Inhomogeneous HeII reionization in Hydrodynamic Simulations
  The reionization of the second electron of helium shapes the physical state
of intergalactic gas at redshifts between 2 < z < 5. Because performing full in
situ radiative transfer in hydrodynamic simulations is computationally
expensive for large volumes, the physics of HeII reionization is often
approximated by a uniform UV background model that does not capture the
patchiness of reionization. We have devised a model that implements the effects
of HeII reionization using semi-analytic calculations of the thermal state of
intergalactic gas-- a way to bypass a full radiative transfer simulation while
still realizing the physics of HeII reionization that affects observables such
as the Lyman alpha forest. Here we present a publicly-available code that
flexibly models inhomogeneous HeII reionization in simulations at a negligible
computational cost. Because many of the parameters of HeII reionization are
uncertain, our model is customizable from a set of free parameters. We show
results from this code in MP-Gadget, where this model is implemented. We
demonstrate the resulting temperature evolution and temperature-density
relation of intergalactic gas-- consistent with recent measurements and
previous radiative transfer simulations. We show that the impact of HeII
reionization gives rise to subtle signatures in the one-dimensional statistics
of the Lyman alpha forest at the level of several per cent, in agreement with
previous findings. The flexible nature of these simulations are ideal for
studies of HeII reionization and future observations of the HeII Lyman alpha
forest.
