The Origin of the Stellar Angular Momentum-Mass Relation: Connecting
  Galaxies and Halos in IllustrisTNG
  The IllustrisTNG simulations reproduce well the observed scaling relation
between specific angular momentum (sAM) $j_{\rm s}$ and mass $M_{\rm s}$ of
galaxies, i.e., the $j_{\rm s}$-$M_{\rm s}$ relation. This relation develops in
disk galaxies at redshift $z\lesssim 1$ via forming disk structures whose sAM
are nearly conserved from their parent dark matter halos. We provide a simple
model that describes well the connection between halos and galaxies, giving
${\rm log} j_{\rm s} = 0.54\ {\rm log} M_{\rm s} - 2.53 + b$, where $b$
quantifies how well sAM is retained. This model suggests that the power-law
index 0.54 of the $j_{\rm s}$-$M_{\rm s}$ relation is determined by (i) the
change of the halo angular momentum-mass relation $j \propto M^{\alpha}$, whose
index $\alpha$ grows from 0.68 to 0.80 under the effect of baryonic processes,
and (ii) the mass dependence of the luminous-dark mass ratio. This model is
different from the traditional expectation using a constant luminous-dark mass
ratio and $\alpha=2/3$. We further suggest that a significant loss of sAM,
which manifests as a decrease in $b$, leads to the parallel offset of
elliptical galaxies (relative to disk galaxies) in the $j_{\rm s}$-$M_{\rm s}$
relation.
