Globular Cluster Populations: First Results from S4G Early-type Galaxies

Using 3.6 μm images of 97 early-type galaxies, we develop and verify methodology to measure globular cluster populations from the S⁴G survey images. We find that (1) the ratio, T _N, of the number of clusters, N _CL, to parent galaxy stellar mass, M _*, rises weakly with M _* for early-type galaxies with M _* > 10¹⁰ M _⊙ when we calculate galaxy masses using a universal stellar initial mass function (IMF) but that the dependence of T _N on M _* is removed entirely once we correct for the recently uncovered systematic variation of IMF with M _*; and (2) for M _* < 10¹⁰ M _⊙, there is no trend between N _CL and M _*, the scatter in T _N is significantly larger (approaching two orders of magnitude), and there is evidence to support a previous, independent suggestion of two families of galaxies. The behavior of N _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 ~0.0014 for M _* > 10¹⁰ M _⊙ and can be as large as ~0.02 for less massive galaxies. These are the first results from the S⁴G sample of galaxies and will be enhanced by the sample of early-type galaxies now being added to S⁴G and complemented by the study of later-type galaxies within S⁴G.