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The velocity dispersion of globular clusters (GCs) around ultra-diffuse galaxies (UDGs) in the Virgo cluster spans a wide range, including cases where GC kinematics suggest haloes as massive as (or even more massive than) that of the Milky Way around these faint dwarfs. We analyse the catalogues of GCs derived in post-processing from the TNG50 cosmological simulation to study the GC system kinematics and abundance of simulated UDGs in galaxy groups and clusters. UDGs in this simulation reside exclusively in dwarf-mass haloes with M200 ≲ 1011.2 M⊙. When considering only GCs gravitationally bound to simulated UDGs, we find GCs properties that overlap well with several observational measurements for UDGs. In particular, no bias towards overly massive haloes is inferred from the study of bound GCs, confirming that GCs are good tracers of UDG halo mass. However, we find that contamination by intracluster GCs may, in some cases, substantially increase velocity dispersion estimates when performing projected mock observations of our sample. We caution that targets with less than 10 GC tracers are particularly prone to severe uncertainties. Measuring the stellar kinematics of the host galaxy should help confirm the unusually massive haloes suggested by GC kinematics around some UDGs.

 

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. 

We present new radial velocity measurements from the Magellan and the Anglo-Australian Telescopes for 175 previously known and 121 newly confirmed globular clusters (GCs) around NGC 5128, the nearest accessible massive early-type galaxy atD= 3.8 Mpc. Remarkably, 28 of these newly confirmed GCs are at projected radii$>50\prime$(≳54 kpc), extending to ∼130 kpc, in the outer halo where few GCs had been confirmed in previous work. We identify several subsets of GCs that spatially trace halo substructures that are visible in red giant branch star maps of the galaxy. In some cases, these subsets of GCs are kinematically cold, and may be directly associated with and originate from these specific stellar substructures. From a combined kinematic sample of 645 GCs, we see evidence for coherent rotation at all radii, with a higher rotation amplitude for the metal-rich GC subpopulation. Using the tracer mass estimator, we measure a total enclosed mass of 2.5 ± 0.3 × 10¹²M_⊙within ∼120 kpc, an estimate that will be sharpened with forthcoming dynamical modeling. The combined power of stellar mapping and GC kinematics makes NGC 5128 an ongoing keystone for understanding galaxy assembly at mass scales inaccessible in the Local Group.

 

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 deep Hubble Space Telescope (HST) imaging of five faint dwarf galaxies associated with the nearby spiral NGC 253 (D ≈ 3.5 Mpc). Three of these are newly discovered dwarf galaxies, while all five were found in the Panoramic Imaging Survey of Centaurus and Sculptor, a Magellan+Megacam survey to identify faint dwarfs and other substructures in resolved stellar light around massive galaxies outside of the Local Group. Our HST data reach ≳3 magnitudes below the tip of the red giant branch for each dwarf, allowing us to derive their distances, structural parameters, and luminosities. All five systems contain mostly old, metal-poor stellar populations (age ∼12 Gyr, [M/H] ≲ −1.5) and have sizes ( r h ∼ 110–3000 pc) and luminosities ( M V ∼ −7 to −12 mag) largely consistent with Local Group dwarfs. The three new NGC 253 satellites are among the faintest systems discovered beyond the Local Group. We also use archival H i data to place limits on the gas content of our discoveries. Deep imaging surveys such as our program around NGC 253 promise to elucidate the faint end of the satellite luminosity function and its scatter across a range of galaxy masses, morphologies, and environments in the decade to come. 

Abstract We use panoramic optical spectroscopy obtained with the Very Large Telescope/MUSE to investigate the nature of five candidate extremely isolated low-mass star-forming regions (Blue Candidates; hereafter, BCs) toward the Virgo cluster of galaxies. Four of the five (BC1, BC3, BC4, and BC5) are found to host several H ii regions and to have radial velocities fully compatible with being part of the Virgo cluster. All the confirmed candidates have mean metallicity significantly in excess of that expected from their stellar mass, indicating that they originated from gas stripped from larger galaxies. In summary, these four candidates share the properties of the prototype system SECCO 1, suggesting the possible emergence of a new class of stellar systems, intimately linked to the complex duty cycle of gas within clusters of galaxies. A thorough discussion of the nature and evolution of these objects is presented in a companion paper, where the results obtained here from the MUSE data are complemented with Hubble Space Telescope (optical) and Very Large Array (H i ) observations. 

ABSTRACT Globular clusters (GCs) are often used to estimate the dark matter content of galaxies, especially dwarf galaxies, where other kinematic tracers are lacking. These estimates typically assume spherical symmetry and dynamical equilibrium, assumptions that may not hold for the sparse GC population of dwarfs in galaxy clusters. We use a catalogue of GCs tagged on to the Illustris simulation to study the accuracy of GC-based mass estimates. We focus on galaxies in the stellar mass range 108–1011.8 M⊙ identified in nine simulated Virgo-like clusters. Our results indicate that mass estimates are, on average, accurate in systems with GC numbers NGC ≥ 10 and where the uncertainty of individual GC line-of-sight velocities is smaller than the inferred velocity dispersion, σGC. In cases where NGC ≤ 10, however, biases may result, depending on how σGC is computed. We provide calibrations that may help alleviate these biases in methods widely used in the literature. As an application, we find a number of dwarfs with $M_{*} \sim 10^{8.5}\, \mathrm{M}_{\odot }$ – comparable with the ultra-diffuse galaxy NGC 1052-DF2 (DF2), notable for the low σGC of its 10 GCs – that have $\sigma _{\rm GC} \sim 7\!-\!15\, {\rm km \,s}^{-1}$. These DF2 analogues correspond to relatively massive systems at their infall time (M200 ∼ 1–3 × 1011 M⊙), which have retained only 3–17 GCs and have been stripped of more than 95 per cent of their dark matter. Our results suggest that extreme tidal mass loss in otherwise normal dwarf galaxies may be a possible formation channel for ultra-diffuse objects such as DF2.