A Universal Angular Momentum Profile for Galactic Halos
Abstract
We study the angular momentum profiles of a statistical sample of halos drawn from a highresolution Nbody 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}μj/(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 solidbody rotation, most halos have a higher fraction of their mass in the low and highj tails of the distribution. Highλ 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 wellaligned 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 tidaltorque theory combined with extended PressSchechter 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.
 Publication:

The Astrophysical Journal
 Pub Date:
 July 2001
 DOI:
 10.1086/321477
 arXiv:
 arXiv:astroph/0011001
 Bibcode:
 2001ApJ...555..240B
 Keywords:

 Cosmology: Dark Matter;
 Galaxies: Formation;
 Galaxies: Halos;
 Galaxies: Structure;
 Astrophysics
 EPrint:
 19 pages LaTeX, uses emulateapj5, 22 embedded figures, 1 separate figure, Submitted to ApJ, version with higher quality figures available at http://www.astronomy.ohiostate.edu/~james/PAPER/parts.html