The formation of disc galaxies

Abstract

We investigate the influence of the cooling epoch on the formation of galaxies in a cold dark matter dominated universe. Isolated haloes, with circular speeds typical of spiral galaxies, have been selected from a low-resolution numerical simulation for resimulation at higher resolution. Initial conditions for each halo consist of a high-resolution region containing dark matter and gas, and a nested hierarchy of particles representing the mass distribution of the original low-resolution simulation. These initial conditions are evolved with two smoothed particle hydrodynamics (SPH) codes, TREESPHGRAPESPH, so that discrepancies due to differences in evolutionary and star formation algorithms can be analysed. In previous SPH simulations, strong outward transport of angular momentum has led to the formation of disc-like systems with much smaller angular momenta than observed in real disc galaxies. Here we investigate whether this problem can be circumvented if feedback processes prevent disc formation until late epochs. In some of our models, the gas is evolved adiabatically until a specified redshift zcool, at which point the gas is allowed to cool radiatively and star formation may begin. In other models, cooling is continuous throughout the evolution but suppressed by a factor chosen to allow discs to grow roughly linearly with time. The results of varying the cooling epoch for each of five different haloes are analysed. When cooling and star formation are initiated at redshift zcool = 4, stellar discs are destroyed during merger events and we observe catastrophic transport of angular momentum similar to that seen in previous work. With cooling suppressed until zcool = 1, discs can form by the present day with angular momenta comparable to those of observed disc galaxies. We conclude that feedback processes, which prevent gas from collapsing until late epochs, are an essential ingredient in disc galaxy formation.