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

Abstract

The IllustrisTNG simulations reproduce the observed scaling relation between the stellar specific angular momentum (sAM) j s and mass M s of central galaxies. We show that the local j s – M s relation log j s = 0.55 log M s + 2.77 develops at z ≲ 1 in disk-dominated galaxies. We provide a simple model that describes well such a connection between halos and galaxies. The index of 0.55 of the j s – M s relation comes from the product of the indices of the j tot ∝ M tot 0.81 , M tot ∝ M s 0.67 , and j s ∝ j tot relations, where j tot and M tot are the overall sAM and mass of the halo, respectively. A non-negligible deviation from tidal torque theory, which predicts j tot ∝ 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 ∼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.