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 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 $\mathrm{log}\ {j}_{{\rm{s}}}=0.55\ \mathrm{log}\ {M}_{{\rm{s}}}+2.77$ 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 ${j}_{\mathrm{tot}}\propto {M}_{\mathrm{tot}}^{0.81}$ , ${M}_{\mathrm{tot}}\propto {M}_{{\rm{s}}}^{0.67}$ , 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 ${j}_{\mathrm{tot}}\propto {M}_{\mathrm{tot}}^{2/3}$ , 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.