Warm photo-ionized IGM: a Clue for galaxy and cluster formation history?

In this paper we investigate the overcooling problem and propose some possible solutions. We show that the overcooling problem is generic to the hierarchical picture of structure formation, as long as the cooling is actually possible in small halos at high redshift. Solutions to this problem are likely to be associated with some feedback mechanism, and probably imply the existence of a warm IGM containing most of the cosmological baryons. We concentrate on some possible solutions, mainly photoionization and bulk-heating of the IGM. We show photoionization can act as a significant feedback mechanism but is not strong enough to solve the entire overcooling problem. We therefore assume that the IGM is maintained hot by some energy injection provided by supernova and galaxy formation is then limited by this feedback mechanism. Such a self regulated scheme allows us to compute the thermal history of the IGM... (abridged)