The Mass Missing Problem in Clusters: Dark Matter or Modified Dynamics?

The widely accepted dark matter hypothesis offers a seductive solution to missing mass problems (galaxies, clusters of galaxies, gravitational collapse in structure formation,...). However the physical nature of the Dark Matter itself is still unknown. Alternatively, it has been proposed that apparent dynamical evidence of dark matter is due to a modification of Newton’s law of gravitation. Here we revisit the Modified Newtonian Dynamics (MOND) theories at the scale of galaxy clusters. Using hydrodynamical simulations, we derived quantities such as the density and the temperature of the ICM. We compared those MOND simulated predictions to high quality X-ray density and temperature profiles observed down to ∼0.5 the virial radius. If the density profiles seems in acceptable agreement, the simulated temperature show a constant increase with the radius whereas the observed profiles show a flat to a mild decrease shape down to ∼0.5 Rvirial. We also computed the dynamical MOND mass for 8 X-ray clusters observed with XMM-Newton. If the MOND mass helps to lower the discrepancy with the baryonic mass by ∼20%, still ∼80% of the mass in clusters is unaccounted by baryons. In order to solve this problem and to reconcile MOND with clusters observations, we investigated the possibility of an added dark baryonic component. We assumed a component of massive neutrinos to fill the remaining discrepancy between the observed MOND dynamical mass and the baryonic mass. This led us to derive a tied observational lower limit for the neutrino mass, mν = 1.06 eV.