A Universal Angular Momentum Profile for Galactic Halos

Abstract

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 v μ j /( j 0 + j ). This profile is defined by one shape parameter (μ or j 0 ) 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 j 0 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.