The evolution of a pre-heated intergalactic medium

We analyse the evolution of the Intergalactic Medium (IGM) by means of an extended set of large box size hydrodynamical simulations which include pre–heating. We focus on the properties of the z ∼ 2 Lyman-α forest and on the population of clusters and groups of galaxies at z = 0. We investigate the distribution of voids in the Lyman-α flux and the entropy–temperature relation of galaxy groups, comparing the simulation results to recent data from high-resolution quasar spectra and from 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 δh is increased to a minimum value Kfl. While the entropy level observed in the central regions of galaxy groups requires a fairly strong pre–heating, with Kfl > 100 keV cm , the void statistics of the Lyman-α forest impose that this pre–heating should take place only in relatively high–density regions, δh∼ > 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 ∼ < 1.