The Globular Cluster Systems in the Coma Ellipticals. IV: WFPC2 Photometry for Five Giant Ellipticals

TL;DR

This study analyzes globular cluster systems in five giant ellipticals in the Coma Cluster, revealing similarities in luminosity functions, a wide range of specific frequencies, and a dominance of metal-rich clusters in the most massive galaxies, challenging previous formation models.

Contribution

It provides detailed photometric analysis of GC systems in Coma ellipticals, highlighting differences in specific frequencies and metallicity distributions, and compares these to Virgo galaxies, offering new insights into galaxy formation.

Findings

GC luminosity functions are similar across galaxies.

NGC 4874 hosts over 30,000 clusters, the largest known system.

Metal-rich clusters dominate in the most massive galaxies.

Abstract

We analyze photometric data in V and I for the globular cluster (GC) systems in five of the giant ellipticals in the Coma Cluster: NGC 4874, 4881, 4889, 4926, and IC 4051. We find that the GC luminosity functions are quite similar to one another, with a turnover derived from a composite sample of more than 9,000 GCs at V = 27.71 +- 0.07 (M_V = -7.3). Both a simple Gaussian curve and an evolved Schechter function fit the bright half of the GCLF equally well, though the Coma GCLF is broader and has a higher ``cutoff mass'' (M_c ~ 3 x 10^6 M_Sun) than in any of the Virgo giants. These five Coma members exhibit a huge range in GC specific frequency, from a low of S_N = 0.7 for NGC 4881 up to 12 for IC 4051 and NGC 4874. No single formation scenario appears able to account for these differences in otherwise-similar galaxies. The supergiant NGC 4874 has the richest globular cluster system known, probably holding more than 30,000 clusters; its true extent is not yet determined and may extend well out into the Coma potential well. For the three biggest GC systems (NGC 4874, 4889, IC 4051), all three populations are dominated by red, metal-rich clusters. Their metallicity distributions also may all have the normal bimodal form, with the two sequences at the expected mean colors <V-I>(blue) = 0.98 and <V-I>(red) = 1.15. However, the color distributions and relative numbers of metal-rich clusters show intriguing counterexamples to a trend established by Peng et al. 2008 (ApJ 681, 197) for the Virgo galaxies. At the very highest-density and most massive regimes represented by the Coma supergiants, formation of metal-rich clusters seems to have been especially favored.