The Collapse and Formation of Galaxies. III. The Origin of the Hubble Sequence

We compare observations of galaxies to previously reported simulations of galaxy formation to determine the physical scaling of the control parameters. In the simulations we found that the initial velocity dispersion of the dark matter is the main factor determining whether the final state of the luminous galaxy is a disk or a spheroid. "Warm" collapses lead to disk galaxies, while "cold" collapses produce spheroidal galaxies. The borderline between "warm'' and "cold" collapse was a velocity dispersion, measured at the time of turnaround, of 20% of the final virial value. We consider the simple proposal that velocity dispersion of the dark matter at turnaround was nearly independent of mass and that the Hubble sequence arises as a "virial velocity sequence." We find ample evidence for this proposal. The observed difference in virial velocities of L_*_ elliptical and disk galaxies, the velocity dispersion of the dark matter at the time of turnaround was ~35 km s^-1^. We then use the observed central phase densities of dark halos to determine the velocity dispersion at turnaround and find a value consistent with 35 km s^-1^.