of photometric properties and morphometric criteria. Additionally, the $H$-band observations were used to constrain the stellar masses of the studied galaxies. Based on the spectroscopic classification, we confirm one GC in UDG3, two in UDG7, and four in UDG11, while UDG9 has no spectroscopically confirmed bright GCs. We identify four intra-cluster GCs in the vicinity of UDG3 and UDG11, and one ultra-compact dwarf with a radial velocity only $Δv = -85 \pm 10\mathrm{km\ s^{-1}}$ relative to UDG7, suggesting it may be bound to it. Considering completeness corrections and accounting for possible contamination, from photometry we estimate that the number of GCs ranges between 0 and $\sim40$ for the investigated UDGs. Their specific frequencies suggest that three out of four UDGs are either GC-rich, similar to those in the Coma cluster, or belong to an intermediate population as seen in the Perseus cluster. Dark matter content estimates, inferred from GC counts and stellar mass, indicate that these galaxies are dark-matter dominated, with dynamical-to-stellar mass ratios of $M_{\mathrm{dyn}} / M_\star \sim 10-1000$.

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arXiv2025

Do Ultra-Diffuse Galaxies Follow the Globular Cluster-Halo Mass Relation?

The stellar mass-halo mass relation and the globular cluster (GC) number-halo mass relation are two scaling relations that relate fundamental properties of normal galaxies. Ultra-Diffuse Galaxies (UDGs), some of which, have rich GC systems and relatively low stellar masses can not follow the mean trend of both relations simultaneously; it is thus important to understand which relationship is followed by UDGs. Using independent halo masses determined from kinematic fitting to large radii, we identify three UDGs and two UDG-like galaxies from the literature and examine which scaling relation they follow. We find that the galaxies follow the GC number-halo mass relation but deviate in a systematic way from the stellar mass-halo mass relation, which depends on their GC count. This scatter off the relation is towards higher halo masses, or equivalently lower stellar masses. The galaxies exhibiting the largest offsets may represent `failed galaxies' that have experienced quenched star formation with later assembly.

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arXiv2025

A Comprehensive Look at PUDG-R21: Stellar Population and Kinematics of a Globular Cluster-Rich Ultra-Diffuse Galaxy in the Perseus Cluster

We present the analysis of the stellar populations and kinematics of the globular cluster (GC) rich ultra-diffuse galaxy, PUDG-R21, using spectroscopic observations obtained with the Keck Cosmic Web Imager (KCWI). The recessional velocity is measured to be 5536$\pm$10 km s$^{\mathrm{-1}}$, confirming its association with the Perseus cluster. The galaxy exhibits mild rotation of 15.6$\pm$10 km s$^{\mathrm{-1}}$ and a stellar velocity dispersion of 19.4$\pm$3.5 km s$^{\mathrm{-1}}$ within the galaxy effective radius. From this, we infer a dynamical mass of M$_{\mathrm{dyn}}=9.3\pm3.3\times10^{8}$ M$_{\odot}$. Based on a halo mass derived from PUDG-R21 GC counts, we find our dynamical mass is consistent with a cored dark matter profile. The integrated stellar population analysis reveals a predominantly old stellar population of 10.4$\pm$1.2 Gyr, with intermediate-low metallicity ([M/H]=-0.64$\pm$0.12 dex) and elevated alpha abundances ([Mg/Fe]=0.38$\pm$0.25 dex). The inferred star formation history suggests rapid stellar assembly, likely truncating prior to or during the galaxy's infall into the cluster at an early epoch ($\sim$10 Gyr ago). The analysis of stellar population gradients (age and metallicity) indicates a flat profile out to one effective radius. Here, we consider the involvement of two star formation events, initially forming a large population of metal-poor globular clusters, and then the latter contributing to the more metal-enriched diffuse stellar body. The evidence of subsequent star formation suggests this galaxy is more like an extension of the classical dwarf population than the much discussed failed galaxy UDGs.

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