The Origin of the Relation Between Stellar Angular Momentum and Stellar Mass in Nearby Disk-dominated Galaxies

The IllustrisTNG simulations reproduce the observed scaling relation between stellar specific angular momentum (sAM) j(s) and mass M-s of central galaxies. We show that the local j(s)-Ms relation log j(s)=0.55 log M-s+2.77 develops at z less than or similar to 1 in disk-dominated galaxies. We provide a simple model that describes well such a connection between halos and galaxies. The index 0.55 of the j(s)-M-s relation comes from the product of the indices of the j(tot) proportional to M-tot(0.81), M-tot proportional to M-s(0).67, and j(s)proportional to j(tot) relations, where j(tot) and M-tot are overall sAM and mass of a halo. A non-negligible deviation from the tidal torque theory, which predicts j(tot)proportional to M-tot(2/3), should be included. This model further suggests that the stellar-to-halo mass ratio of disk galaxies increases monotonically following a nearly power-law function that is consistent with the latest dynamical measurements. Biased collapse, in which galaxies form from the inner and lower sAM portion of their parent halos, has a minor effect at low redshifts. The retention factor of angular momentum reaches similar to 1 in disk galaxies with strong rotations, and it correlates inversely with the mass fraction of the spheroidal component, which partially explains the morphological dependence of the j(s)-M-s relation.