  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.
