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 ΛCDM cosmology. We find that the cumulative mass distribution of specific angular momentum j in a halo of mass Mv is well fit by a universal function, M( j) = Mvµj/(j0 +j). This profile is defined by one shape parameter (µ or j0) in addition to the global spin parameter λ. It follows a power-law M( j) ∝ j over most of the mass, and flattens at large j, with the flattening more pronounced for small values of µ (or large j0 at a fixed λ). 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-λ halos tend to have high µ 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 ∼ 80 % of the halos. The more misaligned halos tend to have low-µ values. When averaged over spherical shells encompassing mass M, the halo j profiles are fit by j(M) ∝ 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.