Low-Mass X-ray Binaries and Globular Clusters in Early-Type Galaxies. II. Globular Cluster Candidates and their Mass-Metallicity Relation

TL;DR

This study catalogs globular cluster candidates in early-type galaxies, revealing a positive mass-metallicity relation especially for metal-poor clusters, and discusses implications for understanding their properties and use as distance indicators.

Contribution

It introduces a new technique for fitting the globular cluster luminosity function and analyzes the mass-metallicity relation in GCs, highlighting its dependence on metallicity bimodality.

Findings

Significant positive correlation between mass and metallicity in GCs.

The GCLF turnover magnitude shows intrinsic scatter, affecting its use as a distance measure.

Sl is a more reliable measure of GC population richness than Sn.

Abstract

We present an astrometry and photometry catalogue of globular cluster (GC) candidates detected with HST WFPC2 in a sample of 19 early-type galaxies, appropriate for comparison to the low-mass X-ray binary (LMXB) 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 colour-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)log M, with a 1-sigma 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 ~0.3mag (1-sigma), limiting its accuracy as a standard distance measure.