Globular Cluster Systems I: V-I Color Distributions

We have compiled data for the globular cluster systems of 50 galaxies from the HST WFPC2 archive, of which 43 are type S0 or earlier. In this paper, we present the data set and derive the V-I color distributions. We derive the first four moments of the color distributions, as well as a measure for their non--unimodality. The number of globular clusters in each galaxy ranges from 18 (in NGC 2778) to 781 (NGC 5846). For those systems having more than 100 clusters, seven of sixteen (44%) show significant bimodality. Overall, roughly half of all the systems in our sample show hints of a bimodal color distribution. In general, the distributions of the faint galaxies are consistent with unimodality, whereas those of the brighter galaxies are not. In comparing the moments of the V-I distributions with various galaxy properties for the early-type galaxies, we find the following difference in the correlations between the field and cluster galaxy populations: the peak V-I color of the globular cluster distribution correlates well with the central velocity dispersion---and hence the Mg2 index and total luminosity---for galaxies in cluster environments; there exists no such correlation for field galaxies. This difference between cluster and field galaxies possibly reflects different formation scenarios for their globular cluster systems. Among the explanations for such a correlation, we consider either a larger age spread in the field populations or the possibility that cluster galaxies are always affected by significant accretion whereas some field galaxies could host pure ``in situ'' formed populations.