Mercado et al., available on arXiv

Abstract: The Radial Acceleration Relation (RAR) connects the total gravitational acceleration of a galaxy at a given radius, atot(r), with that accounted for by baryons at the same radius, abar(r). The shape and tightness of the RAR for rotationally-supported galaxies have characteristics in line with MOdified Newtonian Dynamics (MOND) and can also arise within the Cosmological Constant + Cold Dark Matter (ΛCDM) paradigm. We use zoom simulations of 20 galaxies with stellar masses of Mstar~10^(7−11) Msun to demonstrate that the observed average and scatter about the RAR is reproduced in FIRE-2 simulations. We highlight the existence of many observed galaxies with non-monotonic RAR tracks that “hook” down from the average relation. These hooks are challenging to explain in MOND, but we see them in all of our simulated galaxies that are dark-matter dominated and have feedback-induced cores in their dark matter haloes. We show analytically that downward hooks are expected in such cored haloes because they have non-monotonic acceleration profiles. We also make RAR predictions for the outer reaches of our simulated galactic haloes, extending the relation to accelerations below those traced by disc galaxy rotation curves. In this regime, our simulations predict “bends” off of the MOND-inspired extrapolation of the RAR, which, at large radii, approach atot(r)~abar(r)/fb, where fb is the cosmic baryon fraction. Future efforts to search for these bends at low accelerations around real galaxies will provide tests for MOND and LCDM.