The Globular Cluster Systems of the Early-Type Galaxies NGC 3379, NGC 4406, and NGC 4594 and Implications for Galaxy Formation

摘要

We have investigated the global properties of the globular cluster (GC) systems of three early-type galaxies: the Virgo Cluster elliptical NGC 4406, the field elliptical NGC 3379, and the field S0 galaxy NGC 4594. These galaxies were observed as part of a wide-field CCD survey of the GC populations of a large sample of normal galaxies beyond the Local Group. Images obtained with the Mosaic detector on the Kitt Peak 4 m telescope provide radial coverage to at least 24', or ~70–100 kpc. We use BVR photometry and image classification to select GC candidates and thereby reduce contamination from non-GCs, as well as Hubble Space Telescope WFPC2 data to help quantify the contamination that remains. The GC systems of all three galaxies have color distributions with at least two peaks and show modest negative color gradients. The proportions of blue GCs range from 60% to 70% of the total populations. The GC specific frequency (SN) of NGC 4406 is 3.5 ± 0.5, ~20% lower than past estimates and nearly identical to SN for the other Virgo Cluster elliptical included in our survey, NGC 4472. SN for NGC 3379 and NGC 4594 are 1.2 ± 0.3 and 2.1 ± 0.3, respectively; these are similar to past values, but the errors have been reduced by a factor of 2–3. We compare our results for the early-type sample (including NGC 4472) to models for the formation of massive galaxies and their GC systems. Of the scenarios we consider, a hierarchical merging picture, in which metal-poor GCs form at high redshift in protogalactic building blocks and metal-rich GC populations are built up over time during subsequent gas-rich mergers, appears most consistent with the data.