The effect of non-gravitational gas heating in groups and clusters of galaxies

We present a detailed study of a set of gas-dynamical simulations of galaxy groups and clusters in a flat, �CDM model with m = 0.3, aimed at exploring the effect of non– gravitational heating on the observable properties of the intracluster medium (ICM). We use GASOLINE, a version of the code PKDGRAV that includes an SPH description of hydrodynamics to simulate the formation of four cosmic halos with virial temperatures in the range 0.5∼ T ∼ 8 keV. These simulations resolve the structure and properties of the intra–cluster medium (ICM) down to a small fraction of the virial radius, Rvir. At our resolution X–ray luminosities, (LX), of runs with gravitational heating only are in good agreement, over almost two orders of magnitude in mass, with analytical predictions, that assume a universal profile for CDM halos. For each simulated structure, non–gravitational heating of the ICM is implemented in two different ways: (1) by imposing a minimum entropy floor, Sfl, at a given redshift, that we take in the range 16 z 65; (2) by gradually heating gas within collapsed regions, proportionally to the supernova rate expected from semi–analytical modeling of galaxy formation in halos having mass equal to that of the simulated systems. Our main results are the following. (a) An extra heating energy Eh∼ 1 keV per gas particle within Rvir at z = 0 is required to reproduce the observed LX–T relation, independent of whether it is provided in an impulsive way to create an entropy floor Sfl = 50–100 keV cm 2 , or is modulated in redshift according to the star formation rate; our SN feedback recipe provides at most Eh ≃ 1/3 keV/part and, therefore, its effect on the LX–T relation is too small to account for the observed LX–T relation. (b) The required heating implies, in small groups with T ∼ 0.5 keV, a baryon fraction as low as ∼ 40% of the cosmic value at Rvir/2; this fraction increases to about 80% for a T ≃ 3 keV cluster. (c) Temperature profiles are almost scale free across the whole explored mass range, with T decreasing by a factor of three at the virial radius. (d) The mass–temperature relation is almost unaffected by non–gravitational heating and follows quite closely the M ∝ T 3/2 scaling; however, when compared with data on the M500–Tew relation, it has a ∼ 40% higher normalization. This discrepancy is independent of the heating scheme adopted. The inclusion of cooling in a run of a small group steepens the central profile of the potential well while removing gas from the diffuse phase. This has the effects of increasing Tew by ∼ 30%, possibly reconciling the simulated and the observed M500–Tew relations, and of decreasing LX by ∼ 40%. However, in spite of the inclusion of SN feedback energy, almost 40% of the gas drops out from the hot diffuse phase, in excess of current observational estimates of the amount of cold baryons in galaxy systems. Likely, only a combination of different heating sources (SNe and AGNs) and cooling will be able to reproduce both the LX–Tew and M500–Tew relations, as observed in groups and clusters, while balancing the cooling runaway.