A robust age indicator for old stellar populations

We derive new spectral H gamma index definitions that are robust age indicators for old and relatively old stellar populations and therefore have great potential for solving the age-metallicity degeneracy of galaxy spectra. To study this feature as a function of age, metallicity, and resolution, we have used a new spectral synthesis model that predicts spectral energy distributions of single-age, single-metallicity stellar populations at resolution FWHM similar to 1.8 Angstrom (which can be smoothed to different resolutions), allowing direct measurements of the equivalent widths of particular absorption features. We show that H gamma's strong age-disentangling power is due to a compensating effect: at a specified age, H gamma strengthens with metallicity owing to an adjacent metallic absorption, but on the other hand, the adopted pseudocontinua are depressed by the effects of strong neighboring Fe I lines on both sides of H gamma. Despite the fact that this effect depends strongly on the adopted resolution and galaxy velocity dispersion sigma, we propose a system of indicators that are completely insensitive to metallicity and stable against resolution, allowing the study of galaxies up to sigma similar to 300 km s(-1). An extensive analysis of the characteristics of these indices indicates that observational spectra of very high signal-to-noise ratio and relatively high dispersion are required to gain this unprecedented age-discriminating power. Once such spectra are obtained, accurate and reliable estimates for the luminosity-weighted average stellar ages of these galaxies will become possible for the first time, without assessing their metallicities. We measured this index for two globular clusters, a number of low-luminosity elliptical galaxies, and a standard S0 galaxy. We find a large spread in the average stellar ages of a sample of low-luminosity ellipticals. In particular, these indices yield 4 Gyr for M32. This value is in excellent agreement with the age provided by the extraordinary fit to the full spectrum of this galaxy that we achieve in this paper.