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Abstract We use Hubble Space Telescope imaging to study the globular cluster system of the Virgo Cluster ultradiffuse galaxy (UDG) VCC 615. We select globular cluster candidates through a combination of size and color, while simultaneously rejecting contamination from background galaxies that would be unresolved in ground-based imaging. Our sample of globular cluster candidates is essentially complete down to a limiting magnitude of F814W = 24.0, ≈90% down the globular cluster luminosity function (GCLF). We estimate a total globular cluster population for VCC 615 of $N_{\mathrm{GC}}=25.1_{-5.4}^{+6.5}$, resulting in a specific frequency of $S_N=55.5_{-12.0}^{+14.5}$, quite high compared to normal galaxies of similar luminosity, but consistent with the large specific frequencies found in some other UDGs. The abundant cluster population suggests the galaxy is enshrouded by a massive dark halo, consistent with previous dynamical mass estimates using globular cluster kinematics. While the peak of the GCLF appears slightly brighter than expected (by ≈0.3–0.5 mag), this difference is comparable to the 0.3 mag uncertainty in the measurement, and we see no sign of an extremely luminous population of clusters similar to those detected in the UDGs NGC1054-DF2 and -DF4. However, we do find a relatively high fraction ( $32_{-4}^{+5}$%) of large clusters with half-light radiiR_h > 9 pc. The galaxy's offset nucleus appears photometrically distinct from the globular clusters, and is more akin to ultracompact dwarfs (UCDs) in Virgo. Over time, VCC615’s already diffuse stellar body may be further stripped by cluster tides, leaving the nucleus intact to form a new Virgo UCD. 

Abstract We report the results of the deepest search to date for dwarf galaxies around NGC 3109, a barred spiral galaxy with a mass similar to that of the Small Magellanic Cloud (SMC), using a semiautomated search method. Using the Dark Energy Camera, we survey a region covering a projected distance of ∼70 kpc of NGC 3109 (D= 1.3 Mpc,R_vir∼ 90 kpc,M∼ 10⁸M_*) as part of the MADCASH and DELVE-DEEP programs. We introduce a newly developed semiresolved search method, used alongside a resolved search, to identify crowded dwarf galaxies around NGC 3109. Using both approaches, we successfully recover the known satellites Antlia and Antlia B. We identified a promising candidate, which was later confirmed to be a background dwarf through deep follow-up observations. Our detection limits are well defined, with the sample ∼80% complete down toM_V∼ −8.0, and include detections of dwarf galaxies as faint asM_V∼ −6.0. This is the first comprehensive study of a satellite system through resolved stars around an SMC mass host. Our results show that NGC 3109 has more bright (M_V∼ −9.0) satellites than the mean predictions from cold dark matter models, but well within the host-to-host scatter. A larger sample of LMC/SMC-mass hosts is needed to test whether or not the observations are consistent with current model expectations. 

Abstract We present Keck/DEIMOS spectroscopy of the first complete sample of ultradiffuse galaxies (UDGs) in the Virgo cluster. We select all UDGs in Virgo that contain at least 10 globular cluster (GC) candidates and are more than 2.5 σ outliers in scaling relations of size, surface brightness, and luminosity (a total of 10 UDGs). We use the radial velocity of their GC satellites to measure the velocity dispersion of each UDG. We find a mixed bag of galaxies, from one UDG that shows no signs of dark matter, to UDGs that follow the luminosity–dispersion relation of early-type galaxies, to the most extreme examples of heavily dark matter–dominated galaxies that break well-known scaling relations such as the luminosity–dispersion or U-shaped total mass-to-light ratio relations. This is indicative of a number of mechanisms at play forming these peculiar galaxies. Some of them may be the most extended version of dwarf galaxies, while others are so extreme that they seem to populate dark matter halos consistent with that of the Milky Way or even larger. Even though Milky Way stars and other GC interlopers contaminating our sample of GCs cannot be fully ruled out, our assessment of this potential problem and simulations indicate that the probability is low and, if present, unlikely to be enough to explain the extreme dispersions measured. Further confirmation from stellar kinematics studies in these UDGs would be desirable. The lack of such extreme objects in any of the state-of-the-art simulations opens an exciting avenue of new physics shaping these galaxies. 

We use deep Hubble Space Telescope imaging to study the evolutionary state of of the Virgo Cluster ultradiffuse galaxy (UDG) VCC 615. Using the tip of the red giant branch method, we pinpoint the galaxy's location with Virgo to be on the far side of the cluster, near the Virgo virial radius. When combined with the galaxy's measured line of sight velocity, we find the galaxy to be on an outbound orbit, having likely passed near the cluster core within the past billion years. Given the galaxy's largely undisturbed morphology, we argue that the galaxy has experienced no recent and sudden transformation into a UDG due to the cluster potential, but rather is a long-lived UDG whose relatively wide orbit and large dynamical mass protect it from stripping and destruction by Virgo cluster tides. Our Hubble imaging resolves individual globular clusters within VCC 615 down to a limiting half-light radius of 1 pc, allowing for a clean determination of the size distribution and luminosity function of the galaxy's globular cluster population. The total mass of the galaxy derived from the size of its globular cluster population is comparable to our previous work estimate from the mass from the globular cluster kinematics. Finally, we also derive the structural properties of the galaxy's nucleus and find them similar to the properties of ultracompact galaxies (UCDs) in the Virgo core, suggesting a dynamical link between UCDs and nucleated UDGs in cluster environments. 

ABSTRACT We use spectral energy distribution fitting to place constraints on the stellar populations of 59 ultra-diffuse galaxies (UDGs) in the low-to-moderate density fields of the MATLAS survey. We use the routine prospector, coupled with archival data in the optical from the Dark Energy Camera Legacy Survey, and near- and mid-infrared imaging from the Wide-field Infrared Survey Explorer, to recover the stellar masses, ages, metallicities, and star formation time-scales of the UDGs. We find that a subsample of the UDGs lies within the scatter of the mass–metallicity relation (MZR) for local classical dwarfs. However, another subsample is more metal-poor, being consistent with the evolving MZR at high redshift. We investigate UDG positioning trends in the mass–metallicity plane as a function of surface brightness, effective radius, axis ratio, local volume density, mass-weighted age, star formation time-scale, globular cluster (GC) counts, and GC specific frequency. We find that our sample of UDGs can be separated into two main classes: Class A: comprised of UDGs with lower stellar masses, prolonged star formation histories (SFHs), more elongated, inhabiting less dense environments, hosting fewer GCs, younger, consistent with the classical dwarf MZR, and fainter. Class B: UDGs with higher stellar masses, rapid SFHs, rounder, inhabiting the densest of our probed environments, hosting on average the most numerous GC systems, older, consistent with the high-redshift MZR (i.e. consistent with early-quenching), and brighter. The combination of these properties suggests that UDGs of Class A are consistent with a ‘puffed-up dwarf’ formation scenario, while UDGs of Class B seem to be better explained by ‘failed galaxy’ scenarios. 

Systematic studies1-4 have revealed hundreds of ultra-compact dwarf galaxies (UCDs5) in the nearby Universe. With half-light radii rh of approximately 10-100 parsecs and stellar masses M* ≈ 106-108 solar masses, UCDs are among the densest known stellar systems6. Although similar in appearance to massive globular clusters7, the detection of extended stellar envelopes4,8,9, complex star formation histories10, elevated mass-to-light ratio11,12 and supermassive black holes13-16 suggest that some UCDs are remnant nuclear star clusters17 of tidally stripped dwarf galaxies18,19, or even ancient compact galaxies20. However, only a few objects have been found in the transient stage of tidal stripping21,22, and this assumed evolutionary path19 has never been fully traced by observations. Here we show that 106 galaxies in the Virgo cluster have morphologies that are intermediate between normal, nucleated dwarf galaxies and single-component UCDs, revealing a continuum that fully maps this morphological transition and fills the `size gap' between star clusters and galaxies. Their spatial distribution and redder colour are also consistent with stripped satellite galaxies on their first few pericentric passages around massive galaxies23. The `ultra-diffuse' tidal features around several of these galaxies directly show how UCDs are forming through tidal stripping and that this evolutionary path can include an early phase as a nucleated ultra-diffuse galaxy24,25. These UCDs represent substantial visible fossil remnants of ancient dwarf galaxies in galaxy clusters, and more low-mass remnants probably remain to be found. 

Abstract We use deep Hubble Space Telescope imaging to derive a distance to the Virgo Cluster ultradiffuse galaxy (UDG) VCC 615 using the tip of the red giant branch (TRGB) distance estimator. We detect 5023 stars within the galaxy, down to a 50% completeness limit of F814W ≈ 28.0, using counts in the surrounding field to correct for contamination due to background sources and Virgo intracluster stars. We derive an extinction-corrected F814W tip magnitude of m tip , 0 = 27.19 − 0.05 + 0.07 , yielding a distance of d = 17.7 − 0.4 + 0.6 Mpc. This places VCC 615 on the far side of the Virgo Cluster ( d Virgo = 16.5 Mpc), at a Virgocentric distance of 1.3 Mpc and near the virial radius of the main body of Virgo. Coupling this distance with the galaxy’s observed radial velocity, we find that VCC 615 is on an outbound trajectory, having survived a recent passage through the inner parts of the cluster. Indeed, our orbit modeling gives a 50% chance the galaxy passed inside the Virgo core ( r < 620 kpc) within the past gigayear, although very close passages directly through the cluster center ( r < 200 kpc) are unlikely. Given VCC 615's undisturbed morphology, we argue that the galaxy has experienced no recent and sudden transformation into a UDG due to the cluster potential, but rather is a long-lived UDG whose relatively wide orbit and large dynamical mass protect it from stripping and destruction by the Virgo cluster tides. Finally, we also describe the serendipitous discovery of a nearby Virgo dwarf galaxy projected 90″ (7.2 kpc) away from VCC 615. 

Abstract We present a study of the stellar populations of globular clusters (GCs) in the Virgo Cluster core with a homogeneous spectroscopic catalog of 692 GCs within a major-axis distance R maj = 840 kpc from M87. We investigate radial and azimuthal variations in the mean age, total metallicity, [Fe/H], and α -element abundance of blue (metal-poor) and red (metal-rich) GCs using their co-added spectra. We find that the blue GCs have a steep radial gradient in [Z/H] within R maj = 165 kpc, with roughly equal contributions from [Fe/H] and [ α /Fe], and flat gradients beyond. By contrast, the red GCs show a much shallower gradient in [Z/H], which is entirely driven by [Fe/H]. We use GC-tagged Illustris simulations to demonstrate an accretion scenario where more massive satellites (with more metal- and α -rich GCs) sink further into the central galaxy than less massive ones, and where the gradient flattening occurs because of the low GC occupation fraction of low-mass dwarfs disrupted at larger distances. The dense environment around M87 may also cause the steep [ α /Fe] gradient of the blue GCs, mirroring what is seen in the dwarf galaxy population. The progenitors of red GCs have a narrower mass range than those of blue GCs, which makes their gradients shallower. We also explore spatial inhomogeneity in GC abundances, finding that the red GCs to the northwest of M87 are slightly more metal-rich. Future observations of GC stellar population gradients will be useful diagnostics of halo merger histories.