The stellar populations of early-type galaxies in the Fornax cluster

Abstract

We have measured central line strengths for a magnitude-limited sample of early-type galaxies in the Fornax cluster, comprising 11 elliptical (E) and 11 lenticular (S0) galaxies, more luminous than MB = -17. When compared with single-burst stellar population models we find that the centres of Fornax ellipticals follow a locus of fixed age and have metallicities varying roughly from half solar to twice solar. The centres of (lower luminosity) lenticular galaxies, however, exhibit a substantial spread to younger luminosity-weighted ages, indicating a more extended star formation history. Galaxies with old stellar populations show tight scaling relations between metal-line indices and the central velocity dispersion. Remarkably also, the Fe lines are well correlated with σ0. Our detailed analysis of the stellar populations suggests that these scaling relations are driven mostly by metallicity. Galaxies with a young stellar component do generally deviate from the main relation. In particular, the lower luminosity S0s show a large spread. Our conclusions are based on several age/metallicity diagnostic diagrams in the Lick/IDS system comprising established indices such as Mg2 and Hβ as well as new and more sensitive indices such as HγA and Fe3, a combination of three prominent Fe lines. The inferred difference in the age distribution between lenticular and elliptical galaxies is a robust conclusion, as the models generate consistent relative ages using different age and metallicity indicators, even though the absolute ages remain uncertain. The absolute age uncertainty is mainly caused by the effects of non-solar abundance ratios which are not yet accounted for by the stellar population models. Furthermore, we find that elliptical galaxies and the bulge of one bright S0 are overabundant in magnesium, where the most luminous galaxies show the strongest overabundances. The stellar populations of young and faint S0s are consistent with solar abundance ratios or a weak Mg underabundance. Two of the faintest lenticular galaxies in our sample have blue continua and extremely strong Balmer-line absorption, suggesting star formation <2Gyr ago.