  We derive new spectral H_gamma index definitions which are robust age
indicators for old and relatively old stellar populations and thus have great
potential for solving the age-metallicity degeneracy of galaxy spectra. To
study H_gamma as a function of age, metallicity and resolution, we used a new
spectral synthesis model which predicts SEDs of single-age, single-metallicity
stellar populations at resolution FWHM=1.8A (which can be smoothed to different
resolutions), allowing direct measurements of the equivalent widths of
particular absorption features. We find that the H_gamma strong age
disentangling power strongly depends strongly on the adopted resolution and
galaxy velocity dispersion. We propose a system of indices which are completely
insensitive to metallicity and stable against resolution, allowing the study of
galaxies up to ~300 km/s. Observational spectra of very high S/N 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, in agreement with the age provided by an extraordinary fit to the full
spectrum of this galaxy that we achieve here.
