The fragmentation of 'pancakes' in a dark matter-dominated universe

Galaxy formation is studied within the context of the pancake model. The collapse, cooling, and fragmentation of a massive cloud composed of baryons and collisionless dark matter in an FRW universe with omega(0) = 1, omega(b) = 0.1, and H0 = 75 km/s per Mpc is modeled using 2D numerical simulations. Perturbations having a Poisson spectrum are imposed on the cloud at the start of the calculation to provide a noise source for fragmentation. The Zeus-2D hydrodynamics code with cosmological and cooling terms is coupled to a cloud-in-cell code that evolves the dark matter. A nonuniform grid is used in the direction of collapse to resolve the central thin cooling layer. The gas cloud collapses to a pancake at redshift approximately equal to 5. It cools and undergoes instabilities causing it to fragment into protogalactic objects about 12.5 kpc, which then merge into larger objects. 16 refs.