Preheating of the Intergalactic Medium by Gravitational Collapse and Ultraviolet Background

Abstract

Abstract The preheating of the intergalactic medium by structure collapse and ultraviolet background (UVB) is investigated in cosmological hydrodynamical simulations. When gravitational collapse is the sole heating mechanism, we find that (1) 60% and 45% of the IGM are heated up to S > 8 and 17 keV cm2, respectively, at z = 0, but the fractions drop rapidly to a few percent at z = 2; (2) the entropy of the circumhalo gas is higher than the virial entropy for more than 75% of the halos with masses since z = 2, but the fraction higher than the entropy, , required in the preventive model of galaxy formation is only 15%–20% for halos with at z = 0, and decreases as redshift increases; (3) assuming a metallicity of , the fraction of halos whose circumhalo gas has a cooling time longer than the Hubble time is merely 5%–10% at , and even less at for halos with ; and (4) gas in the filaments undergoes the strongest preheating. Furthermore, we show that the UVB cannot enhance the fraction of the IGM with , but can increase the fraction of low-mass halos ( ) having to ∼70% at z = 0 and that having to 15%–30% at . Our results indicate that preheating due to gravitational collapse and UVB is inadequate to fulfill the needs of the preventative model, especially for halos with . Nevertheless, these two mechanisms might cause large-scale galactic conformity.