Semi-empirical analysis of Sloan Digital Sky Survey galaxies: II. The bimodality of the galaxy population revisited

We revisit the bimodal distribution of the galaxy population commonly seen in the local Universe. Here we address the bimodality observed in galaxy properties in terms of spectral synthesis products, such as mean stellar ages and stellar masses, derived from the application of this powerful method to a volume-limited sample of about 50 thousand luminous galaxies from the SDSS Data Release 2. In addition, galaxies are classified according to their emission line properties in three distinct spectral classes: star-forming galaxies, with young stellar populations; passive galaxies, dominated by old stellar populations; and, hosts of active nuclei, which comprise a mix of young and old stellar populations. We show that the extremes of the distribution of some galaxy properties are associated to star-forming galaxies at one side, and passive galaxies at another. We found that the mean light-weighted stellar age of galaxies is the direct responsible to the bimodality seen in the galaxy population, and that stellar mass is a coadjuvant in the sense that only less massive galaxies are currently forming stars. Our results also give support to the existence of a `downsizing' in galaxy formation, where massive galaxies seen nowadays have stellar populations formed at early times