The radial acceleration relation at the EDGE of galaxy formation

A tight correlation between the baryonic and observed acceleration of galaxies has been reported over a wide mass range (108 < Mbar/M⊙ < 1011 M⊙); this is known as the radial acceleration relation (RAR). This has been interpreted as evidence that dark matter is actually a manifestation of a modified, weak-field gravity theory. In this work, we studied the radially resolved RAR of 12 nearby dwarf galaxies, with baryonic masses in the 104 < Mbar/M⊙ < 107.5 M⊙ range, using a combination of literature data and data from the MUSE-Faint survey. We used stellar line-of-sight velocities and the Jeans modelling code G<ns0:sc>RAV</ns0:sc>S<ns0:sc>PHERE</ns0:sc> to infer the mass distributions of these galaxies, allowing us to compute the RAR. We compare the results with the EDGE simulations of isolated dwarf galaxies with similar stellar masses in a Λ cold dark matter cosmology. We find that most of the observed dwarf galaxies lie systematically above the low-mass extrapolation of the RAR. Each galaxy traces a locus in the RAR space that can have a multi-valued observed acceleration for a given baryonic acceleration, while there is significant scatter from galaxy to galaxy. Our results indicate that the RAR does not apply to low-mass dwarf galaxies, and that the inferred baryonic acceleration of these dwarfs does not contain enough information, on its own, to derive the observed acceleration. The simulated EDGE dwarfs behave similarly to the real data, with a higher observed acceleration at a fixed baryonic acceleration than the extrapolated RAR. We show that, in the context of modified, weak-field gravity theories, these results cannot be explained by differential tidal forces from the Milky Way or by the galaxies being far from dynamical equilibrium, since none of the galaxies in our sample seem to experience strong tides. As such, our results provide further evidence of the need for invisible dark matter in the smallest dwarf galaxies.