Cosmological self-similar shock waves and galaxy formation

Abstract

A new family of similarity solutions is found for a point blast in a £2 = 1 Friedmann cosmology (Einstein-de Sitter universe). The given solutions are valid several Hubble times after an explosion. Initially the shock wave formed satisfies the Sedov solution for adiabatic evolution in a constant medium, but it is modified by the cosmological expansion to form a self-similar thin dense shell which cools by adiabatic expansion and is unstable to gravitational fragmentation and collapse. The shock solution is applied to the explosive amplification model of galaxy formation of Ostriker and Cowie. It is shown that earlier than 1 + z ~ 9, inverse Compton and free-free cooling prevent galactic-scale shocks from forming. At smaller redshifts, galactic shocks cool adiabatically to form cosmological dense-shelled shocks. Line cooling becomes important in the dense shells, cooling them to ~ 10 4 K and setting the Jeans mass to galactic scales. One generation of explosive amplification from an initial "seed" galaxy produces a shell of ~ 3 x 10 11 M 0 in galaxies, with radius and velocity dispersion typical of small groups. There may be the resulting shell of ~2 x 10 13 M 0 has radius clusters of galaxies.