Globular Cluster Systems in Brightest Cluster Galaxies: A Near-Universal Luminosity Function?

TL;DR

This study analyzes globular cluster systems in seven brightest cluster galaxies, revealing a consistent log-normal luminosity function shape across diverse environments and identifying potential ultra-compact dwarf candidates.

Contribution

First large-scale HST survey of BCG globular clusters showing a universal luminosity function shape and preliminary evidence of spatial dependence of the LF turnover point.

Findings

Luminosity function well-described by a log-normal distribution

LF shape is nearly identical across all seven galaxies

Presence of superluminous objects consistent with UCDs

Abstract

We present the first results from our HST Brightest Cluster Galaxy (BCG) survey of seven central supergiant cluster galaxies and their globular cluster (GC) systems. We measure a total of 48000 GCs in all seven galaxies, representing the largest single GC database. We find that a log-normal shape accurately matches the observed luminosity function (LF) of the GCs down to the GCLF turnover point, which is near our photometric limit. In addition, the LF has a virtually identical shape in all seven galaxies. Our data underscore the similarity in the formation mechanism of massive star clusters in diverse galactic environments. At the highest luminosities (log L > 10^7 L_Sun) we find small numbers of "superluminous" objects in five of the galaxies; their luminosity and color ranges are at least partly consistent with those of UCDs (Ultra-Compact Dwarfs). Lastly, we find preliminary evidence that in the outer halo (R > 20 kpc), the LF turnover point shows a weak dependence on projected distance, scaling as L_0 ~ R^-0.2, while the LF dispersion remains nearly constant.