Globular Cluster Populations: First Results from S$^4$G Early-Type Galaxies

TL;DR

This study develops a method to measure globular cluster populations in early-type galaxies using 3.6μm images, revealing how cluster-to-galaxy mass ratios vary with galaxy mass and IMF corrections, and identifying two galaxy families.

Contribution

First to measure globular cluster populations in early-type galaxies from S$^4$G images, analyzing their relation to galaxy mass and IMF variations.

Findings

T_N ratio increases weakly with M_* for massive galaxies when using a universal IMF.

T_N ratio dependence on M_* disappears after IMF correction.

In low-mass galaxies, N_CL shows no trend and high scatter, indicating diverse cluster formation efficiencies.

Abstract

Using 3.6$\mu$m images of 97 early-type galaxies, we develop and verify methodology to measure globular cluster populations from the S$^4$G survey images. We find that 1) the ratio, T$_{\rm N}$, of the number of clusters, N$_{\rm CL}$, to parent galaxy stellar mass, M$_*$, rises weakly with M$_*$ for early-type galaxies with M$_* > 10^{10}$ M$_\odot$ when we calculate galaxy masses using a universal stellar initial mass function (IMF), but that the dependence of T$_{\rm N}$ on M$_*$ is removed entirely once we correct for the recently uncovered systematic variation of IMF with M$_*$, and 2) for M$_* < 10^{10}$ M$_\odot$ there is no trend between N$_{\rm CL}$ and M$_*$, the scatter in T$_{\rm N}$ is significantly larger (approaching 2 orders of magnitude), and there is evidence to support a previous, independent suggestion of two families of galaxies. The behavior of N$_{\rm CL}$ in the lower mass systems is more difficult to measure because these systems are inherently cluster poor, but our results may add to previous evidence that large variations in cluster formation and destruction efficiencies are to be found among low mass galaxies. The average fraction of stellar mass in clusters is $\sim$ 0.0014 for M$_* > 10^{10}$ M$_\odot$ and can be as large as $\sim 0.02$ for less massive galaxies. These are the first results from the S$^4$G sample of galaxies, and will be enhanced by the sample of early-type galaxies now being added to S$^4$G and complemented by the study of later type galaxies within S$^4$G.