LOW-MASS X-RAY BINARIES AND GLOBULAR CLUSTERS IN EARLY-TYPE GALAXIES. II. GLOBULAR CLUSTER CANDIDATES AND THEIR MASS-METALLICITY RELATION

Abstract

We present an astrometry and photometry catalogue of globular cluster (GC) candidates detected with the Hubble Space Telescope Wide Field Planetary Camera 2 in a sample of 19 early-type galaxies, appropriate for comparison with the low-mass X-ray binary populations observed with Chandra. In a companion paper, we present the Chandra data and investigate the relation between these populations. We demonstrate that, although there is little evidence of a color-magnitude correlation for the GCs, after estimating mass and metallicity from the photometry under the assumption of a single-age simple stellar population, there is a significant positive correlation between mass and metallicity. We constrained [Z/H] = (-2.1 ± 0.2) + (0.25 ± 0.04)log10M, with a 1σ intrinsic scatter of 0.62 dex in metallicity. If GCs are bimodal in metallicity, this relation is consistent with recent suggestions of a mass-metallicity relation only for metal-poor clusters. Adopting a new technique to fit the GC luminosity function (GCLF) accounting for incompleteness and the Eddington bias, we compute the V-band local GC specific frequency (SN ) and specific luminosity (SL ) of each galaxy. We show that SL is the more robust measure of the richness of a GC population where a significant fraction is undetected due to source detection incompleteness. We find that the absolute magnitude of the GCLF turnover exhibits intrinsic scatter from galaxy to galaxy of ~0.3 mag (1σ), limiting its accuracy as a standard distance measure.