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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, with magnitude limit cutoff M_r = -20.5, containing
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,
essentially galaxy colours, 4000 A break index, and mean stellar ages, are
associated to star-forming galaxies at one side, and passive galaxies at
another. We find that the mean light-weighted stellar age of galaxies is the
direct responsible for the bimodality seen in the galaxy population. The
stellar mass, in this view, has an additional role since most of the
star-forming galaxies present in the local universe are low-mass galaxies. 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.Comment: 18 pages, 19 figures, accepted for publication in MNRA
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