Extragalactic globular clusters in the near-infrared

V. IC 4051 and NGC 3311

¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany e-mail: mhempel@eso.org
² Michigan State University, Department of Physics and Astronomy, East Lansing, 48824 MI, USA e-mail: hempel@pa.msu.edu
³ Departamento de Física, Facultad de Ciencias Físicas y Matemáticas, Universidad de Concepción, Casilla 160-C, Concepción, Chile e-mail: doug@kukita.cfm.udec.cl
⁴ Spitzer Science Center, California Institute of Technology, 220-6 Pasadena, CA 91125, USA e-mail: hoard@ipac.caltech.edu
⁵ Department of Physics and Astronomy, McMaster University, Hamilton, Ontario L8S 4M1, Canada e-mail: harris@physics.mcmasters.ca

Received: 18 August 2004
Accepted: 28 April 2005

Abstract

We present the results of combined optical and near-infrared photometry for the globular cluster systems of the giant ellipticals IC 4051 and NGC 3311. We use the reduced age-metallicity degeneracy in  vs.  color-color diagrams to derive the cumulative age distribution within the red sub-population of globular clusters and to search for age sub-populations. The age distribution is then compared to the one determined for simulated globular cluster systems in order to set constraints on the relative age and size of these globular cluster sub-populations. In both galaxies we find a significant fraction of globular clusters with ages between 2-5 Gyr. We also investigate the metallicity distribution in both systems. Small number statistics prevent us from making any definite statements concerning NGC 3311, but we find that the derived metallicity distribution of the IC 4051 clusters strongly depends on the assumed age distribution. Based on our most likely result that finds a large number of young/intermediate age clusters (~2 Gyr) within the selected globular cluster sample, we find metallicity peaks at ~-0.2 for the old clusters and +0.8 for the young clusters. Only few very metal poor clusters are found. However, the metallicity distribution within the young/intermediate globular cluster population is significantly affected by our choice of the applied Single Stellar Population model. The mean metallicity of the second generation of globular clusters changes from the above mentioned and extremely high +0.8 dex to +0.2 dex. Note that the model dependency becomes less severe with an increasing age of the cluster population.