Inhomogeneous He II reionization in hydrodynamic simulations

The reionization of the second electron of helium shapes the physical state of intergalactic gas at redshifts between 2 less than or similar to z less than or similar to 5. Because performing full in situ radiative transfer in hydrodynamic simulations is computationally expensive for large volumes, the physics of He II reionization is often approximated by a uniform ultraviolet background model that does not capture the spatial inhomogeneity of reionization. We have devised a model that implements the effects of He II 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 He II reionization that affects observables such as the Lyman alpha forest. Here, we present a publicly available code that flexibly models inhomogeneous He II reionization in simulations at a negligible computational cost. Because many of the parameters of He II 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 He II reionization gives rise to subtle signatures in the 1D statistics of the Lyman alpha forest at the level of several percent, in agreement with previous findings. The flexible nature of these simulations is ideal for studies of He II reionization and future observations of the He II Lyman alpha forest.