A Robust Age Indicator for Old Stellar Populations

We derive new spectral H γ index definitions which 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 which predicts spectral energy distributions of single-age, single-metallicity stellar populations at resolution FWHM ∼ 1 . 8˚A (which can be smoothed to different resolutions), allowing direct measurements of the equivalent widths of particular absorption features. We show that H γ strong age disentangling power is due to a compensating effect: at specified age, H γ strengthens with metallicity due 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 γ . Despite the fact that this effect depends strongly on the adopted resolution and galaxy velocity dispersion σ , we propose a system of indicators which are completely insensitive to metallicity and stable against resolution, allowing the study of galaxies up to σ ∼ 300 kms − 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 M 32. This value is in excellent agreement with the age provided by an extraordinary fit to the full spectrum of this galaxy that we achieve in this paper.