Testing dark matter and modifications to gravity using local Milky Way observables

Galactic rotation curves are often considered the first robust evidence for the existence of dark matter. However, even in the presence of a dark matter halo, other galactic-scale observations, such as the baryonic Tully-Fisher relation and the radial acceleration relation, remain challenging to explain. This has motivated long-distance, infrared modifications to gravity as an alternative to the dark matter hypothesis as well as various dark matter theories with similar phenomenology. In general, the standard lore has been that any model that reduces to the phenomenology of modified Newtonian dynamics (MOND) on galactic scales explains essentially all galaxy-scale observables. We present a framework to test precisely this statement using local Milky Way observables, including the vertical acceleration field, the rotation curve, the baryonic surface density, and the stellar disk profile. We focus on models that predict scalar amplifications of gravity, i.e., models that increase the magnitude but do not change the direction of the gravitational acceleration. We find that models of this type are disfavored relative to a simple dark matter halo model because the Milky Way data requires a substantial amplification of the radial acceleration with little amplification of the vertical acceleration. We conclude that models which result in a MOND-like force struggle to simultaneously explain both the rotational velocity and vertical motion of nearby stars in the Milky Way.