The ACS Virgo Cluster Survey. XII. The Luminosity Function of Globular Clusters in Early‐Type Galaxies

We analyze the luminosity function of the globular clusters (GCs) belongingto the early-type galaxies observed in the ACS Virgo Cluster Survey. We haveobtained estimates for a Gaussian representation of the GC luminosity function(GCLF) for 89 galaxies. We have also fit the GCLFs with an "evolved Schechterfunction", which is meant to reflect the preferential depletion of low-massGCs, primarily by evaporation due to two-body relaxation, from an initialSchechter mass function similar to that of young massive clusters. We find asignificant trend of the GCLF dispersion with galaxy luminosity, in the sensethat smaller galaxies have narrower GCLFs. We show that this narrowing of theGCLF in a Gaussian description is driven by a steepening of the GC massfunction above the turnover mass, as one moves to smaller host galaxies. Weargue that this behavior at the high-mass end of the GC mass function is mostlikely a consequence of systematic variations of the initial cluster massfunction. The GCLF turnover mass M_TO is roughly constant, at ~ 2.2 x 10^5M_sun in bright galaxies, but it decreases slightly in dwarfs with M_B >~ -18.We show that part of the variation could arise from the shorter dynamicalfriction timescales in smaller galaxies. We probe the variation of the GCLF toprojected galactocentric radii of 20-35 kpc in the Virgo giants M49 and M87,finding that M_TO is essentially constant over these spatial scales. Our fitsof evolved Schechter functions imply average dynamical mass losses (Delta) overa Hubble time that fall in the range 2 x 10^5 <~ (Delta/M_sun) < 10^6 per GC.We agree with previous suggestions that if the full GCLF is to be understood inmore detail GCLF models will have to include self-consistent treatments ofdynamical evolution inside time-dependent galaxy potentials. (Abridged)