Globular Clusters in Virgo Ellipticals: Unexpected Results for Giants and Dwarfs from Advanced Camera for Surveys Imaging

Abstract

We have analyzed archival Hubble Space Telescope Advanced Camera for Surveys images in g and z of the globular cluster (GC) systems of 53 ellipticals in the Virgo Cluster, spanning massive galaxies to dwarf ellipticals (dEs). Several new results emerged. (1) In the giant ellipticals (gEs) M87 and NGC 4649, there is a correlation between luminosity and color for individual blue (metal-poor) GCs, such that more massive GCs are more red (metal-rich). A plausible interpretation of this result is self-enrichment, and a speculative suggestion is that these GCs once possessed dark matter halos. (2) The dispersion in color is nearly twice as large for the metal-rich GCs as for the metal-poor GCs. However, there is evidence for a nonlinear relation between g - z and metallicity, and the dispersion in metallicity may be the same for both subpopulations. (3) Very luminous, intermediate-color GCs are common in gEs. These objects may be remnants of many stripped dwarfs, analogs of ω Cen in the Galaxy. (4) There is a continuity of GC system colors from gEs to some dEs; in particular, many dEs have metal-rich GC subpopulations. We also confirm the GC color-galaxy luminosity relations found previously for both metal-poor and metal-rich GC subpopulations. (5) There are large differences in GC specific frequency among dEs, independent of the presence of a nucleus and the fraction of metal-rich GCs. Over -15 < M B < -18 we find little correlation between specific frequency and M B . But we do find evidence for two separate S N classes of dEs: those with B-band S N ∼ 2 and those with populous GC systems that have S N ranging from ∼5 to 20 with median S N ∼ 10. Together, these points suggest multiple formation channels for dEs in the Virgo Cluster. (6) The peak of the GC luminosity function (GCLF) is the same for both gEs and dEs. This is contrary to expectations of dynamical friction on massive GCs, unless the primordial GCLF varies between gEs and dEs. Among gEs the GCLF turnover varies by a surprisingly small 0.05 mag, an encouraging result for its use as an accurate standard candle. © 2006. The American Astronomical Society. All rights reserved.