1-3 -(6) Five of the 13 DGTOs in Virgo are associated with a single galaxy: M87.This finding suggests that proximity to the Virgo center may be of critical importance for the formation of these objects, although we find M87 to be more abundant in DGTOs than would be expected on the basis of its luminosity, the size of its globular cluster system, or the local galaxy density.These results show that distinguishing bonafide UCDs from high-luminosity globular clusters requires a careful analysis of their detailed structural and dynamical properties, particularly their mass-to-light ratios. In general, the properties of the UCDs in our sample are consistent with models in which these objects form through tidal stripping of nucleated dwarf galaxies.
Imaging surveys with the Hubble Space Telescope (HST) have shown that ≈ 50-80% of low-and intermediate-luminosity galaxies contain a compact stellar nucleus at their center, regardless of host galaxy morphological type. We combine HST imaging for early-type galaxies from the ACS Virgo Cluster Survey with ground-based long-slit spectra from KPNO to show that the masses of compact stellar nuclei in Virgo Cluster galaxies obey a tight correlation with the masses of the host galaxies. The same correlation is obeyed by the supermassive black holes (SBHs) found in predominantly massive galaxies. The compact stellar nuclei in the Local Group galaxies M33 and NGC 205 are also found to fall along this same scaling relation. These results indicate that a generic by-product of galaxy formation is the creation of a central massive object (CMO) -either a SBH or a compact stellar nucleus -that contains a mean fraction, ≈ 0.2%, of the total galactic mass. In galaxies with masses greater than M gal ∼ a few 10 10 M ⊙ , SBHs appear to be the dominant mode of CMO formation.
The ACS Virgo Cluster Survey is a Hubble Space Telescope program to obtain high-resolution imaging in widely separated bandpasses ( F475W % g and F850LP % z) for 100 early-type members of the Virgo Cluster, spanning a range of %460 in blue luminosity. We use this large, homogenous data set to examine the innermost structure of these galaxies and to characterize the properties of their compact central nuclei. We present a sharp upward revision in the frequency of nucleation in early-type galaxies brighter than M B % À15 (66% P f n P 82%) and show that groundbased surveys underestimated the number of nuclei due to surface brightness selection effects, limited sensitivity and poor spatial resolution. We speculate that previously reported claims that nucleated dwarfs are more concentrated toward the center of Virgo than their nonnucleated counterparts may be an artifact of these selection effects. There is no clear evidence from the properties of the nuclei, or from the overall incidence of nucleation, for a change at M B $ À17:6, the traditional dividing point between dwarf and giant galaxies. There does, however, appear to be a fundamental transition at M B $ À20:5, in the sense that the brighter, ''core-Sérsic'' galaxies lack resolved (stellar) nuclei. A search for nuclei that may be offset from the photocenters of their host galaxies reveals only five candidates with displacements of more than 0B5, all of which are in dwarf galaxies. In each case, however, the evidence suggests that these ''nuclei'' are, in fact, globular clusters projected close to the galaxy photocenter. Working from a sample of 51 galaxies with prominent nuclei, we find a median half-light radius of hr h i ¼ 4:2 pc, with the sizes of individual nuclei ranging from 62 pc down to 2 pc (i.e., unresolved in our images) in about a half-dozen cases. Excluding these unresolved objects, the nuclei sizes are found to depend on nuclear luminosity according to the relation r h / L 0:50AE0:03 . Because the large majority of nuclei are resolved, we can rule out low-level AGNs as an explanation for the central luminosity excess in almost all cases. On average, the nuclei are %3.5 mag brighter than a typical globular cluster. Based on their broadband colors, the nuclei appear to have old to intermediate age stellar populations. The colors of the nuclei in galaxies fainter than M B % À17:6 are tightly correlated with their luminosities, and less so with the luminosities of their host galaxies, suggesting that their chemical enrichment histories were governed by local or internal factors. Comparing the nuclei to the ''nuclear clusters'' found in late-type spiral galaxies reveals a close match in terms of size, luminosity, and overall frequency. A formation mechanism that is rather insensitive to the detailed properties of the host galaxy properties is required to explain this ubiquity and homogeneity. The mean of the frequency function for the nucleus-to-galaxy luminosity ratio in our nucleated galaxies, hlog i ¼ À2:49 AE 0:09 dex ( ¼ 0:59 AE 0:10), i...
Abstract. We examine the properties of ten compact objects in the vicinity of M87 using HST imaging and Keck spectroscopy. We find three objects to be larger and brighter than typical globular clusters; they resemble the nuclei of nucleated dEs in Virgo, having similar massto-light ratios, luminosities and colors. Their properties are consistent with models of tidally stripping of nucleated dEs. One object could be an old "stellar supercluster", formed through the amalgamation of multiple young massive clusters. Two other objects appear to be massive versions of otherwise "normal" globular clusters.
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