Astronomy and Astrophysics, volume 401, 429-432 (2003/4-2)
The impact of α/Fe enhanced stellar evolutionary tracks on the ages of elliptical galaxies.
THOMAS D. and MARASTON C.
Abstract (from CDS):
We complement our study of α/Fe enhanced stellar population models of Lick absorption indices (Thomas et al., 2003MNRAS.339..897T) by comparing two sets of α/Fe enhanced models. In both models the impact on Lick indices due to α/Fe enhancement is accounted for through a modification of the stellar absorption line-strengths using the response functions of Tripicco & Bell (1995AJ....110.3035T). One set of models, however, uses solar-scaled, the other α/Fe enhanced stellar evolutionary tracks. Since the α/Fe enhanced tracks are hotter than the solar-scaled ones (Salasnich et al., 2000A&A...361.1023S), the correspondent stellar population models have slightly weaker metallic indices (i.e. Mg b, <Fe> etc.) and stronger Balmer line indices (Hβ) (Maraston et al., 2003A&A...400..823M). Here we explore quantitatively the impact of this effect on the α/Fe ratios, metallicities and ages that are derived for elliptical galaxies. We find that the modest decrease of the metallic indices Mg b and <Fe> balance each other, such that fully consistent α/Fe ratios are derived for stellar systems using α/Fe enhanced models with either solar-scaled or α/Fe enhanced stellar tracks. The decrease of the metallic indices and the increase of Hβ conspire in a way that also consistent metallicities are obtained. The derived ages, instead, are significantly different. The inclusion of α/Fe enhanced stellar tracks leads to the derivation of ages as high as ∼30Gyr for elliptical galaxies. For the same objects, ages not older than 15Gyr are obtained, if α/Fe enhanced models using solar-scaled tracks are adopted. This may indicate that current stellar evolutionary models overestimate the bluing of stellar evolutionary tracks due to α/Fe enhanced chemical mixtures at super-solar metallicities.
stars: evolution - stars: abundances - galaxies: stellar content - galaxies: elliptical and lenticular, cD - galaxies: formation