The impact of helium reionization on the structure of the intergalactic
  medium
  We examine the impact of helium reionization on the structure of the
intergalactic medium (IGM). We model the reionization using a radiative
transfer (RT) code coupled to the combined gravity hydrodynamics code Enzo.
Neutral hydrogen and helium are initially ionized by a starburst spectrum,
which is allowed to gradually evolve into a power law spectrum over the
redshift interval 3 < z < 4. The temperature-density relation of the gas is
found to fan out and flatten following HeII reionization, with an inversion for
overdensities above 5. Peculiar velocities of up to 10 km/s are induced by the
increased pressure, with the gas density field distorted over large coherent
regions by 10-20%, and the dark matter by levels of 1%. The
photoionization-induced flows may thus distort the matter power spectrum at
comoving wavenumbers k > 0.5 h/Mpc by a few percent by z = 2. Absorption
spectra for HI and HeII are drawn from the simulations, and absorption lines
are fit to the spectra. A median Doppler parameter of 35 km/s is obtained for
the HI absorption systems at z = 3. Dividing into subsamples optically thick
and optically thin at line centre reveals that the optically thick systems
undergo only mild evolution while the optically thin systems evolve rapidly
following HeII reionization. A comparison between HeII and HI absorption
features shows a broad distribution in the HeII and HI column density ratio,
peaking near the measured value and only slightly narrower than measured. A
comparison with approximate simulation methods shows moderately good agreement
in the absorption line properties, but not to the precision to which they may
be measured.
