Angular Momentum Distribution Within Dark Halos And Implications For Disk Galaxy Formation

We study the angular momentum profile both for dark matter and for gas within virial-ized halos, using statistical samples drawn from different sets of high-resolution cosmological simulations.For the pure dark matter case, two typical cold dark matter (CDM) models have been analysed. In contrast with the recent claims of Bullock et al., we find that the degree of misalignment of angular momentum within a halo is very high. About 50 percent of halos have more than 10 percent of halo mass in the mass of negative angular momentum j. Our results, however, are broadly in good agreement with a recent work of van den Bosch et al.. We also study the angular momentum profile of halos in a Warm Dark Matter (WDM) model and a Self-Interacting Dark Matter (SIDM) model. We find that the angular momentum profile of halos in the WDM is statistically indistinguishable from that in the CDM model, but the angular momentum of halos in the SIDM is reduced by the self-interaction of dark matter.For the gas and dark matter case, three simulations have been analyzed, one is the "non-radiative" simulation, and the other two have radiative cooling. We find that the gas component on average has a larger spin and contains a smaller fraction of mass with negative angular momentum than its dark matter counterpart in the non-radiative model. For both the cooling models, after the mass of negative angular momentum is excluded, the angular momentum profile of the hot gas component approximately follows the universal function originally proposed by Bullock et al. for dark matter, although the shape parameter μ is much larger for hot gas and is comfortably in the range required by observations of disk galaxies. Since disk formation is related to the distribution of hot gas that will cool, our study may explain the fact that the disk component of observed galaxies contains a smaller fraction of low angular momentum material than dark matter in halos.