SIMULATIONS OF DISSIPATIVE GALAXY FORMATION IN HIERARCHICALLY CLUSTERING UNIVERSES .2. DYNAMICS OF THE BARYONIC COMPONENT IN GALACTIC HALOES

We present self-consistent 3D simulations of the formation of virialized systems containing both gas and dark matter. Using a fully Lagrangian code based on the smooth particle hydrodynamics technique and a tree data structure, we follow the evolution of regions of comoving radius 2-3 Mpc with proper inclusion of the tidal effects of surrounding material. Initial conditions at high redshifts assume an Einstein-de Sitter universe, a biased cold dark matter perturbation spectrum (b = 2.5), and a baryonic mass fraction of 10 per cent. The gas is initially cold and radiates in the manner expected for a plasma of primordial composition. We neglect star formation and associated processes. We find that most of the gas settles rapidly into centrifugally supported discs at the centres of small dark matter clumps. These are not disrupted during later evolution, and survive the merging of their dark haloes. They either remain distinct, or merge without reheating at a later time. These results confirm that significant energy input from non-gravitational sources is required to produce a cold baryonic fraction consistent with the observed abundance of gas and stars in galaxies. In our simulations, much of the final disc of gas accumulates through mergers of dense clumps rather than through the smooth infall envisaged in earlier models. This process involves substantial angular momentum loss and large collapse factors. It produces discs that are more concentrated than real spiral discs and have circular velocities up to 70 per cent greater than that of the surrounding halo. Nevertheless, the surface density profile of these discs can be reasonably well fitted by an exponential, and their rotation curves are flat out to approximately 5 disc scalelengths. The high efficiency of cooling also results in very low predicted X-ray luminosities for the residual haloes of hot gas.