  In cosmological simulations without baryons, the relation between the
specific angular momentum $j_{\rm h}$ and mass $M_{\rm h}$ of galactic dark
matter halos has the well-established form $j_{\rm h} \propto M_{\rm h}^{2/3}$.
This is invariably adopted as the starting point in efforts to understand the
analogous relation between the specific angular momentum $j_{\ast}$ and mass
$M_{\ast}$ of the stellar parts of galaxies, which are often re-expressed
relative to the corresponding halo properties through the retention fractions
$f_j = j_{\ast} / j_{\rm h}$ and $f_M = M_{\ast} / M_{\rm h}$. An important
caveat here is that the adopted $j_{\rm h} \propto M_{\rm h}^{2/3}$ relation
could, in principle, be modified by the gravitational back-reaction of baryons
on dark matter (DM). We have tested for this possibility by comparing the
$j_{\rm h}$-$M_{\rm h}$ relations in the IllustrisTNG100 and TNG50 simulations
that include baryons (full-physics runs) with their counterparts that do not
(DM-only runs). In all cases, we find scaling relations of the form $j_{\rm h}
\propto M_{\rm h}^{\alpha}$, with $\alpha \approx 2/3$ over the ranges of mass
and redshift studied here: $M_{\rm h} \geq 10^{10} \, M_{\odot}$ and $0 \leq z
\leq 2$. The values of $\alpha$ are virtually identical in the full-physics and
DM-only runs at the same redshift. The only detectable effect of baryons on the
$j_{\rm h}$-$M_{\rm h}$ relation is a slightly higher normalization, by 12%-15%
at $z=0$ and by 5% at $z=2$. This implies that existing estimates of $f_j$
based on DM-only simulations should be adjusted downward by similar amounts.
Finally, we discuss briefly some implications of this work for studies of
galaxy formation.
