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 M_v is well fitted by a universal function, M(<j)=M_vμj/(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 fitted 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. Finally, we briefly explore implications of the M(<j) profile on the formation of galactic disks assuming that j is conserved during an adiabatic baryonic infall. The implied gas density profile deviates from an exponential disk, with a higher density at small radii and a tail extending to large radii. The steep central density profiles may imply disk scale lengths that are smaller than observed. This is reminiscent of the ``angular momentum problem'' seen in hydrodynamic simulations, even though we have assumed perfect j conservation. A possible solution is to associate the central excesses with bulge components and the outer regions with extended gaseous disks.