A Robust Age Indicator for Old Stellar Populations

We derive new spectral H_gamma index definitions which are robust ageindicators for old and relatively old stellar populations and thus have greatpotential for solving the age-metallicity degeneracy of galaxy spectra. Tostudy H_gamma as a function of age, metallicity and resolution, we used a newspectral synthesis model which predicts SEDs of single-age, single-metallicitystellar populations at resolution FWHM=1.8A (which can be smoothed to differentresolutions), allowing direct measurements of the equivalent widths ofparticular absorption features. We find that the H_gamma strong agedisentangling power strongly depends strongly on the adopted resolution andgalaxy velocity dispersion. We propose a system of indices which are completelyinsensitive to metallicity and stable against resolution, allowing the study ofgalaxies up to ~300 km/s. Observational spectra of very high S/N and relativelyhigh dispersion, are required to gain this unprecedented age discriminatingpower. Once such spectra are obtained, accurate and reliable estimates for theluminosity-weighted average stellar ages of these galaxies will become possiblefor the first time, without assessing their metallicities. We measured thisindex for two globular clusters, a number of low-luminosity elliptical galaxiesand a standard S0 galaxy. We find a large spread in the average stellar ages ofa sample of low-luminosity ellipticals. In particular these indices yield 4 Gyrfor M32, in agreement with the age provided by an extraordinary fit to the fullspectrum of this galaxy that we achieve here.