Discovery of Ultracompact Dwarf Galaxies in the Virgo Cluster

We have discovered nine ultra-compact dwarf galaxies (UCDs) in the VirgoCluster, extending samples of these objects outside the Fornax Cluster. Usingthe 2dF multi-fiber spectrograph on the Anglo-Australian Telescope, the newVirgo members were found among 1500 color-selected, star-like targets with 16.0< b_j < 20.2 in a two-degree diameter field centered on M87 (NGC4486). Thenewly-found UCDs are comparable to the UCDs in the Fornax Cluster, with sizes<~ 100 pc, -12.9 < M_B < -10.7, and exhibiting red, absorption-line spectra,indicative of an older stellar population. The properties of these objectsremain consistent with the tidal threshing model for the origin of UCDs fromthe surviving nuclei of nucleated dwarf ellipticals disrupted in the clustercore, but can also be explained as objects that were formed by mergers of starclusters created in galaxy interactions. The discovery that UCDs exist in Virgoshows that this galaxy type is probably a ubiquitous phenomenon in clusters ofgalaxies; coupled with their possible origin by tidal threshing, the UCDpopulation is a potential indicator and probe of the formation history of agiven cluster. We also describe one additional bright UCD with M_B = -12.0 in the core ofthe Fornax Cluster. We find no further UCDs in our Fornax Cluster SpectroscopicSurvey down to b_j = 19.5 in two additional 2dF fields extending as far as 3degrees from the center of the cluster. All six Fornax bright UCDs identifiedwith 2dF lie within 0.5 degree (projected distance of 170 kpc) of the centralelliptical galaxy NGC1399.Comment: 26 pages; 12 figures; accepted for publication in the Astronomical Journa