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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 2.5σ 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.

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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 NGC=25.1−5.4+6.5 , resulting in a specific frequency of SN=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 radii Rh > 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.

We study the size and structure of globular cluster (GC) systems of 118 early-type galaxies from the NGVS, MATLAS, and ACSVCS surveys. Fitting Sérsic profiles, we investigate the relationship between effective radii of GC systems (R e,gc) and galaxy properties. GC systems are 2–4 times more extended than host galaxies across the entire stellar mass range of our sample (108.3 M ⊙ < M * < 1011.6 M ⊙). The relationship between R e,gc and galaxy stellar mass exhibits a characteristic “knee” at a stellar mass of M p ≃ 1010.8, similar to the galaxy R e –stellar mass relationship. We present a new characterization of the traditional blue and red GC color subpopulations, describing them with respect to host galaxy (g′−i′) color (Δgi): GCs with similar colors to their hosts have a “red” Δgi, and those significantly bluer GCs have a “blue” Δgi. The GC populations with red Δgi, even in dwarf galaxies, are twice as extended as the stars, suggesting that formation or survival mechanisms favor the outer regions. We find a tight correlation between R e,gc and the total number of GCs, with intrinsic scatter ≲0.1 dex spanning two and three orders of magnitude in size and number, respectively. This holds for both red and blue subpopulations, albeit with different slopes. Assuming that N GC,Total correlates with M 200, we find that the red GC systems have effective radii of roughly 1%–5% R 200, while the blue GC systems in massive galaxies can have sizes as large as ∼10% R 200. Environmental dependence on R e,gc is also found, with lower-density environments exhibiting more extended GC systems at fixed mass.

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, Rvir ∼ 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 to MV ∼ −8.0, and include detections of dwarf galaxies as faint as MV ∼ −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 (MV ∼ −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.

We present an analysis of the spatial distribution of globular cluster (GC) systems of 118 nearby early-type galaxies in the Next Generation Virgo Cluster Survey and Mass Assembly of early-Type GaLAxies with their fine Structures survey programs, which both used MegaCam on the Canada–France–Hawaii Telescope. We describe the procedure used to select GC candidates and fit the spatial distributions of GCs to a two-dimensional Sérsic function, which provides effective radii (half number radii) and Sérsic indices, and estimate background contamination by adding a constant term to the Sérsic function. In cases where a neighboring galaxy affects the estimation of the GC spatial distribution in the target galaxy, we fit two two-dimensional Sérsic functions, simultaneously. We also investigate the color distributions of GCs in our sample by using Gaussian mixture modeling. For GC systems with bimodal color distributions, we divide the GCs into blue and red subgroups and fit their respective spatial distributions with Sérsic functions. Finally, we measure the total number of GCs based on our fitted Sérsic function, and calculate the GC specific frequency.

We present deep optical and near-infrared observations of the host galaxies of 34 fast radio bursts (FRBs) detected by the Commensal Real-time ASKAP Fast Transient (or CRAFT) survey on the Australian SKA Pathfinder (ASKAP) to compare the locations of FRBs relative to their host light distributions. Incorporating three additional FRBs from the literature, for a total of four repeating and 33 apparently nonrepeating FRBs, we determine their projected galactocentric offsets and find a median of 4.2−2.5+5.7 kpc ( 1.0−0.6+1.5re ). We model their host surface-brightness profiles and develop synthetic spatial distributions of their globular clusters (GCs) based on host properties. We calculate the likelihood the observed location of each FRB is consistent with the smooth light of its host galaxy, residual (primarily spiral) substructure, or GC distributions. The majority of FRBs favor locations within the disks of their galaxies, while only 11% ± 5% favor a GC origin, primarily those with galactocentric offsets ≳3re. At z < 0.15, where spiral structure is apparent in 86% of our sample of FRB hosts, we find ≈20%–46% of FRBs favor an association with spiral arms. Assuming FRBs derive from magnetars, our results support multiple formation channels, with the majority of progenitors associated with massive stars and a minority formed through dynamical channels. However, the moderate fraction of FRBs associated with spiral structure indicates that high star formation efficiency of the youngest and most massive stars is not a predominant driver in the production of FRB progenitors.

We present a study of morphologies, based on deep u*g′i′z′ imaging of the Virgo Cluster from the Next Generation Virgo Cluster Survey, for 3689 Virgo cluster members spanning a mass range of ∼1011–105M⊙. Our analysis introduces a new, two-component visual classification scheme developed to capture the morphological diversity of galaxies over more than 6 orders of magnitude in stellar mass. Our morphological classifications use two parameters to describe the global structure and star formation activity of each galaxy. Structural subcodes denote features such as spiral arms, bars, disks, shells, and streams, while star formation subcodes indicate the form and location of the current star formation activity (e.g., in cores, clumps, filaments). These visual classifications rely on deep g′ -band images, supplemented by u*g′i′ ­ color images, as well as unsharp-masked images for a subset of objects. We compare our classifications to previous results for bright member galaxies that used more established schemes, finding good agreement. We also measure quantitative classification statistics (e.g., CASGM20) for a subset of the brighter galaxies, and present catalogs for some galaxy types of special interest, including structurally compact galaxies, ultradiffuse galaxies, candidate ultracompact dwarf transition objects, as well as candidate postmerger systems. These morphological classifications may be useful as a training set in the application of machine learning tools to the next generation of wide-field imaging surveys.

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 mtip,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.

Nuclear star clusters (NSCs) are the densest stellar systems in the Universe. These clusters can be found at the centre of all galaxy types but tend to favour galaxies of intermediate stellar mass around 10 9 M ⊙ (refs.  1 , 2 ). At present, two main processes are under debate to explain their formation: in situ star formation from gas infall 3 and migration and merging of globular clusters (GCs) caused by dynamical friction 4 . Studies 5 , 6 , 7 , 8 – 9 of NSC stellar populations suggest that the former predominates in massive galaxies, whereas the latter prevails in dwarf galaxies, and both contribute equally at intermediate mass. However, until now, no ongoing merger of GCs has been observed to confirm this scenario. Here we report the serendipitous discovery of five dwarf galaxies with complex nuclear regions, characterized by multiple nuclei and tidal tails, using high-resolution images from the Hubble Space Telescope. These structures have been reproduced in complementary N -body simulations, supporting the interpretation that they result from migrating and merging of star clusters. The small detection rate and short simulated timescales (below 100 Myr) of this process may explain why this has not been observed previously. This study highlights the need for large surveys with high resolution to fully map the migration scenario steps. High-resolution images from the Hubble Space Telescope show evidence of star cluster migration and merger in dwarf galaxies.