Three-dimensional hydrodynamical calculations on the fragmentation of pancakes and galaxy formation

The fragmentation of pancakes is numerically investigated by a three-dimensional smoothed particle hydrodynamics (SPPH) combined with a usual N-body scheme, where the ionization equilibria are solved and all kinds of the cooling and heating processes are explicitly included for primordial gas. We consider dark-matter-dominated pancakes as well as baryon-dominated ones, in a range of pancake mass M pan =10 13-15 M ○. . We find a three-step fragmentation scenario: First, the fragmentation is triggered by the filamentary mode instability in a central cooled dense layer. Second, they are torn to numerous lumps, forming chainlike structures