Galaxy clusters in presence of dark energy: a kinetic approach

Context. The external regions of galaxy clusters may be under strong influence of the dark energy, which was discovered by observations of supernovae Ia at redshift z < 1. The presence of the dark energy in the gravitational equilibrium equation, with the Einstein Λ term, balances the gravity, and extends the equilibrium configuration more in radius. Aims. We investigate the features of the equilibrium configurations to analyse how the presence of the dark energy affects the density profiles and radial extension by specifying the conditions for which the gravitational equilibrium begins. Methods. We derived the kinetic equation for an equilibrium configuration in presence of dark energy and solved the gravitational equilibrium equation by considering a Maxwell-Boltzmann distribution function with a cut-off in the framework of the Newtonian regime, because the observed velocities of galaxies inside a cluster are much lower than the velocity of light. Results. The prevalence of dark energy effects on the gravity shows a wide region in the W0–ρΛ diagram where equilibrium solutions are not possible. In these particular conditions, the galaxies located in the external regions of a cluster can flow out, following the accelerating expansion of the Universe.