Angular Momentum in the Formation of Disc Galaxies

Within the current framework of disc galaxy formation, we discuss the resulted surface-density profiles according to the theoretical angular momentum distributions (AMDs) presented by Bullock et al. Astrophys. J. 555 ,(2001) 240(B01) for the Λ CDM cosmology in both spherical and cylindrical coordinates. It is found that the derived surface density distribution of a disc in the outer region is in general similar to an exponential disc for both the theoretical AMDs. In the central region, the results from both the theoretical AMDs are inconsistent with observations whether the disc bar-instability is taken into account or not. The cylindrical form of the theoretical AMD leads to the bar-instability more easily for a galaxy than that for spherical AMD, which could result in a more massive bulge. After comparing the model predictions with our Milky Way Galaxy, we find that the theoretical AMDs predict larger mass fractions of baryons with low angular momentum than the observed ones, which would lead to the disc sizes being smaller. Two possible processes which could solve the angular momentum problem are discussed.