The Origin of Angular Momentum in Dark Matter Halos

We propose a new explanation for the origin of angular momentum in galaxiesand their dark halos, in which the halos obtain their spin through thecumulative acquisition of angular momentum from satellite accretion. In ourmodel, the build-up of angular momentum is a random walk process associatedwith the mass assembly history of the halo's major progenitor. We assume nocorrelation between the angular momenta of accreted objects. Using the extendedPress-Schechter approximation, we calculate the growth of mass, angularmomentum, and spin parameter $\lambda$ for many halos. Our random walk modelreproduces the key features of the angular momentum of halos found in N-bodysimulations: a lognormal distribution in $\lambda$ with an average of$<\lambda> \approx 0.04$, independent of mass and redshift. The evolution ofthe spin parameter in individual halos in this model is quite different fromthe steady increase with time of angular momentum in the tidal torque picture.We find both in N-body simulations and in our random walk model that the valueof $\lambda$ changes significantly with time for a halo's major progenitor. Ittypically has a sharp increase due to major mergers, and a steady declineduring periods of gradual accretion of small satellites. The model predictsthat on average the $\lambda$ of halos which had major mergers after redshift$z=2$ should be substantially larger than the $\lambda$ of those which did not.Perhaps surprisingly, this suggests that halos that host late-formingelliptical galaxies should rotate faster than halos of spiral galaxies.Comment: 11 pages, 10 figures, wording improved and typo corrected, matches accepted ApJ versio