Probing the radial acceleration relation and the strong equivalence principle with the Coma cluster ultra-diffuse galaxies

The tight radial acceleration relation (RAR) obeyed by rotationally supported disk galaxies is one of the most successful a priori predictions modified Newtonian dynamics (MOND) paradigm on galaxy scales. Another important consequence MOND as classical modification gravity that strong equivalence principle (SEP) -- which requires small, free-falling, self-gravitating system not to depend external gravitational field in it embedded should be broken. Multiple tentative detections this so-called effect (EFE) have been made past, but systems sensitive are with low internal accelerations residing clusters within field. Here, we show ultra-diffuse (UDGs) Coma cluster do lie RAR, and their velocity dispersion profiles full agreement isolated predictions, especially when including some degree anisotropy. However, breaking SEP via EFE seriously deteriorates agreement. We discuss various possibilities explain context MOND, combination tidal heating higher baryonic masses. also speculate our results could mean screened UDGs. fact would happen precisely clusters, where fails, relevant nature residual missing mass galaxies.