The Origin of the Stellar Angular Momentum-Mass Relation: Connecting Galaxies and Halos in IllustrisTNG

Kavli Affiliate: Luis C. Ho

| First 5 Authors: Min Du, Luis C. Ho, Hao-Ran Yu, Victor P. Debattista,

| Summary:

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 $zlesssim 1$ 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.

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