Shock waves and vorticity generation in protogalactic medium

Abstract

A theory of secondary vorticity is suggested on grounds of an isotropic Hot Big Bang. Vortical motions are generated in front of the shock waves, which appear as a result of supersonic hydrodynamic processes induced by gravitational instability at a later epoch of cosmological expansion. Inside large layers of gas compressed by shocks subsonic turbulence with both vortical and acoustic modes develops. Every such layer may be treated as a protocluster; and eddies, if strong enough, would give rise to protogalaxies. An effective mechanism of vorticity generation — scattering of density inhomogeneities on shock fronts — is examined in detail. Quantitative estimates show that the vortices that are due to this mechanism can be at least the order of magnitude to account for the angular mometa of spiral galaxies. The spectrum of initial irrotational perturbations remains open to discussion, but characteristic amplitudes required do not contradict any observational or theoretical restrictions.