Extragalactic Globular Cluster Systems

Abstract

We have conducted a comprehensive study of ages, metallicities, and [alpha/Fe] ratios of extragalactic globular clusters in elliptical, lenticular, and spiral galaxies based on the Lick index system. As a first step, Lick indices were measured for metal-rich Galactic globular clusters which most closely resemble the high-metallicity stellar populations in early-type galaxies. In the following, these Milky Way globular clusters serve as fundamental ingredients of an empirical calibration of synthetic Lick indices (e.g. Mg, Fe, all Balmer indices, etc.) of Simple Stellar Population models that extend up to solar metallicity, for the first time. The model predictions are used to derive ages, metallicities, and [alpha/Fe] of extragalactic globular clusters. In extragalactic globular cluster systems of early-type galaxies we find an increasing formal age dispersion towards higher metallicities. Metal-rich ([Z/H] > +0.8 dex) globular clusters in early-type galaxies show lower mean ages t ~ 6 Gyr than their metal-poor counterparts with a mean age t ~ 11 Gyr. A universal lower bound for globular cluster metallicities is found for all studied galaxies at [Z/H] -2.3 dex. Mean [alpha/Fe] ratios of globular clusters in early-type galaxies have super-solar values (~0.44 dex). There is evidence for an [alpha/Fe] age relation in globular cluster systems of early-type galaxies, in the sense that younger objects have lower [alpha/Fe] ratios. We find a formal offset of 0.3 dex between globular clusters formally younger than 5 Gyr and the old globular cluster sub-population. A comparison of globular cluster systems in galaxies of different morphological type reveals that spiral galaxies host significantly older cluster systems (t ~ 9.3 Gyr) than lenticular and elliptical galaxies (both t ~ 7.7 Gyr). The highest metallicities reached by globular clusters in spiral galaxies are around solar while those for globular clusters in elliptical galaxies reach 2 - 3 times the solar metallicity. Mean [alpha/Fe] ratios for globular clusters in spiral and lenticular galaxies are found around solar, while globular clusters in elliptical galaxies are clearly enhanced in [alpha/Fe] reaching 0.4 dex. This indicates different formation timescales/scenarios for globular cluster system in elliptical on the one hand and spiral and lenticular galaxies, on the other hand. A significant 0.2 dex offset in mean [alpha/Fe] between globular cluster systems in field and group/cluster galaxies is found. The latter have higher mean [alpha/Fe] ratios around 0.4 dex which seems to be driven by the metal-poor globular cluster sub-population.