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