Synthetic metal line indices for elliptical galaxies from super metal rich alpha-enhanced stellar models

Abstract

There are strong indications from recent papers (e.g. Worthey et al. 1992) that the abundance ratio of Mg/Fe, and consequently also O/Fe in giant elliptical galaxies is not solar. The line strengths of two Fe lines at 5270 and 5335 A are weaker than one expects from the strength of the Mg b line if [Mg/Fe] = 0. We have synthesized absorption line indices to derive the Mg and Fe abundances of these galaxies. For these models we have calculated new evolutionary tracks of high metallicity stars with a range of Mg/Fe abundances. This is the first time that such tracks have been generated. Integrating along isochrones to synthesize metal line strengths we find that for a typical bright giant elliptical [Mg/Fe] has to be between +0.3 and +0.7. We show that this result is independent of other parameters such as age, total metal content and mixing length parameter. The total metal content is super-solar, but the iron metallicity of elliptical galaxies not necessarily has to be larger than solar. For the formation of elliptical galaxies our result on the Mg and Fe abundances has the implication that most of the enrichment of the gas has to come from SNe II, which have more massive progenitors and as such produce relatively more O and Mg than Fe. It means that most of the stars have to be formed within a period of $3 \times 10^8$ years, so that there can only be one major collapse phase of the galaxy.