An optical/NIR survey of globular clusters in early-type galaxies

II. Ages of globular cluster systems and the relation to galaxy morphology

¹ Astronomical Institute, University of Utrecht, Princetonplein 5, 3584 Utrecht, The Netherlands
e-mail: A.L.Chies@uu.nl
² Department of Physics and Astronomy, McMaster University, Hamilton L8S 4M1, Canada
³ Space Telescope European Coordinating Facility, European Southern Observatory, Garching, Germany
⁴ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
⁵ UCO/Lick Observatory, University of California, Santa Cruz, CA 95064, USA
⁶ Departamento de Física, ICEx, Universidade Federal de Minas Gerais, Av. Antônio Carlos 6627, Belo Horizonte 31270-901, MG, Brazil

Received: 2 September 2010
Accepted: 21 October 2010

Abstract

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.