Kavli Affiliate: Luis C. Ho
| First 5 Authors: Min Du, Luis C. Ho, Hao-Ran Yu, Victor P. Debattista,
| Summary:
The IllustrisTNG simulations reproduce the observed scaling relation between
stellar specific angular momentum (sAM) $j_{rm s}$ and mass $M_{rm s}$ of
central galaxies. We show that the local $j_{rm s}$-$M_{rm s}$ relation ${rm
log} j_{rm s} = 0.55 {rm log} M_{rm s} + 2.77$ develops at $zlesssim 1$
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 $j_{rm
s}$-$M_{rm s}$ relation comes from the product of the indices of the $j_{rm
tot}propto M_{rm tot}^{0.81}$, $M_{rm tot}propto M_{rm s}^{0.67}$, 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 $j_{rm tot}propto M_{rm tot}^{2/3}$, 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 $sim 1$ 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 $j_{rm s}$-$M_{rm s}$ relation.
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