The angular momentum distribution of gas and dark matter in galactic halos

We report results of a series of nonradiative N-body/SPH simulations in a Delta CDM cosmology, designed to study the growth of angular momentum in galaxy systems. A sample of 41 halos of differingmass and environment were selected from a cosmological N-body simulation of size 32.5 h(-1) Mpc and resimulated at higher resolution with the tree-SPH code GADGET. We find that the spin of the baryonic component correlates well with the spin of the dark matter, but there is a misalignment of typically 20 degrees between these two components. The spin of the baryonic component is also on average larger than that of the darkmatter 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. This trend can be explained as a result of increasing thermalization of the virializing gas with decreasing 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 global angular momentum properties, in particular the anticorrelation of f with the spin parameter k and the shape of the angular momentum distributions ( AMDs). We investigate in detail the AMDs of the gas and the dark matter components of the halo. We compare and contrast various techniques to determine the AMDs. We show that the technique of adding thermal velocities to streaming motions ( broadening) is unsuitable for making comparisons between gas and dark matter AMDs because the shape of the broadened AMDs is predominantly determined by the dispersion and is insensitive to the underlying nonbroadened AMD. In order to bring both gas and darkmatter to the same footing, we smooth the angular momentum of the particles over a fixed number of neighbors. The AMDs obtained by this method have a smooth and extended truncation as compared to earlier methods. We find that an analytical function in which the differential distribution of specific angular momentum j is given by P( j) = [1/j(d)(alpha)Gamma(alpha)]( j)(alpha-1) e(-j/jd), where j(d) = j(tot)/alpha, 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 dark matter follows roughly a lognormal distribution. The mean and standard deviation of log alpha for gas are -0.04 and 0.11, respectively. About 90%-95% of halos have alpha < 1: 3, while exponential disks in Navarro-Frenk- White ( 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 that is consistent with the findings of earlier works. Parameter alpha for gas is correlated with that for dark matter (DM), but they have a significant scatter [alpha(gas)/alpha DM] = 1:09 +/- 0:2. Parameter alpha(gas) is also biased toward slightly higher values compared to alpha(DM). The angular momentum in halos is also found to have a significant spatial asymmetry with the asymmetry being more pronounced for dark matter.