Mergers and the Formation of Disk Galaxies in Hierarchically Clustering Universes

Abstract

New simulations are presented which investigate the formation of smaller groups of galaxies in a CDM like universe. The simulations follow the evolution of dark matter and gas and are performed with an SPH code adapted for the special-purpose hardware GRAPE. The mass resolution in the baryonic component is 5 10^6 Msol, the spatial resolution is 1.5 kpc. A sufficiently large volume is simulated in order to accurately include the tidal field and the mass inflow. The mass resolution enables us to resolve the galaxies formed in the simulation. Individual properties of the galaxies, each consisting of several thousand particles, can also be investigated. It turns out, that in the center of the dark haloes the gas accumulates to form a rotationally supported disk. However, in comparison with observations, disks which form in numerical simulations are too concentrated: Due to the merging of substructures, angular momentum is transported from the gas to the dark halo, and far too much gas is accumulated at very small radii. It is argued that changing the cosmogony (e.g. to low Omega) or adding photoionisation due to an external UV background do not provide an easy fix to the angular momentum problem. Feedback processes caused by supernovae or stellar winds seem to be the most likely solution to this problem.