The impact of alpha/Fe enhanced stellar evolutionary tracks on the ages of elliptical galaxies

We complement our study of α/Fe enhanced stellar population models of Lick absorption indices (Thomas et al. 2003) 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 (1995). 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. 2000), the correspondent stellar population models have slightly weaker metallic indices (i.e. Mgb, <Fe> etc.) and stronger Balmer line indices (Hβ) (Maraston et al. 2003). 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 H 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 ∼30 Gyr for elliptical galaxies. For the same objects, ages not older than 15 Gyr 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.