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 re-simulation at higher resolution with dark matter and gas components. The initial conditions are evolved with two smoothed particle hydrodynamics codes, TREESPH and GRAPESPH. 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. The results of varying the cooling epoch for each of five different haloes are analysed. When cooling and star formation occur at early times, stellar discs are destroyed during merger events and we observe similar catastrophic transport of angular momentum as seen in previous work. With cooling suppressed until z=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.