Globular cluster systems. I. V-I color distributions

We have compiled data for the globular duster systems of 50 galaxies from t he Hubble Space Telescope Wide Field Planetary Camera 2 archive, of which 4 3 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 col or distributions, as well as a measure for their nonunimodality. The number of globular clusters in each galaxy ranges from 18 (in NGC 2778) to 781 (N GC 5846). For those systems having more than 100 dusters, seven of 16 (44%) show significant bimodality. Overall, roughly half of all the systems in o ur sample show hints of a bimodal color distribution. In general, the distr ibutions of the faint galaxies are consistent with unimodality, whereas tho se of the brighter galaxies are not. We also find a number of systems with narrow color distributions-with both mean red and blue colors-suggesting th at systems exist with only metal-rich or only metal-poor globular dusters. We discuss their possible origins. In comparing the moments of the V - I di stributions with various galaxy properties for the early-type galaxies, we find the following difference in the correlations between the field and clu ster galaxy populations: the peak V - I color of the globular cluster distr ibution correlates well with the central velocity dispersion-and hence the Mg-2 index and total luminosity-for galaxies in cluster environments; there exists no such correlation for field galaxies. This difference between clu ster and held galaxies possibly reflects different formation scenarios for their globular duster systems. Among the explanations for such a correlatio n, we consider either a larger age spread in the held populations or the po ssibility that cluster galaxies are always affected by significant accretio n whereas some field galaxies could host pure "in situ" formed populations.