Star formation histories of early-type galaxies. I. Higher order Balmer lines as age indicators

We have obtained blue integrated spectra of 175 nearby early-type galaxies, covering a wide range in galaxy velocity dispersion and emphasizing those with sigma<100 km s(-1). Galaxies have been observed both in the Virgo Cluster and in lower density environments. The main goals are the evaluation of higher order Balmer lines as age indicators and differences in stellar populations as a function of mass, environment, and morphology. In this first paper, our emphasis is on presenting the methods used to characterize the behavior of the Balmer lines through evolutionary population synthesis models. Lower σ galaxies exhibit a substantially greater intrinsic scatter, in a variety of line-strength indicators, than do higher σ galaxies, with the large intrinsic scatter setting in below a σ of 100 km s(-1). Moreover, a greater contrast in scatter is present in the Balmer lines than in the lines of metal features. Evolutionary synthesis modeling of the observed spectral indexes indicates that the strong Balmer lines found primarily among the low-σ galaxies are caused by young age, rather than by low metallicity. Thus we find a trend between the population age and the central velocity dispersion, such that low-σ galaxies have younger luminosity-weighted mean ages. We have repeated this analysis using several different Balmer lines and find consistent results from one spectral indicator to another.