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.