“Four ways of looking at the radial acceleration relation”
The radial acceleration relation (RAR) is a strong correlation of late-type galaxy dynamics, seeming to describe a mysterious link between baryonic and total dynamical mass. Its properties are so striking that it has even been argued to manifest a new law of gravity in galaxies, the so-called Modified Newtonian Dynamics (MOND). I will describe four recent works analysing the RAR in novel ways. The first marginalises over the galaxy parameters upon which the RAR variables depend, revealing an extremely small intrinsic scatter of 0.034 dex. The second applies a technique called symbolic regression to generate and score all possible functional forms for the RAR, suggesting the data is insufficient to infer Newtonian and deep-MOND limits with statistical confidence. The third uses machine learning to present evidence that the RAR is fundamental: it has no residual correlations, is the tightest projection of galaxies’ dynamical parameter space and is uniquely capable of explaining all other dynamical correlations. The last shows that under a “classic” MOND interpretation, the RAR is inconsistent with planetary ephemerides and the kinematics of local wide binaries. I close with an abortive attempt to tie this all together.