Comparison of the production rate of ionizing photons and the rate of actual ionizations, including secondary collisional ionizations. This is the maximum ideal case where all AGN are assumed to accrete at a rate of λEdd = 1.0 and the X-ray photon index is Γ = 2.5. The difference between the two curves increases as redshift increases, where the IGM becomes more and more neutral. The hydrogen ionization fraction, x, used to calculate ṅAGN in Eq. (4) is due solely to the star-forming galaxies calculated by Robertson et al. (2013). Important to note that even though the high-energy photons cause additional ionizations, the number remains below ṅ ≈ 2 − 4 × 1050 Mpc-3 as seen in the work of Alvarez et al. (2012) for the galactic ionization production rate. The inset shows the ratio of ionizations to ionizing photons.
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