Testing refracted gravity with the kinematics of galaxy clusters

Refracted gravity (RG) is a classical theory of gravity where a gravitational permittivity, ϵ ( ρ ), namely, a monotonically increasing function of the local density, ρ , is introduced in the Poisson equation to mimic the effect of dark matter at astrophysical scales. We used high-precision spectroscopic data of two massive galaxy clusters, MACS J1206.2-0847 at a redshift of z = 0.44 and Abell S1063 (RXC J2248.7-4431) at z = 0.35, to determine the total gravitational potential in the context of RG. We also sought to constrain the three (supposedly universal) free parameters of this model. Using an upgraded version of the MG-MAMPOSS T algorithm, we performed a kinematic analysis that combines the velocity distribution of the cluster galaxies and the velocity dispersion profile of the stars within the brightest cluster galaxy (BCG). This unprecedented dataset used was obtained from an extensive spectroscopic campaign carried out with the VIMOS and MUSE spectrographs at the ESO VLT. We found that RG describes the kinematics of these two clusters as well as Newtonian gravity, although the latter is slightly preferred. However, (i) each cluster requires a different set of the three free RG parameters and (ii) the two sets are inconsistent with other results in the literature at different scales. We discuss the limitation of the method used to constrain the RG parameters, as well as possible systematic effects, which can give rise to the observed tension; notably, these are deviations from the spherical symmetry and from the dynamical equilibrium of the clusters.