Fluctuations in the Ionizing Background During and After Helium Reionization

Understand

The radiation background above the ionization edge of HeII varies strongly during and after helium reionization, because the attenuation length of such photons is relatively short (<40 Mpc) and because the ionizing sources (quasars) are rare.

• Here we construct analytic and Monte Carlo models to examine these fluctuations, including, for the first time, those during the reionization era itself.
• In agreement with detailed numerical simulations, our analytic model for the post-reionization Universe predicts order-of-magnitude fluctuations in the HeII ionization rate.
• Observations of the hardness ratio between HeII/HI show even larger fluctuations, which may be due to more complicated radiative transfer effects.