A Backwards Approach to the Formation of Disk Galaxies I. Stellar and Gas Content

A simple chemical enrichment code is described where the two basic mechanisms driving the evolution of the ages and metallicities of the stellar populations are the star formation efficiency and the fraction of gas ejected from the galaxy. Using the observed Tully-Fisher relation in different passbands as a constraint, it is found that a steep correlation between the maximum disk rotational velocity (vROT) and star formation efficiency (Ceff) must exist — Ceff ∝ vROT — either for a linear or a quadratic Schmidt law. Outflows do not play a major role. This result is in contrast with what we have found for early-type systems, where the Faber-Jackson constraint in different bands allows a significant range of outflows and requires a large star formation efficiency regardless of galaxy mass. The extremely low efficiencies found at low masses translate into a large spread in the distribution of stellar ages in these systems, as well as a large gas mass fraction independently of the star formation law. The model predictions are consistent with the star formation rates in low-mass local galaxies. However, our predictions for gas mass are in apparent conflict with the estimates of atomic hydrogen content observed through the flux of the 21cm line of H I. The presence of large masses of cold molecular hydrogen — especially in systems with low mass and metallicity — is predicted, up to ratios M(H2)/M(HI)∼ 4, in agreement with a recent tentative detection of warm H2. The redshift evolution of disk galaxies is explored, showing that a significant change in the slope of the Tully-Fisher relation (L ∝ vγROT) is expected because of the different age distributions of the stellar components in high and low-mass disk galaxies. The slope measured in the rest frame B,K-bands is found to change from γB ∼ 3,γK ∼ 4 at z = 0 up to ∼ 4.5,5 at z ∼ 1, with a slight dependence on formation redshift. Subject headings: galaxies: evolution — galaxies: formation — galaxies: spiral — galaxies: stellar content — galaxies: fundamental parameters.