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 (ΛCDM) paradigm. We use zoom simulations of 20 galaxies with stellar masses of M ⋆ ≃ 107–11 M⊙ 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 ΛCDM-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 ≈ 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 ΛCDM. [ABSTRACT FROM AUTHOR]