Backwards galaxy formation with only a few parameters

We present a simple phenomenological model of star formation in galaxies that describes the process with a set of a few parameters. The star formation efficiency (C eff) and the fraction of gas and metals ejected in outflows (B out) are assumed to be the main drivers of star formation. The Tully-Fisher and the Faber-Jackson relations in different passbands are used as constraints in the analysis of disk and elliptical galaxies, respectively. We find that a steep correlation between C eff and maximum rotation velocity is needed in disk systems, whereas elliptical galaxies can be explained by a high star formation efficiency, uncorrelated with the central velocity dispersion. Gas outflows are not important in disks, whereas a correlation of outflows with galaxy mass must be invoked in early-type galaxies to account for the observed colors. A simple explanation for these correlations can be given with respect to the difference in the dynamical formation histories of disks and elliptical galaxies. A phe-nomenolgical model allows us to evolve the system " backwards " in time. A significant redshift evolution of the slope of the Tully-Fisher relation is predicted, with the rest frame B-band slope (γ B = ∆ log L B /∆v rot) steepening from γ B ∼ 3 at z = 0 up to 4.5 at z ∼ 1.