Star Formation Histories of Early-Type Galaxies. I. Higher Order Balmer Lines as Age Indicators

(shortened) We have obtained blue integrated spectra of 175 nearby early-typegalaxies, covering a wide range in galaxy velocity dispersion, and emphasizingthose with sigma < 100 km/s. Galaxies have been observed both in the Virgocluster and in lower-density environments. The main goals are the evaluation ofhigher order Balmer lines as age indicators, and differences in stellarpopulations as a function of mass, environment and morphology. In this firstpaper our emphasis is on presenting the evolutionary population synthesismodels. Lower-sigma galaxies exhibit a substantially greater intrinsic scatter,in a variety of line strength indicators, than do higher-sigma galaxies, withthe large intrinsic scatter setting in below a sigma of 100 km/s. Modeling ofthe observed spectral indices indicates that the strong Balmer lines foundprimarily among the low-sigma galaxies are caused by young age, rather than bylow metallicity. Thus we find a trend between the population age and thevelocity dispersion, such that low sigma galaxies have youngerluminosity-weighted mean ages. We have repeated this analysis using severaldifferent Balmer lines, and find consistent results from one spectral indicatorto another.Comment: To appear in AJ. Full version of Table 3 is available as a separate fil