VLT Spectroscopy of Globular Cluster Systems, II. Spectroscopic Ages, Metallicities, and [alpha/Fe] Ratios of Globular Clusters in Early-Type Galaxies

Abstract

An analysis of ages, metallicities, and [alpha/Fe] ratios of globular cluster systems in early-type galaxies is presented, based on Lick index measurements summarized in Puzia et al. (2004, Paper I of this series). In the light of calibration and measurement uncertainties, age-metallicity degeneracy, and the relative dynamic range of Lick indices, as well as systematics introduced by abundance ratio variations (in particular variations of [alpha/Fe] ratios), we find that the most reliable age indicator for our dataset is a combination of the Lick Balmer-line indices HgammaA, Hbeta, and HdeltaA. [MgFe]' is used as a spectroscopic metallicity indicator which is least affected by [alpha/Fe] variations. We introduce an interpolation routine to simultaneously derive ages, metallicities, and [alpha/Fe] ratios from diagnostic grids constructed from Lick indices. From a comparison of high-quality data with SSP model predictions, we find that ~2/3 of the globular clusters in early-type galaxies are older than 10 Gyr, up to 1/3 have ages in the range ~5-10 Gyr, and only a few cluster are younger than ~5 Gyr. Our sample of globular clusters covers metallicities from [Z/H] = -1.3 up to ~0.5 dex. We find that metal-rich globular clusters show on average a smaller mean age and a larger age scatter than their metal-poor counterparts. [alpha/Fe] diagnostic plots show that globular cluster systems in early-type galaxies have super-solar alpha/Fe abundance ratios with a mean [alpha/Fe] = 0.47+/-0.06 dex and a dispersion of about 0.3 dex. We find evidence for a correlation between [alpha/Fe] and metallicity, in the sense that more metal-rich clusters exhibit lower alpha-element enhancements. [abridged]