An interpretation of the line-strength indexes in old stellar populations using an evolutionary synthesis approach

Abstract

Evolutionary population synthesis models with dif­ferent metallicities have been computed in order to interpret the observed Mg_2 and Hβ, line-strength indices in old stellar popu­lations. Both indices are found to be quite insensitive to changes in the slope of the IMF and the upper mass limit. These models have been applied to three different cases, with the following results: a) Galactic globular clusters do not exhibit a significant dis­persion in age, and the metallicities of the computed models resemble those estimated for the clusters. The indices for the metal-poor globulars ([Fe/H] < -1) cannot be attained with these models due to the lack of low metallicity evolutionary tracks and stellar spectra libraries. b) The line-strength gradients observed in the elliptical gal­axy NGC 5813 are due, essentially, to intrinsic variations in metallicity, and they cannot be explained just from changes in the remaining parameters of the stellar population. c) In order to synthesize the M 32 indices we must introduce a star formation elapsed for a long time-scale , the star forrnation being still significant ≈ 5 Gyr ago.