We study the ionization and thermal evolution of the intergalactic medium during the epoch of He II reionization by means of radiation hydrodynamical cosmological simulations .
We post-process baryonic density fields from a standard optically-thin IGM simulation with a homogeneous galaxy-dominated UV background ( UVB ) which reionizes H I and He I at z=6.5 but does not have any contribution to the ionization of He II .
Therefore , we suppress the He II photoheating contribution to the gas temperature due to the homogeneous UVB .
Quasars are introduced as point sources throughout the 100 Mpc simulation volume located at cold dark matter ( CDM ) density peaks consistent with the Pei luminosity function .
We assume an intrinsic quasar spectrum J ( \nu ) \propto \nu ^ { -1.8 } and a luminosity proportional to the halo mass .
We evolve the spatial distribution of the He II ionizing radiation field at h \nu = 4 , 8 , and 16 Ryd using a time-implicit variable tensor Eddington factor radiative transfer scheme .
Simultaneously , we also solve for the local ionization of He II to He II and the associated photoheating of the gas including opacity effects .
We find that the percolation of the He III regions is essentially complete by z=2.5 .
When comparing to a self-consistent optically thin simulation at the same redshift , in which He II is also ionized by the uniform UVB , we find that inclusion of opacity effects results in higher IGM temperature by a factor of approximately 1.7 at the mean gas density level .
We construct synthetic absorption line spectra from which we derive statistical parameters of the He II Ly \alpha forest .
We use 300 long ( \Delta z = 0.2 ) random lines of sight to compute at \bar { z } = 2.5 \pm 0.1 a mean He II Ly \alpha line transmission of \bar { F } = 0.304 \pm 0.002 .
The error corresponds to a significant one standard deviation in the transmitted flux due to the sightline to sightline variance equal to \simeq 11 \% the mean value .
The opacity effect on the gas temperature is shown by comparing the broadening width of the H I and He II Ly \alpha lines to the results from the self-consistent optically thin simulation .
We find a shift by approximately 1.25 km/s to higher b-parameter values for both H I and He II .
Finally , we estimate the relative broadening width between the two forests and find that the He II median b-parameter is about 0.8 times the median H I broadening width .
This implies that the He II absorbers are physically extended consistent with conclusions from observed lines of sight .