Negligible Effects of Baryons on the Angular Momentum Scaling Relations of Galactic Dark Matter Halos

In cosmological simulations without baryons, the relation between the specific angular momentum j _h and mass M _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 _* and mass M _* of the stellar parts of galaxies, which are often re-expressed relative to the corresponding halo properties through the retention fractions f _j = j _*/j _h and f _M = M _*/M _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 _h-M _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 α ≈ 2/3 over the ranges of mass and redshift studied here: M _h ≥ 10¹⁰ M _⊙ and 0 ≤ z ≤ 2. The values of α are virtually identical in the full-physics and DM-only runs at the same redshift. The only detectable effect of baryons on the j _h-M _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.