Strong Balmer Lines in Old Stellar Populations: No Need for Young Ages in Ellipticals?

Abstract

Comparing models of Simple Stellar Populations (SSPs) with observed line strengths generally provides a tool for breaking the age-metallicity degeneracy in elliptical galaxies. Because of the wide range of Balmer line strengths observed, ellipticals have been interpreted as exhibiting an appreciable scatter in age. In this paper, we analyze Composite Stellar Population models with a simple mix of an old metal-rich and an old metal-poor component. We show that these models simultaneously produce strong Balmer lines and strong metallic lines without invoking a young population. The key to this result is that our models are based on SSPs that better match the steep increase of Hβ in metal-poor globular clusters than models in the literature. Hence, the scatter of Hβ observed in cluster and luminous field elliptical galaxies can be explained by a spread in the metallicity of old stellar populations. We check our model with respect to the so-called G-dwarf problem in ellipticals. For a galaxy subsample covering a large range in 1500-V colors, we demonstrate that the addition of an old metal-poor subcomponent does not invalidate other observational constraints like colors and the flux in the mid-UV.