Exploring the star formation history of elliptical galaxies: beyond simple stellar populations with a new line strength estimator

We study the stellar populations of a sample of 14 elliptical galaxies in the Virgo cluster. Using spectra with high signal-to-noise ratio (S/N greater than or similar to 100 A-1) we propose an alternative approach to the standard side-band method to measure equivalent widths (EWs). Our boosted median continuum is shown to map the EWs more robustly than the side-band method, minimizing the effect from neighbouring absorption lines and reducing the uncertainty at a given S/N. Our newly defined line strengths are more successful at disentangling the age-metallicity degeneracy. We concentrate on Balmer lines (H beta, H gamma, H delta), the G band and the combination [MgFe] as the main age and metallicity indicators. We go beyond the standard comparison of the observations with simple stellar populations (SSPs) and consider four different models to describe the star formation histories, either with a continuous star formation rate or with a mixture of two different SSPs. These models improve the estimates of the more physically meaningful mass-weighted ages. Composite models are found to give more consistent fits among individual line strengths and agree with an independent estimate using the spectral energy distribution. A combination of age- and metallicity-sensitive spectral features allows us to constrain the average age and metallicity. For a Virgo sample of elliptical galaxies our age and metallicity estimates correlate well with stellar mass or velocity dispersion, with a significant threshold around 5 x 1010 M(circle dot) above which galaxies are uniformly old and metal rich. This threshold is reminiscent of the one found by Kauffmann et al. in the general population of Sloan Digital Sky Survey galaxies at a stellar mass 3 x 1010 M(circle dot). In a more speculative way, our models suggest that it is formation epoch and not formation time-scale what drives the mass-age relationship of elliptical galaxies.