The age structure of globular cluster systems in early-type galaxies and its dependence on host galaxy properties

Aims.We present the results of an optical – near-infrared photometric survey of globular cluster systems in early-type galaxies. Our project aims to study correlations between the age structure of the globular cluster systems and environmental as well as structural parameters of their host galaxies. Methods.To derive the age structure in globular cluster systems we use combined optical and near-infrared photometry. This combination facilitates the detection of young globular cluster sub-populations if their age differs significantly from the old (~13 Gyr) population, i.e. if their age is $\la$7 Gyr. We investigate globular cluster systems in galaxies located in the central regions of galaxy clusters (NGC 1399, M 87, NGC 4478), galaxies which are members of (sub-) groups (NGC 4365, NGC 5846) and isolated galaxies (NGC 3115 and NGC 7192). Further, we look for correlations between globular cluster system properties and structural parameters of the galaxies (velocity dispersion σ, color ($(V-K), (B-V)$), and luminosity MV). For a quantitative analysis we define the “Methuselah” parameter, ${\tt Mth}$ – based on the fraction of young and old metal-rich globular clusters weighted by age, characterizing the age structure of a globular cluster system. Results.We find that, within our galaxy sample, group galaxies appear to host an intermediate-age globular cluster population, whereas the globular cluster systems in cluster galaxies and in rather isolated galaxies are dominated by an old globular cluster population. A weak correlation of ${\tt Mth}$ with the Hβ index of the integrated light of the host galaxies is the only trend found with respect to internal galaxy parameters