VLT spectroscopy of globular cluster systems

II. Spectroscopic ages, metallicities, and [ /Fe] ratios of globular clusters in early-type galaxies

¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA e-mail: [tpuzia;goudfroo]@stsci.edu
² Sternwarte der Ludwig-Maximilians-Universität, Scheinerstr. 1, 81679 München, Germany
³ European Southern Observatory, 85749 Garching bei München, Germany e-mail: [mkissler;hempel]@eso.org
⁴ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching bei München, Germany e-mail: [bender;saglia]@mpe.mpg.de
⁵ University of Oxford, Astrophysics, Keble Road, Oxford, OX1 3RH, UK e-mail: [dthomas;maraston]@astro.ox.ac.uk

Received: 5 January 2004
Accepted: 13 May 2005

Abstract

An analysis of ages, metallicities, and [ α/Fe]  ratios of globular cluster systems in early-type galaxies is presented, based on Lick index measurements summarized in Puzia et al. (2004, A&A, 415, 123, 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 [ α/Fe]  ratios), we find that the most reliable age indicator for our dataset is a combination of the Lick Balmer-line indices H, Hβ, and H. [MgFe]´ is used as a spectroscopic metallicity indicator which is least affected by [ α/Fe]  variations. We introduce an interpolation routine to simultaneously derive ages, metallicities, and [ α/Fe]  ratios from diagnostic grids constructed from Lick indices. From a comparison of high-quality data with SSP model predictions, we find that ~ of the globular clusters in early-type galaxies are older than 10 Gyr, up to have ages in the range ~ 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. [ α/Fe]  diagnostic plots show that globular cluster systems in early-type galaxies have super-solar α/Fe abundance ratios with a mean [ α/Fe]  = 0.47 ± 0.06 dex and a dispersion of ~0.3 dex. We find evidence for a correlation between [ α/Fe] and metallicity, in the sense that more metal-rich clusters exhibit lower α-element enhancements. A discussion of systematics related to the Lick index system shows that the method suffers to some extent from uncertainties due to unknown horizontal branch morphologies at high metallicities. However, these systematics still allow us to make good qualitative statements. A detailed investigation of indices as a function of data quality reveals that the scatter in Balmer index values decreases for higher-quality data. In particular, extremely low Balmer index values that are lower than any SSP model prediction tend to disappear. Furthermore, we find that observed photometric colors are in good agreement with computed SSP colors using ages and metallicities as derived from the spectroscopic line indices.