A universal angular momentum profile for galactic halos

We study the angular momentum profiles of a statistical sample of halos drawn from a high-resolution N-body simulation of the Lambda CDM cosmology. We find that the cumulative mass distribution of specific angular momentum j in a halo of mass M-v is well fitted by a universal function, M(j/(j(0) + j). This profile is defined by one shape parameter (mu or j(0)) in addition to the global spin parameter lambda. It follows a power law M( j over most of the mass and flattens at large j, with the flattening more pronounced for small values of mu (or large j(0) at a fixed lambda). Compared to a uniform sphere in solid-body rotation, most halos have a higher fraction of their mass in the low- and high-j tails of the distribution. High-lambda halos tend to have high mu values, corresponding to a narrower, more uniform j distribution. The spatial distribution of angular momentum in halos tends to be cylindrical and is well-aligned within each halo for similar to 80% of the halos. The more misaligned halos tend to have low mu values. When averaged over spherical shells encompassing mass M, the halo j profiles are fitted by j(M) proportional to M-s with s = 1.3 +/- 0.3. We investigate two ideas for the origin of this profile. The first is based on a revised version of linear tidal-torque theory combined with extended Press-Schechter mass accretion, and the second focuses on j transport in minor mergers. Finally, we briefly explore implications of the M(