Abstract
Large samples of galaxy spectra of intermediate spectral resolution and high signal-to-noise ratio are now available (e.g. SDSS). Several libraries of empirical spectra of stars covering wide ranges of values of the atmospheric parameters T, log g, [Fe/H], and spectral type, have been compiled in the last few years and incorporated into population synthesis models (e.g. Bruzual & Charlot 2003, MNRAS, 344, 1000). These two facts have made possible the detailed study of galaxy spectra by means of population synthesis models to an unprecedented degree of detail. Several authors have extracted information about the star formation history in these galaxies by fitting the continuum model spectral energy distributions to the observations (e.g. Heavens et al. 2004, Nature, 428, 625; Cid-Fernandes et al. 2005, MNRAs, 358, 363; Mateu et al. 2005, in preparation). In general, these studies do not consider in detail the degree up to which the synthesis models can reproduce or not the intensity of the most conspicuous absorption lines in galaxy spectra. In this paper I explore in a systematic manner which spectral lines are well understood in terms of current population synthesis models. I use the line strength indices defined in the Lick system, complemented with our own definition of new indices (Stock et al. 2005, in preparation), to select the absorption lines which contain most information about the history of star formation in a given galaxy.