On the star formation rate, initial mass function, and Hubble type of disk galaxies and the age of the universe

Evolutionary models for the disks of large disk galaxies, including effects of star formation, noninstantaneous gas recycling from stars, and infall of low-metallicity gas from the halo, have been calculated and compared with data for nearby, generally large disk galaxies on present disk star-formation rates (based on integrated H alpha luminosities) as a function of disk gas fractions. The data were extracted from the work by Kennicutt, Tamblyn, & Congdon. The result of the comparison suggests that for disk galaxies the Hubble sequence is a disk age sequence, with early-type disks being the oldest and late types the youngest. 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 universe, 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 function (IMF) in galactic disks is 2-3 times more weighted toward high-mass stars than the Scale ''best-fitting'' model for the solar-neighborhood IMF. The more top-heavy model of Kennicutt provides a good fit to observation.