Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. 2Department of Physics and Astronomy, Rutgers, The State University of New Jersey, Piscataway, NJ 08854; mhasegan@physics.rutgers.edu 3Institute for Space Sciences, P.O.Box MG-23, RO 77125, Bucharest-Magurele, Romania 4Claudio Anguita Fellow 5Astrophysics, Denys Wilkinson Building, University of Oxford, 1 Keble Road, Oxford, OX1 3RH, UK 6European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany; ajordan@eso.org 7Herzberg Institute of Astrophysics, National Research Council of Canada, 5071West Saanich Road, Victoria, BC, V9E 287, Canada; Patrick.Cote@nrc-cnrc.gc.ca, Eric.Peng@nrc-cnrc.gc.ca, Laura.Ferrarese@nrccnrc. gc.ca 8California Institute of Technology, Pasadena, CA 91125; george@astro.caltech.edu, milos@ tapir.caltech.edu 9Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218; deanm@stsci.edu 10Department of Physics and Astronomy, The Johns Hopkins University, 3400 North Charles Street,Baltimore, MD 21218-2686; jpb@pha.jhu.edu, smei@pha.jhu.edu 11Department of Physics and Astronomy, University of Hawaii, Hilo, HI 96720; westm@hawaii.edu 12Sherman M. Fairchild Fellow 13Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822;jt@ifa.hawaii.edu 14Department of Physics, Rochester Institute of Technology, 84 Lomb Memorial Drive, Rochester, NY 14623; merritt@mail.rit.edu We investigate the connection between globular clusters and ultra-compact dwarf galaxies (UCDs) by examining the properties of ten compact, highluminosity (−11.8 . MV . −10.8) objects associated with M87 (NGC 4486,VCC1316), the cD galaxy in the Virgo Cluster. These objects, most of which were previously classified as M87 globular clusters, were selected from a combination of ground- and space-based imaging surveys. Our observational database for these objects — which we term DGTOs or “dwarf-globular transition objects” — includes Advanced Camera for Survey (ACS) F475W and F850LP imaging from ACS Virgo Cluster Survey, integrated-light spectroscopy from Keck/ESI and archival F606W WFPC2 imaging. We also present a search for DGTOs associated with other galaxies based on ACS imaging for 100 early-type galaxies in Virgo. Our main findings can be summarized as follows: (1) Out of the six DGTOs in M87 with both ground-based spectroscopy and HST imaging, we find two objects to have half-light radii, velocity dispersions and mass-to-light ratios that are consistent with the predictions of population synthesis models for old, metal-rich, high-luminosity globular clusters. (2) Three other DGTOs are much larger, with half-light radii rh ∼ 20 pc, and have V -band mass-to-light ratios in the range 6 . M/LV . 9. These objects, which we consider to be UCDs, resemble the nuclei of nucleated dwarf elliptical galaxies in the Virgo Cluster, having similar mass-to-light ratios, luminosities, and colors. (3) The classification of the sixth object is more uncertain, but it bears a strong resemblance to simulated “stellar superclusters” which are presumed to form through the amalgamation of multiple young massive clusters. (4) In general, the UCDs in M87 are found to follow the extrapolated scaling relations of galaxies more closely than those of globular clusters. There appears to be a transition between the two types of stellar systems at a mass of ≈ 2 × 106M⊙. We suggest that the presence of dark matter is the fundamental property distinguishing globular clusters from UCDs. (5) We identify a sample of 13 DGTO candidates from the complete ACS Virgo Cluster Survey, selecting on the basis of half-light radius, magnitude and color. For a number of these objects, membership in Virgo can be established through radial velocities or surface brightness fluctuation measurements with our ACS images. Three of these DGTO candidates are embedded in low-surface brightness envelopes. (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.

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Also archived in: arXiv: astro-ph/0503566 v1 25 Mar 2005 Support for program GO-9401 was provided through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. P.C. acknowledges additional support provided by NASA LTSA grant NAG5-11714. A.J. acknowledges additional financial support provided by the National Science Foundation through a grant from the Association of Universities for Research in Astronomy, Inc., under NSF cooperative agreement AST 9613615 and by Fundaci´on Andes under project No.C-13442. M.M. acknowledges additional financial support provided by the Sherman M. Fairchild foundation. D.M. is supported by NSF grant AST-020631, NASA grant NAG5-9046, and grant HST-AR-09519.01-A from STScI. M.J.W. acknowledges support through NSF grant AST-0205960. S.G.D. acknowledges a partial support from the Ajax Foundation.ISSN:1538-4357 Note: imported from RIT’s Digital Media Library running on DSpace to RIT Scholar Works in February 2014.

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