The evolution of a pre-heated intergalactic medium

We analyse the evolution of the intergalactic medium by means of an extended set of large box size hydrodynamical simulations which include pre-heating. We focus on the properties of the z similar to 2 Lyman alpha forest and the population of clusters and groups of galaxies at z = 0. We investigate the distribution of voids in the Lyman alpha flux and the entropy-temperature relation of galaxy groups, comparing the simulation results to recent data from high-resolution quasar spectra and X-ray observations. Pre-heating is included through a simple phenomenological prescription, in which at z = 4 the entropy of all gas particles, whose overdensity exceeds a threshold value delta(h), is increased to a minimum value K-fl. While the entropy level observed in the central regions of galaxy groups requires a fairly strong pre-heating, with K-fl > 100 keV cm(2), the void statistics of the Lyman alpha forest impose that this pre-heating should take place only in relatively high-density regions, delta(h) greater than or similar to 30, in order not to destroy the cold filaments that give rise to the forest. We conclude that any injection of non-gravitational energy in the diffuse baryons should avoid low-density regions at high redshift and/or take place at relatively low redshift z less than or similar to 1.