Galaxy Formation Without Primordial Turbulence: Mechanisms for Generating Cosmic Vorticity

It is shown that neither the tidal interaction between protogalaxies nor turbulent dynamo action can account for the observed vorticities of spiral galaxies. Doroshkevich's suggestion that vorticity generated in shock waves might account for the rotation of spiral galaxies is then investigated. A relation is obtained for the vorticity generated in a generally curved shock front. Applying this result to a collapsing protocluster of galaxies it is found that this mechanism does indeed provide more than adequate vorticity to account for spiral galaxy formation. On the basis of this model there is a natural explanation of the tendency of spiral or elliptical galaxies to cluster together. There should also be a correlation between the relative proportions of spirals and ellipticals and the shape of a cluster itself. The application of this analysis to the Virgo Supercluster gives a different pattern of peculiar velocities for the galaxies in the solar neighbourhood from that expected on the basis of the primordial turbulence model. When more precise data on the anisotropy of the 2.7 K background becomes available, this may enable us to choose between these two theories.