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

Abstract

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 h ∝ M 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 h ∝ M 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 h ∝ M h α , with α ≈ 2/3 over the ranges of mass and redshift studied here: M h ≥ 10 10 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.