Ju l 2 00 5 Globular Clusters in Virgo Ellipticals : Unexpected Results for Giants , Dwarfs , and Nuclei from ACS Imaging

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: (i) In the giant ellipticals (gEs) M87 and NGC 4649, there is a correlation between luminosity and color for individual metal-poor GCs, such that more massive GCs are more metal-rich. A plausible interpretation of this result is self-enrichment, and may suggest that these GCs once possessed dark matter halos. (ii) In some gEs (most notably M87), there is an " interloping " subpopulation of GCs with intermediate colors (1.0 < g − z < 1.25) and a narrow magnitude range (0.5 mag) near the turnover of the GC luminosity function. These GCs look otherwise identical to the classic metal-poor and metal-rich GC subpopulations. (iii) The dispersion in color is nearly twice as large for the metal-rich GCs than 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. (iv) Very luminous, intermediate-color GCs are common in gEs. These objects may be remnants of many stripped dwarfs, analogues of ω Cen in the Galaxy. (v) There is a continuity of GC system colors from gEs to some dEs: in particular, many dEs have metal-rich GC subpopula-tions. We also confirm the GC color–galaxy luminosity relations found previously for both metal-poor and metal-rich GC subpopulations. (vi) 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, there is little correlation between specific frequency and M B (in contrast to previous studies). But we do find evidence for two separate S N classes of dEs: those with B-band S N ∼ 2, and dEs with populous GC systems that have S N ranging from ∼ 5 − 20 with median S N ∼ 10. Together, these points suggest multiple formation channels for dEs in the Virgo Cluster. (vii) The peak of the GC luminosity – 2 – 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 surprising …