New insights from HST studies of globular cluster systems. I. Colors, distances, and specific frequencies of 28 elliptical galaxies

We present an analysis of the globular cluster systems of 28 elliptical galaxies using archival WFPC2 images in the V and I bands. The V - I color distributions of at least 50% of the galaxies appear to be bimodal at the present level of photometric accuracy. We argue that this is indicative of multiple epochs of cluster formation early in the history of these galaxies, possibly due to mergers. We also present the first evidence of bimodality in low-luminosity galaxies and discuss its implication on formation scenarios. The mean color of the 28 cluster systems studied by us is V - I = 1.04 +/- 0.04 (0.01) mag corresponding to a mean metallicity of Fe/H = -1.0 +/- 0.19 (0.04). We find that the turnover magnitudes of the globular cluster luminosity functions (GCLF) of our sample are in excellent agreement with the distance measurements using other methods and conclude that the accuracy of the GCLF is at least as good as the surface brightness fluctuation method. The absolute magnitude of the turnover luminosity of the GCLF is M-V(0) = -7.41 (0.03) in V and M-I(0) = -8.46 (0.03) in I. The mean local specific frequency of our sample of elliptical galaxies within the WFPC2 field-of-view is 2.4 +/- 1.8 (0.4), considerably higher than the 1.0 +/- 0.6 (0.1) derived for a comparable sample of S0s in a similar analysis. It shows no obvious correlation with metallicity, host galaxy mass or membership in a galaxy cluster. The median size of clusters in all galaxies appears to be remarkably constant at similar to2.4 pc. We suggest that in the future it might be possible to use the sizes of clusters in the inner regions of galaxies as a simple geometrical distance indicator.