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-1 Mpc-1 is modeled using two-dimensional 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 z approximately 5. It cools and undergoes instabilities causing it to fragment into protogalactic objects approximately 12.5 kpc, which then merge into larger objects.