Helium Reionization Simulations. II. Signatures of Quasar Activity on the IGM

We have run a new suite of simulations that solve hydrodynamics and radiative transfer simultaneously to study helium II reionization. Our suite of simulations employs various models for populating quasars inside of dark matter halos, which affect the He II reionization history. In particular, we are able to explore the impact that differences in the timing and duration of reionization have on observables. We examine the thermal signature that reionization leaves on the IGM, and measure the temperature-density relation. As previous studies have shown, we confirm that the photoheating feedback from helium II reionization raises the temperature of the IGM by several thousand kelvin. To compare against observations, we generate synthetic Ly$alpha$ forest sightlines on-the-fly and match the observed effective optical depth $tau_{mathrm{eff}}(z)$ of hydrogen to recent observations. We show that when the simulations have been normalized to have the same values of $tau_mathrm{eff}$, the effect that helium II reionization has on observations of the hydrogen Ly$alpha$ forest is minimal. Specifically, the flux PDF and the one-dimensional power spectrum are sensitive to the thermal state of the IGM, but do not show direct evidence for the ionization state of helium. We show that the peak temperature of the IGM typically corresponds to the time of 90%-95% helium ionization by volume, and is a relatively robust indicator of the timing of reionization. Future observations of helium reionization from the hydrogen Ly$alpha$ forest should thus focus on measuring the temperature of the IGM, especially at mean density. Detecting the peak in the IGM temperature would provide valuable information about the timing of the end of helium II reionization.