Cooling shells and galaxy formation in the early universe

The cooling and fragmentation of expanding shells in the early universe are investigated. Contrary to what has been assumed in some earlier works, it is shown that radiative cooling plays an important role in the inverse Compton era and that, due to the UV radiation from the shock front, explosions could not have cooled to the background radiation temperature. On the other hand, molecular cooling, mainly by H2, may be important, yielding equilibrium temperatures of approx.10/sup 3.5/z/sup -0.5/ K. Explosions with a seed mass larger than 109 M/sub sun/ can be amplified and become gravitationally unstable. At large redshifts (z>60) individual supernovae cool before their blast waves can merge to a galactic scale explosion. At small redshifts (z< or approx. =10), this process converges to masses of order 1013--1014 M/sub sun/, which fragment to galactic size masses