The Hβ Index and the Ages of Old Stellar Systems

Abstract

The Hβ and some metal line indices, such as Mg2, Fe52, Fe53 of single-age and single-metallicity populations are computed based on the method of evolutionary population synthesis, with careful consideration of the variation of the horizontal-branch morphology with metallicity and age. We find (a) that while metal lines are little affected, the Hβ index is severely enhanced (up to 30%) by the presence of the blue horizontal-branch stars, frustrating the current age-estimations from this index without careful consideration of these stars, and (b) that there is a systematic trend in the sense that the globular clusters in giant elliptical galaxies appear to be older than those in our Galaxy by several billion years. We also calculate these indices for the stellar populations with a metallicity spread, by adopting metallicity distribution functions predicted by chemical evolution models. The comparison of the models with the observed indices of the central regions of the early-type galaxies yields the results (a) that the ages of the giant elliptical galaxies would be older than the previous estimations by several billion years, and (b) that there is a considerable age spread among elliptical galaxies, in the sense that the giant elliptical galaxies are older than the small ones.