TRENDS IN THE GLOBULAR CLUSTER LUMINOSITY FUNCTION OF EARLY-TYPE GALAXIES

摘要

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 approx > 3 x 10~5 solar mass, 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) approx = 2 x 10~5 solar mass. However, we find that M_(TO) scatters to lower values [≈(1-2) x 10~5 solar mass] in galaxies fainter than M_(B, gal) approx > -18.5, an important consideration if the GCLF is to be used as a distance indicator for dwarf ellipticals.