Trends in the Globular Cluster Luminosity Function of Early-Type Galaxies

Abstract

We present results from a study of the globular cluster luminosity function (GCLF) in a sample of 89 early-type galaxies observed as part of the ACS Virgo Cluster Survey. Using a Gaussian parameterization of the GCLF, we find a highly significant correlation between the GCLF dispersion, σ, and the galaxy luminosity, M_B,gal, in the sense that the GC systems in fainter galaxies have narrower luminosity functions. The GCLF dispersions in the Milky Way and M31 are fully consistent with this trend, implying that the correlation between σ and galaxy luminosity is more fundamental than older suggestions that GCLF shape is a function of galaxy Hubble type. We show that the σ-M_B,gal relation results from a bona fide narrowing of the distribution of (logarithmic) cluster masses in fainter galaxies. We further show that this behavior is mirrored by a steepening of the GC mass function for relatively high masses, M>~3×10⁵ M_solar, a mass regime in which the shape of the GCLF is not strongly affected by dynamical evolution over a Hubble time. We argue that this trend arises from variations in initial conditions and requires explanation by theories of cluster formation. Finally, we confirm that in bright galaxies the GCLF ``turns over'' at the canonical mass scale of M_TO~=2×10⁵ M_solar. However, we find that M_TO scatters to lower values [~(1-2)×10⁵ M_solar] in galaxies fainter than M_B,gal>~-18.5, an important consideration if the GCLF is to be used as a distance indicator for dwarf ellipticals.

Based on observations with the NASA/ESA Hubble Space Telescope (HST) obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.