An optical/NIR survey of globular clusters in early-type galaxies II. Ages of GC 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 suggestedthe presence of a large fraction of intermediate-age (2-8 Gyrs) GCs. Aims: Weinvestigate the age distributions of GC systems in 14 E/S0 galaxies. Methods:We carry out a differential comparison of the (g-z) vs. (g-K) two-colourdiagrams for GC systems in the different galaxies in order to see whether thereare indications of age differences. We also compare the different GC systemswith a few simple stellar population models. Results: No significant differenceis detected in the mean ages of GCs among elliptical galaxies. S0 galaxies onthe other hand, show evidence for younger GCs. Surprisingly, this appears to bedriven by the more metal-poor clusters. The age distribution of GCs in NGC4365seems to be similar to that of other large ellipticals (e.g. NGC4486, NGC4649).Padova SSPs with recently released isochrones for old ages (14 Gyrs) show lessof an offset with respect to the photometry than previously published models.Conclusions: We suggest that E type galaxies assembled most of their GCs in ashorter and earlier period than S0 type galaxies. The latter galaxy type, seemsto have a more extended period of GC formation/assembly.