A Universal Angular Momentum Profile for Galactic Halos

Abstract

[Abridged] We study the angular-momentum profiles of a statistical sample of\nhalos drawn from a high-resolution N-body simulation of the LCDM cosmology. We\nfind that the cumulative mass distribution of specific angular momentum, j, in\na halo of mass Mv is well fit by a universal function, M(<j) = Mv \\mu\nj/(j_0+j). This profile is defined by one shape parameter (\\mu or j_0) in\naddition to the global spin parameter \\lambda. It follows a power-law over most\nof the mass, and flattens at large j, with the flattening more pronounced for\nsmall values of \\mu. Compared to a uniform sphere in solid-body rotation, most\nhalos have a higher fraction of their mass in the low- and high-j tails of the\ndistribution. The spatial distribution of angular momentum in halos tends to be\ncylindrical and is well-aligned within each halo for ~80% of the halos. We\ninvestigate two ideas for the origin of this profile. The first is based on a\nrevised version of linear tidal-torque theory combined with extended\nPress-Schechter mass accretion, and the second focuses on j transport in minor\nmergers. Finally, we briefly explore implications of the M(<j) profile on the\nformation of galactic disks assuming that j is conserved during an adiabatic\nbaryonic infall. The implied gas density profile deviates from an exponential\ndisk, with a higher density at small radii and a tail extending to large radii.\nThe steep central density profiles may imply disk scale lengths that are\nsmaller than observed. This is reminiscent of the "angular-momentum problem"\nseen in hydrodynamic simulations, even though we have assumed perfect j\nconservation. A possible solution is to associate the central excesses with\nbulge components and the outer regions with extended gaseous disks.\n