Angular Momentum Distribution of Hot Gas and Implications for Disk Galaxy Formation

We study the angular momentum profiles both for dark matter and for gaswithin virialized halos, using a statistical sample of halos drawn fromcosmological hydrodynamics simulations. Three simulations have been analyzed,one is the ``non-radiative'' simulation, and the other two have radiativecooling. We find that the gas component on average has a larger spin andcontains a smaller fraction of mass with negative angular momentum than itsdark matter counterpart in the non-radiative model. As to the cooling models,the gas component shares approximately the same spin parameter as its darkmatter counterpart, but the hot gas has a higher spin and is more aligned inangular momentum than dark matter, while the opposite holds for the cold gas.After the mass of negative angular momentum is excluded, the angular momentumprofile of the hot gas component approximately follows the universal functionoriginally proposed by Bullock et al. for dark matter, though the shapeparameter $\mu$ is much larger for hot gas and is comfortably in the rangerequired by observations of disk galaxies. Since disk formation is related tothe distribution of hot gas that will cool, our study may explain the fact thatthe disk component of observed galaxies contains a smaller fraction of lowangular momentum material than dark matter in halos.Comment: 30 pages, 12 figures, 4 tables, accepted for publication in Ap