Pre-heating of the intergalactic medium by gravitational collapse and ultraviolet background

TL;DR

This study uses cosmological hydrodynamical simulations to evaluate how gravitational collapse and ultraviolet background contribute to preheating of the intergalactic medium, impacting galaxy formation and large-scale conformity.

Contribution

It provides new quantitative insights into the limited role of gravitational collapse and UVB in preheating, challenging their effectiveness in galaxy formation models.

Findings

A significant fraction of IGM is heated at low redshift but drops at high redshift.

Most halos have circum-halo gas entropy exceeding virial entropy since z=2.

UVB increases the fraction of low-mass halos with long cooling times and high entropy.

Abstract

The preheating of intergalactic medium(IGM) by structure collapsing and ultraviolet background(UVB) are investigated in cosmological hydrodynamical simulations. When gravitational collapsing is the sole heating mechanism, we find that (1) $60\%, 45\%$ of the IGM are heated up to $S>8, 17$ kev cm$^2$ respectively at $z=0$, but the fractions drop rapidly to a few percents at $z=2$, (2) the entropy of the circum-halo gas $S_{\rm{cir}}$ is higher than the virial entropy for more than $75 \%$ of the halos with masses $M<10^{11.5}$ $M_{\odot}$ since $z=2$, but the fraction higher than the entropy, $S_{\rm{pr}}$, required in preventive model of galaxies formation is only $15-20 \%$ for halos with $M<10^{10.5} M_{\odot}$ at $z=0$, and decreases as redshift increases, (3)assuming a metallicity of $Z \leq 0.03 Z_{\odot}$, the fraction of halos whose circum-halo gas having a cooling time longer than the Hubble time $t_{cool,cir}>t_{\rm{H}}$ is merely $5-10 \%$ at $z \lesssim 0.5$, and even less at $z \geq 1$ for halos with $M<10^{10.5} M_{\odot}$. (4) gas in the filaments undergoes the strongest preheating. Furthermore, we show that the UVB can not enhance the fraction of IGM with $S>17$ kev cm$^2$, but can increase the fraction of low mass halos($<10^{10.5} M_{\odot}$) that having $S_{\rm{cir}}>S_{\rm{pr}}$ to $\sim 70 \%$ at $z=0$, and that having $t_{\rm{cool, cir}}>t_{\rm{H}}$ to $15-30 \%$ at $z \lesssim 0.5$. Our results indicate that preheating due to gravitational collapsing and UVB are inadequate to fulfil the needs of preventative model, especially for halos with $10^{10.5}<M<10^{11.5} M_{\odot}$. Nevertheless, these two mechanisms might cause large scale galactic conformity.