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 $j_{\mathrm{h}}$ and mass $M_{\mathrm{h}}$ of galactic dark matter halos has the well-established form $j_{\mathrm{h}}\propto M_{\mathrm{h}}^{2/3}$. 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 $f_j=j_{\ast }/j_{\mathrm{h}}$ and $f_M=M_{\ast }/M_{\mathrm{h}}$. An important caveat here is that the adopted $j_{\mathrm{h}}\propto M_{\mathrm{h}}^{2/3}$ 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 $j_{\mathrm{h}}$-$M_{\mathrm{h}}$ 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 $j_{\mathrm{h}}\propto M_{\mathrm{h}}^{\hat{I}}\pm$, with $\hat{I}\pm \approx 2/3$ over the ranges of mass and redshift studied here: $M_{\mathrm{h}}\ge 10^{10}{M}_{\odot }$ and $0\le z\le 2$. The values of $\hat{I}\pm$ are virtually identical in the full-physics and DM-only runs at the same redshift. The only detectable effect of baryons on the $j_{\mathrm{h}}$-$M_{\mathrm{h}}$ 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.

8 pages, 3 figures, 1 table. Accepted for publication in ApJL