Hooks & Bends in the Radial Acceleration Relation: Discriminatory Tests for Dark Matter and MOND

TL;DR

This paper investigates the shape of the Radial Acceleration Relation (RAR) in galaxies, revealing features like hooks and bends that challenge MOND and support dark matter models, through simulations and analytical analysis.

Contribution

It introduces simulated galaxy features such as non-monotonic RAR hooks and bends, providing new tests to distinguish between dark matter and MOND theories.

Findings

Simulated galaxies exhibit non-monotonic RAR hooks challenging MOND.

Downward hooks are explained by dark matter halos with cores.

Bends in the RAR at large radii deviate from MOND extrapolation.

Abstract

The Radial Acceleration Relation (RAR) connects the total gravitational acceleration of a galaxy at a given radius, $a_{\rm tot}(r)$, with that accounted for by baryons at the same radius, $a_{\rm bar}(r)$. 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 $M_{\star} \, \simeq 10^{7-11} \, M_{\odot}$ to study the RAR in the \texttt{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 \lcdm-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 $a_{\rm tot} \, \approx \, a_{\rm bar} \, /f_{\rm b}$, where $f_{\rm b}$ 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.