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Recent He II Lyman-alpha forest observations from 2.0 < z < 3.2 show largefluctuations in the optical depth at z > 2.7. These results point to afluctuating He-ionizing background, which may be due to the end of heliumreionization of this era. We present a fast, semi-numeric procedure toapproximate detailed cosmological simulations. We compute the distribution ofdark matter halos, ionization state of helium, and density field at z = 3 inbroad agreement with recent simulations. Given our speed and flexibility, weinvestigate a range of ionizing source and active quasar prescriptions.Spanning a large area of parameter space, we find order-of-magnitudefluctuations in the He II ionization rate in the post-reionization regime.During reionization, the fluctuations are even stronger and develop a bimodaldistribution, in contrast to semi-analytic models and the hydrogen equivalent.These distributions indicate a low-level ionizing background even atsignificant He II fractions.

Semi-numeric simulations of helium reionization and the fluctuating radiation background