The physics of dissipational galaxy formation
The literature on nonlinear stages of galaxy formation is briefly reviewed along with arguments that lead to the consideration of a dissipative formation process. Hydrodynamic calculations are presented for the collapse of protogalactic gas clouds at redshifts less than 10; it is assumed that the clouds do not fragment during their first infall, so that the collapse is halted by shock-front formation about a caustic surface. The cooling, later fragmentation, evaporation, and binding-energy loss of such clouds are investigated, and ways in which these processes may have influenced the formation of elliptical and disk galaxies are discussed. A simple model for the formation of the Hubble sequence is outlined in which mass and perturbation morphology, together with the physics governing the cooling of a hydrogen-helium mixture, determine the type of galaxy that is formed from a whole range of protogalactic clouds.