Figure rotation of dark halos in cold dark matter simulations.

Abstract

We have simulated structure formation on cosmological-scales using N-body simulations run on the University of KwaZulu-Natal's cluster of computers and have used these simulations to investigate aspects of galaxy evolution. In particular, we focus on the rotation of dark matter halos identified in Cold Dark Matter (CDM) simulations. These halos are typical of those thought to surround galaxies. Understanding their morphology and kinematics will help with the interpretation of observations and will constrain models of galaxy formation and evolution. We have determined the mass function of our simulated halos and shown that this agrees well with other simulations and theoretical predictions of this function. We have also explored the evolution of the mass function with redshift, which clearly shows hierarchical structure formation. In considering the angular momentum of our sample of halos, we have found the spin distribution to be well fit by a log normal distribution. After removing all halos that have either recently undergone major mergers or contain a significant amount of substructure from our sample, 75% of the remaining halos were found to undergo coherent rotation over periods of three gigayears. The pattern speeds were found to follow a log normal distribution, with an average value of 0.13h radians per gigayear. The most rapidly rotating halo detected was found to have a pattern speed of 0.41h radians per gigayear. Many halos showed alignment between their rotation and minor axes. We found no correlation between halo properties, such as total mass, and the pattern speed. While the speeds observed were not sufficient to cause spiral structure, the rotation could be relevant for understanding other observations of galaxies.