THE ORIGIN OF THE MASS, DISK-TO-HALO MASS RATIO, AND L-V RELATION OF SPIRAL GALAXIES

We argue thai spiral galaxies form from virialized gas clouds in which the cooling and free-fall times are comparable. This condition can arise for a range of protogalactic masses if rapid fragmentation depletes the original gas distribution. Most of the protogalactic gas forms dark objects which add to, or constitute all of, the dark galactic halo. The remaining gas collapses to form the visible disk. The upper mass of spiral galaxies is given by the condition that all the protogalactic gas collapses to a disk, whereas the lower limit is obtained by demanding that the gaseous disk is dense enough to overcome the gravity of the halo and fragment into stars. Smaller gaseous disks are stable to fragmentation and may be associated with the Lyα absorbers. The derived mass range of spirals is in reasonable agreement with observation. One important consequence of this model is that the disk-to-halo mass ratio increases with galaxy luminosity, as is found observationally. We also show that the L-V relation of spiral galaxies can be explained in this scenario. In hierarchical clustering models, our results suggest that all spirals formed at a similar epoch.