ON THE STAR-FORMATION RATE, INITIAL MASS FUNCTION, AND HUBBLE TYPE OFDISK GALAXIES AND THE AGE OF THE UNIVERSE

Evolutionary models for the disks of large disk galaxies, including ef fects of star formation, noninstantaneous gas recycling from stars, an d infall of low-metallicity gas from the halo, have been calculated an d compared with data for nearby, generally large disk galaxies on pres ent disk star-formation rates (based on integrated H alpha luminositie s) as a function of disk gas fractions. The data were extracted from t he work by Kennicutt, Tamblyn, & Congdon. The result of the comparison suggests that for disk galaxies the Hubble sequence is a disk age seq uence, with early-type disks being the oldest and late types the young est. Under the assumption of a minimum age of the Galactic disk of 10 Gyr, the mean age of Sa/Sab galaxies, and hence the age of the univers e, is found to be at least 17 +/- 2 Gyr. It is furthermore found that the disk star-formation timescale is approximately independent of disk -galaxy type. Finally, it is found that the global initial mass functi on (IMF) in galactic disks is 2-3 times more weighted toward high-mass stars than the Scale ''best-fitting'' model for the solar-neighborhoo d IMF. The more top-heavy model of Kennicutt provides a good fit to ob servation.