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) $j_{\mathrm{s}}$ and mass $M_{\mathrm{s}}$ of central galaxies. We show that the local $j_{\mathrm{s}}$-$M_{\mathrm{s}}$ relation ${\rm log}\ j_{\rm s} = 0.55 \ {\rm log}\ M_{\rm s} + 2.77$developsat$z\lesssim 1$ 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 $j_{\rm s}$-$M_{\rm s}$relationcomesfromtheproductoftheindicesofthe$j_{\rm tot}\propto M_{\rm tot}^{0.81}$,$M_{\rm tot}\propto M_{\rm s}^{0.67}$,and$j_{\rm s}\propto j_{\rm tot}$relations,where$j_{\rm tot}$and$M_{\rm tot}$ are overall sAM and mass of a halo. A non-negligible deviation from the tidal torque theory, which predicts $j_{\mathrm{t}\mathrm{o}\mathrm{t}}\propto M_{\mathrm{t}\mathrm{o}\mathrm{t}}^{2\mathrm{/}3}$, 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 $\sim 1$ 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 $j_{\mathrm{s}}$-$M_{\mathrm{s}}$ relation.