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Register a new userA universal angular momentum profile for galactic halos
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Authors
James S. Bullock
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[Abridged] We study the angular-momentum profiles of a statistical sample of halos drawn from a high-resolution N-body simulation of the LCDM 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) = Mv mu j/(j_0+j). This profile is defined by one shape parameter (mu or j_0) in addition to the global spin parameter lambda. It follows a power-law over most of the mass, and flattens at large j, with the flattening more pronounced for small values of mu. 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. The spatial distribution of angular momentum in halos tends to be cylindrical and is well-aligned within each halo for ~80% of the halos. 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.
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We use high-resolution cosmological hydrodynamic simulations to study the angular momentum acquisition of gaseous halos around Milky Way sized galaxies. We find that cold mode accreted gas enters a galaxy halo with ~70% more specific angular momentum than dark matter averaged over cosmic time (though with a very large dispersion). In fact, we find that all matter has a higher spin parameter when measured at accretion than when averaged over the entire halo lifetime, and is well characterized by lambda~0.1, at accretion. Combined with the fact that cold flow gas spends a relatively short time (1-2 dynamical times) in the halo before sinking to the center, this naturally explains why cold flow halo gas has a specific angular momentum much higher than that of the halo and often forms cold flow disks. We demonstrate that the higher angular momentum of cold flow gas is related to the fact that it tends to be accreted along filaments.
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We summarize recent developments in the study of the origin of halo spin profiles and preliminary implications on disk formation. The specific angular-momentum distributions within halos in N-body simulations match a universal shape, M(<j) propto j/(j_0+j). It is characterized by a power law over most of the mass, and one shape parameter in addition to the spin parameter lambda. The angular momentum tends to be aligned throughout the halo and of cylindrical symmetry. Even if angular momentum is conserved during baryonic infall, the resultant disk density profile is predicted to deviate from exponential, with a denser core and an extended tail. A slightly corrected version of the scaling relation due to linear tidal-torque theory is used to explain the origin of a typical power-law profile in shells, j(M) propto M^s with s gsim 1. While linear theory crudely predicts the amplitudes of halo spins, it is not a good predictor of their directions. Independently, mergers of halos are found to produce a similar profile due to j transfer from the orbit to the product halo via dynamical friction and tidal stripping. The halo spin is correlated with having a recent major merger, though this correlation is weakened by mass loss. These two effects are due to a correlation between the spins of neighboring halos and their orbit, leading to prograde mergers.
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