The Angular Momentum Distribution of Gas and Dark Matter in Galactic Halos

Abstract

(Abridged) We report results of a series of non radiative N-body/SPH simulations in a LCDM cosmology. We find that the spin of the baryonic component is on average larger than that of the dark matter (DM) component and we find this effect to be more pronounced at lower redshifts. A significant fraction f of gas has negative angular momentum and this fraction is found to increase with redshift. We describe a toy model in which the tangential velocities of particles are smeared by Gaussian random motions. This model is successful in explaining some of the angular momentum properties. We compare and contrast various techniques to determine the angular momentum distributions (AMDs). We show that broadening of velocity dispersions is unsuitable for making comparisons between gas and DM. We smooth the angular momentum of the particles over a fixed number of neighbors. We find that an analytical function based on gamma distribution can be used to describe a wide variety of profiles, with just one parameter \alpha. The distribution of the shape parameter $\alpha$ for both gas and DM follows roughly a log-normal distribution. The mean and standard deviation of log(\alpha) for gas is -0.04 and 0.11 respectively. About 90-95% of halos have \alpha<1.3, while exponential disks in NFW halos would require 1.3<\alpha<1.6. This implies that a typical halo in simulations has an excess of low angular momentum material as compared to that of observed exponential disks, a result which is consistent with the findings of earlier works. \alpha for gas is correlated with that of DM but they have a significant scatter =1.09 \pm 0.2. \alpha_Gas is also biased towards slightly higher values compared to \alpha_DM.Comment: 19 pages, 32 figures (replaced to correct a typo in the authors field in the above line, paper unchanged