Angular Momentum and Morphological Sequence of Massive Galaxies through Dark Sage

We study the present-day connection between galaxy morphology and angular momentum using the D ark S age semi-analytic model of galaxy formation. For a given stellar mass in the range 10 10 –10 12 M ⊙ , the model predicts that galaxies with more prominent disks exhibit higher stellar disk specific angular momentum ( j stellar,disk ). However, when we include the gas in the disk, bulge-dominated galaxies have the highest total disk specific angular momentum ( j total,disk ). We attribute this to a large contribution from an extended disk of cold gas in typical bulge-dominated galaxies. Note that while the specific angular momenta ( j = J / M ) of these disks are large, their masses ( M ) are negligible. Thus, the contribution of these disks to the total angular momentum of the galaxy is small. We also find the relationship between the specific angular momentum of the dark matter ( j dark matter ) and morphology to be counterintuitive. Surprisingly, in this stellar mass range, not only do bulge-dominated galaxies tend to live in halos with higher j dark matter than disk-dominated galaxies, but intermediate galaxies (those with roughly equal fractions of bulge and disk mass) have the lowest j dark matter of all. Yet, when controlling for halo mass, rather than stellar mass, the relationship between j dark matter and morphology vanishes. Based on these results, we find that halo mass—rather than angular momentum—is the main driver of the predicted morphology sequence in this high mass range. In fact, in our stellar mass range, disk-dominated galaxies live in dark matter halos that are roughly one-fifth the mass of their bulge-dominated counterparts.