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Register a new userSubstructure in the Globular Cluster Populations of the Virgo Cluster Elliptical Galaxies M84 and M86
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We have carried out a search for substructure within the globular cluster systems of M84 (NGC 4374) and M86 (NGC 4406), two giant elliptical galaxies in the Virgo Cluster. We use wide-field (36 arcmin x 36 arcmin), multi-color broadband imaging to identify globular cluster candidates in these two galaxies as well as several other nearby lower-mass galaxies. Our analysis of the spatial locations of the globular cluster candidates reveals several substructures, including: a peak in the projected number density of globular clusters in M86 that is offset from the system center and may be at least partly due to the presence of the dwarf elliptical galaxy NGC 4406B; a bridge that connects the M84 and M86 globular cluster systems; and a boxy iso-density contour along the southeast side of the M86 globular cluster system. We divide our sample into red (metal-rich) and blue (metal-poor) globular cluster candidates to look for differences in the spatial distributions of the two populations and find that the blue cluster candidates are the dominant population in each of the substructures we identify. We also incorporate the measurements from two radial velocity surveys of the globular clusters in the region and find that the bridge substructure is populated by globular clusters with a mix of velocities that are consistent with either M86 and M84, possibly providing further evidence for interaction signatures between the two galaxies.
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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.
Substructure in globular cluster (GC) populations around large galaxies is expected in galaxy formation scenarios that involve accretion or merger events, and it has been searched for using direct associations between GCs and structure in the diffuse galaxy light, or with GC kinematics. Here, we present a search for candidate substructures in the GC population around the Virgo cD galaxy M87 through the analysis of the spatial distribution of the GC colors.~The study is based on a sample of $sim!1800$ bright GCs with high-quality $u,g,r,i,z,K_s$ photometry, selected to ensure a low contamination by foreground stars or background galaxies.~The spectral energy distributions of the GCs are associated with formal estimates of age and metallicity, which are representative of its position in a 4-D color-space relative to standard single stellar population models.~Dividing the sample into broad bins based on the relative formal ages, we observe inhomogeneities which reveal signatures of GC substructures.~The most significant of these is a spatial overdensity of GCs with relatively young age labels, of diameter $sim!0.1$,deg ($sim!30,$kpc), located to the south of M87.~The significance of this detection is larger than about 5$sigma$ after accounting for estimates of random and systematic errors.~Surprisingly, no large Virgo galaxy is present in this area, that could potentially host these GCs.~But candidate substructures in the M87 halo with equally elusive hosts have been described based on kinematic studies in the past.~The number of GC spectra available around M87 is currently insufficient to clarify the nature of the new candidate substructure.
We present UBVRI surface photometry for 16 dwarf elliptical galaxies in the Virgo Cluster with previously measured kinematic properties. The global optical colors are red, with median values for the sample of 0.24 +/- 0.03 in (U-B), 0.77 +/- 0.02 in (B-V), and 1.02 +/- 0.03 in (V-I). We recover the well known color-magnitude relation for cluster galaxies, but find no significant difference in dominant stellar population between rotating and non-rotating dwarf elliptical galaxies; the average age of the dominant stellar population is 5-7 Gyr in all 16 galaxies in this sample. Analysis of optical spectra confirm these age estimates and indicate Fe and Mg abundances in the range of 1/20th to 1/3 of solar, as expected for low luminosity galaxies. Based on Lick indices and simple stellar population models, the derived [alpha/Fe] ratios are sub-solar to solar, indicating a more gradual chemical enrichment history for dEs as compared to giant elliptical galaxies in the Virgo Cluster. These observations confirm the marked difference in stellar population and stellar distribution between dwarf and giant elliptical galaxies and further substantiate the need for alternative evolutionary scenarios for the lowest mass cluster galaxies. We argue that it is likely that several different physical mechanisms played a significant role in the production of the Virgo cluster dE galaxies including in situ formation, infall of dEs that were once part of Local Group analogs, and transformation of dwarf irregular galaxies by the cluster environment. The observations support the hypothesis that a large fraction of the Virgo cluster dEs are formed by ram pressure stripping of gas from infalling dIs.
It has been proposed that a galaxys nova rate might be enhanced by the production of nova progenitor binaries in the dense cores of its globular clusters (GCs). To explore this idea, relative nova rates in three Virgo elliptical galaxies, M87, M49 and M84, which have significantly different GC specific frequencies ($S_{N}$) of 14, 3.6, and 1.6, respectively, were measured over the course of 4 epochs spanning a period of 14 months. To simplify the analysis, observations of the nearly equidistant galaxies were made on the same nights, with the same integration times, and through the same filter (H$alpha$), so that the relative numbers of novae discovered would reflect the relative nova rates. At the conclusion of our survey we found a total of 27 novae associated with M87, 37 with M49, and 19 with M84. After correcting for survey completeness, we found annual nova rates of $154^{+23}_{-19}$, $189^{+26}_{-22}$, and $95^{+15}_{-14}$, for M87, M49, and M84, respectively, corresponding to $K$-band luminosity-specific nova rates of $3.8pm1.0$, $3.4pm0.6$, and $3.0pm0.6$ novae per year per $10^{10}~L_{K,odot}$. The overall results of our study suggest that a galaxys nova rate simply scales with its luminosity, and is insensitive to its GC specific frequency. Two novae, one in M87 and one in M84, were found to be spatially coincident with known GCs. After correcting for the mass fraction in GCs, we estimate that novae are likely enhanced relative to the field by at least an order of magnitude in the GC systems of luminous Virgo ellipticals.
In the light of the question whether most early-type dwarf (dE) galaxies in clusters formed through infall and transformation of late-type progenitors, we search for an imprint of such an infall history in the oldest, most centrally concentrated dE subclass of the Virgo cluster: the nucleated dEs that show no signatures of disks or central residual star formation. We select dEs in a (projected) region around the central elliptical galaxies, and subdivide them by their line-of-sight velocity into fast-moving and slow-moving ones. These subsamples turn out to have significantly different shapes: while the fast dEs are relatively flat objects, the slow dEs are nearly round. Likewise, when subdividing the central dEs by their projected axial ratio into flat and round ones, their distributions of line-of-sight velocities differ significantly: the flat dEs have a broad, possibly two-peaked distribution, whereas the round dEs show a narrow single peak. We conclude that the round dEs probably are on circularized orbits, while the flat dEs are still on more eccentric or radial orbits typical for an infalling population. In this picture, the round dEs would have resided in the cluster already for a long time, or would even be a cluster-born species, explaining their nearly circular orbits. They would thus be the first generation of Virgo cluster dEs. Their shape could be caused by dynamical heating through repeated tidal interactions. Further investigations through stellar population measurements and studies of simulated galaxy clusters would be desirable to obtain definite conclusions on their origin.
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