Hooks & Bends in the radial acceleration relation: discriminatory tests for dark matter and MOND

The radial acceleration relation (RAR) connects the total gravitational acceleration of a galaxy at a given radius, a(tot)(r), with that accounted for by baryons at the same radius, a(bar)(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 (Lambda CDM) paradigm. We use zoom simulations of 20 galaxies with stellar masses of M-star similar or equal to 10(7-11) M-circle dot to study the RAR in the FIRE-2 simulations. We highlight the existence of simulated galaxies with non-monotonic RAR tracks that 'hook' down from the average relation. These hooks are challenging to explain in Modified Inertia theories of MOND, but naturally arise in all of our Lambda CDM-simulated galaxies that are dark-matter dominated at small radii and have feedback-induced cores in their dark matter haloes. We show, analytically and numerically, that downward hooks are expected in such cored haloes because they have non-monotonic acceleration profiles. We also extend the relation to accelerations below those traced by disc galaxy rotation curves. In this regime, our simulations exhibit 'bends' off of the MOND-inspired extrapolation of the RAR, which, at large radii, approach a(tot) approximate to a(bar)/f(b), where f(b) is the cosmic baryon fraction. Future efforts to search for these hooks and bends in real galaxies will provide interesting tests for MOND and Lambda CDM.