On the evolution of cooling cores in X–ray galaxy clusters

To define a framework for the formation and evolution of the cooling cores in X-ray galaxy clusters, we study how the physical properties change as function of the cosmic time in the inner regions of a 4 keV and 8 keV galaxy cluster under the action of radiative cooling and gravity only. The cooling radius, Rcool, defined as the radius at which the cooling time equals the Universe age at given redshift, evolves from ∼ 0.01R200 at z > 2, where the structures begin their evolution, to ∼ 0.05R200 at z = 0. The values measured at 0.01R200 show an increase of about 15-20 per cent per Gyr in the gas density and surface brightness and a decrease with a mean rate of 10 per cent per Gyr in the gas temperature. The emission-weighted temperature diminishes by about 25 per cent and the bolometric X-ray luminosity rises by a factor ∼ 2 after 10 Gyrs when all the cluster emission is considered in the computation. On the contrary, when the core region within 0.15R500 is excluded, the gas temperature value does not change and the X-ray luminosity varies by 10 − 20 per cent only. The cooling time and gas entropy radial profiles are well represented by power-law functions, tcool = t0 + t0.01(r/0.01R200)γ and K = K0 + K0.1(r/0.1R200)α, with t0 and K0 that decrease with time from 13.4 Gyrs and 270 keV cm in the hot system (8.6 Gyr and 120 keV cm in the cool one) and reach zero after about 8 (3) Gyrs. The slopes vary slightly with the age, with γ ≈ 1.3 and α ≈ 1.1. The behaviour of the inner slopes of the gas temperature and density profiles are the most sensitive and unambiguous tracers of an evolving cooling core. Their values after 10 Gyrs of radiative losses, Tgas ∝ r and ngas ∝ r, are remarkably in agreement with the observational constraints available for nearby X-ray luminous cooling core clusters. Because our simulations do not consider any AGN heating, they imply that the feedback process does not greatly alter the gas density and temperature profiles as generated by radiative cooling alone.