The Angular Momentum of Gas in Proto-galaxies: I – Implications for the Formation of Disk Galaxies

We use non-radiative N-body/SPH simulations of structure formation in a ΛCDM cosmology to compare the angular momentum distributions of dark matter and gas in a large sample of halos. We show that the two components have identical spin parameter distributions and that their angular momentum distributions within individual halos are very similar, all in excellent agreement with standard assumptions. Despite these similarities, however, we find that the angular momentum vectors of the gas and dark matter are poorly aligned, with a median misalignment angle of ∼ 30 degrees, which might have important implications for spin correlation statistics used in weak lensing studies. We present distributions for the component of the angular momentum that is aligned with the total angular momentum of each halo, and find that for between 5 and 50 percent of the mass this component is negative. This disagrees with the ‘Universal’ angular momentum distribution suggested by Bullock et al. (2001), for which the mass fraction with negative specific angular momentum is zero. We discuss the implications of our results for the formation of disk galaxies. Since galactic disks generally do not contain counter-rotating stars or gas, disk formation cannot occur under detailed conservation of specific angular momentum. We suggest that the material with negative specific angular momentum combines with positive angular momentum material to build a bulge component, and show that in such a scenario the remaining material can form a disk with a density distribution that is very close to exponential. Subject headings: cosmology: dark matter — galaxies: formation — galaxies: structure — galaxies: halos.