We present a study of ultra-diffuse galaxies (UDGs) in the Virgo Cluster based on deep imaging from the Next Generation Virgo Cluster Survey (NGVS). Applying a new definition for the UDG class based on galaxy scaling relations, we define samples of 4
4 and 26 UDGs using expansive and restrictive selection criteria, respectively. Our UDG sample includes objects that are significantly fainter than previously known UDGs: i.e., more than half are fainter than $langlemurangle_e sim27.5$ mag arcsec$^{-2}$. The UDGs in Virgos core region show some evidence for being structurally distinct from normal dwarf galaxies, but this separation disappears when considering the full sample of galaxies throughout the cluster. UDGs are more centrally concentrated in their spatial distribution than other Virgo galaxies of similar luminosity, while their morphologies demonstrate that at least some UDGs owe their diffuse nature to physical processes---such as tidal interactions or low-mass mergers---that are at play within the cluster environment. The globular cluster (GC) systems of Virgo UDGs have a wide range in specific frequency ($S_N$), with a higher mean $S_N$ than normal Virgo dwarfs, but a lower mean $S_N$ than Coma UDGs at fixed luminosity. Their GCs are predominantly blue, with a small contribution from red clusters in the more massive UDGs. The combined GC luminosity function is consistent with those observed in dwarf galaxies, showing no evidence of being anomalously luminous. The diversity in their morphologies and their GC properties suggests no single process has given rise to all objects within the UDG class. Based on the available evidence, we conclude that UDGs are simply those systems that occupy the extended tails of the galaxy size and surface brightness distributions.
Stellar shells around galaxies could provide precious insights into their assembly history. However, their formation mechanism remains poorly empirically constrained, in particular the type of galaxy collisions at their origin. We present MUSE@VLT da
ta of the most prominent outer shell of NGC 474, to constrain its formation history. The stellar shell spectrum is clearly detected, with a signal-to-noise ratio of around 65 pix$^{-1}$. We use a full spectral fitting method to determine the line-of-sight velocity and the age and metallicity of the shell and associated point-like sources within the MUSE field of view. We detect six GC candidates and eight PN candidates which are all kinematically associated to the stellar shell. We show that the shell has an intermediate metallicity, [M/H] = $-0.83^{+0.12}_{-0.12}$ and a possible $alpha$-enrichment, [$alpha$/Fe] ~ 0.3. Assuming the material of the shell comes from a lower mass companion, and that the latter had no initial metallicity gradient, such a stellar metallicity would constrain the mass of the progenitor to be around 7.4 x 10^8 M$_odot$, implying a merger mass ratio of about 1:100. However our census of PNs and earlier photometry of the shell would suggest a much higher ratio, around 1:20. Given the uncertainties, this difference is significant only at the ~1sigma level. We discuss the characteristics of the progenitor, in particular whether the progenitor could also be composed of stars from the low metallicity outskirts from a more massive galaxy. Ultimately, the presented data does not allow us to put a firm constraint on the progenitor mass. We show that at least two globular cluster candidates possibly associated with the shell are quite young, with ages below 1.5~Gyr. We also note the presence of a young (~1Gyr) stellar population in the center of NGC 474. The two may have resulted from the same event.
Ultra-diffuse galaxies (UDGs) are unusual galaxies with low luminosities, similar to classical dwarf galaxies, but sizes up to $sim!5$ larger than expected for their mass. Some UDGs have large populations of globular clusters (GCs), something unexpec
ted in galaxies with such low stellar density and mass. We have carried out a comprehensive study of GCs in both UDGs and classical dwarf galaxies at comparable stellar masses using HST observations of the Coma cluster. We present new imaging for 33 Dragonfly UDGs with the largest effective radii ($>2$ kpc), and additionally include 15 UDGs and 54 classical dwarf galaxies from the HST/ACS Coma Treasury Survey and the literature. Out of a total of 48 UDGs, 27 have statistically significant GC systems, and 11 have candidate nuclear star clusters. The GC specific frequency ($S_N$) varies dramatically, with the mean $S_N$ being higher for UDGs than for classical dwarfs. At constant stellar mass, galaxies with larger sizes (or lower surface brightnesses) have higher $S_N$, with the trend being stronger at higher stellar mass. At lower stellar masses, UDGs tend to have higher $S_N$ when closer to the center of the cluster, i.e., in denser environments. The fraction of UDGs with a nuclear star cluster also depends on environment, varying from $sim!40$% in the cluster core, where it is slightly lower than the nucleation fraction of classical dwarfs, to $lesssim20%$ in the outskirts. Collectively, we observe an unmistakable diversity in the abundance of GCs, and this may point to multiple formation routes.
We present Keck/DEIMOS spectroscopy of globular clusters (GCs) around the ultra-diffuse galaxies (UDGs) VLSB-B, VLSB-D, and VCC615 located in the central regions of the Virgo cluster. We spectroscopically identify 4, 12, and 7 GC satellites of these
UDGs, respectively. We find that the three UDGs have systemic velocities ($V_{sys}$) consistent with being in the Virgo cluster, and that they span a wide range of velocity dispersions, from $sim 16$ to $sim 47$ km/s, and high dynamical mass-to-light ratios within the radius that contains half the number of GCs ($ 407^{+916}_{-407}$, $21^{+15}_{-11}$, $60^{+65}_{-38}$, respectively). VLSB-D shows possible evidence for rotation along the stellar major axis and its $V_{sys}$ is consistent with that of the massive galaxy M84 and the center of the Virgo cluster itself. These findings, in addition to having a dynamically and spatially ($sim 1$ kpc) off-centered nucleus and being extremely elongated, suggest that VLSB-D could be tidally perturbed. On the contrary, VLSB-B and VCC615 show no signals of tidal deformation. Whereas the dynamics of VLSB-D suggest that it has a less massive dark matter halo than expected for its stellar mass, VLSB-B and VCC615 are consistent with a $sim 10^{12}$ M$_{odot}$ dark matter halo. Although our samples of galaxies and GCs are small, these results suggest that UDGs may be a diverse population, with their low surface brightnesses being the result of very early formation, tidal disruption, or a combination of the two.
Small distortions in the images of Einstein rings or giant arcs offer the exciting prospect of detecting dark matter haloes or subhaloes of mass below $10^9$M$_{odot}$, most of which are too small to have made a visible galaxy. A very large number of
such haloes are predicted to exist in the cold dark matter model of cosmogony; in contrast other models, such as warm dark matter, predict no haloes below a mass of this order which depends on the properties of the warm dark matter particle. Attempting to detect these small perturbers could therefore discriminate between different kinds of dark matter particles, and even rule out the cold dark matter model altogether. Globular clusters in the lens galaxy also induce distortions in the image which could, in principle, contaminate the test. Here, we investigate the population of globular clusters in six early type galaxies in the Virgo cluster. We find that the number density of globular clusters of mass $sim10^6$M$_{odot}$ is comparable to that of the dark matter perturbers (including subhaloes in the lens and haloes along the line-of-sight). We show that the very different degrees of mass concentration in globular clusters and dark matter haloes result in different lensing distortions. These are detectable with milli-arcsecond resolution imaging which can distinguish between globular cluster and dark matter halo signals.
Globular clusters (GCs) are some of the most visible tracers of the merging and accretion history of galaxy halos. Metal-poor GCs, in particular, are thought to arrive in massive galaxies largely through dry, minor merging events, but it is rare to s
ee a direct connection between GCs and visible stellar streams. NGC 474 is a post-merger early-type galaxy with dramatic fine structures made of concentric shells and radial streams that have been more clearly revealed by deep imaging. We present a study of GCs in NGC 474 to better establish the relationship between merger-induced fine structure and the GC system. We find that many GCs are superimposed on visible streams and shells, and about 35% of GCs outside $3R_{rm e,galaxy}$ are located in regions of fine structure. The spatial correlation between the GCs and fine structure is significant at the 99.9% level, showing that this correlation is not coincidental. The colors of the GCs on the fine structures are mostly blue, and we also find an intermediate-color population that is dominant in the central region, and which will likely passively evolve to have colors consistent with a traditional metal-rich GC population. The association of the blue GCs with fine structures is direct confirmation that many metal-poor GCs are accreted onto massive galaxy halos through merging events, and that progenitors of these mergers are sub-L* galaxies.
Observations of nearby galaxy clusters at low surface brightness have identified galaxies with low luminosities, but sizes as large as L* galaxies, leading them to be dubbed ultra-diffuse galaxies (UDGs). The survival of UDGs in dense environments li
ke the Coma cluster suggests that UDGs could reside in much more massive dark halos. We report the detection of a substantial population of globular clusters (GCs) around a Coma UDG, Dragonfly 17 (DF17). We find that DF17 has a high GC specific frequency of S_N=26+/-13. The GC system is extended, with an effective radius of 12+/-2, or 5.6+/-0.9 kpc at Coma distance, 70% larger than the galaxy itself. We also estimate the mean of the GC luminosity function to infer a distance of 97 (+17/-14) Mpc, providing redshift-independent confirmation that one of these UDGs is in the Coma cluster. The presence of a rich GC system in DF17 indicates that, despite its low stellar density, star formation was intense enough to form many massive star clusters. If DF17s ratio of total GC mass to total halo mass is similar to those in other galaxies, then DF17 has an inferred total mass of ~10^11 solar masses, only ~10% the mass of the Milky Way, but extremely dominated by dark matter, with M/L_V~1000. We suggest that UDGs like DF17 may be pure stellar halos, i.e., galaxies that formed their stellar halo components, but then suffered an early cessation in star formation that prevented the formation of any substantial central disk or bulge.
