An optical/NIR survey of globular clusters in early-type galaxies - II. Ages of globular cluster?systems and the relation to galaxy morphology

摘要

Context. Some photometric studies of extragalactic globular cluster (GC) systems using the optical and near-infrared colour combination have suggested the presence of a large fraction of intermediate-age (2?8?Gyr) GCs. Aims. We investigate the age distributions of GC?systems in 14 E/S0 galaxies. Methods. We carried out a differential comparison of the (g???z) vs. (g???K) two-colour diagrams for GC?systems in the different galaxies in order to see whether there are any indications of age differences. We also compared the different GC?systems with a few simple stellar population models. Results. No significant difference is detected in the mean ages of GCs among elliptical galaxies. S0 galaxies, on the other hand, show evidence for younger GCs. Surprisingly, this appears to be driven by the more metal-poor clusters. The age distribution of GCs in NGC?4365 seems to be similar to that of other large ellipticals (e.g. NGC?4486, NGC?4649). Padova SSPs with recently released isochrones for old ages (14?Gyr) show less of an offset with respect to the photometry than previously published models. Conclusions. We suggest that E type galaxies assembled most of their GCs in a shorter and earlier period than S0 type galaxies. The latter galaxy type seems to have a more extended period of GC?formation/assembly.