Assuming Ionization Equilibrium and the Impact on the Lyα Forest Power Spectrum during the End of Reionization at 8 ≥ z ≥ 5

Abstract

We explore how the assumption of ionization equilibrium modulates the modeled intergalactic medium at the end of the hydrogen epoch of reionization using the cosmological radiation hydrodynamic Technicolor Dawn simulation. In neutral and partially ionized regions where the metagalactic ultraviolet background is weak, the ionization timescale t ion ≡ Γ−1 exceeds the Hubble time. Assuming photoionization equilibrium in such regions artificially boosts the ionization rate, accelerating reionization. By contrast, the recombination time t rec < t ion in photoionized regions, with the result that assuming photoionization equilibrium artificially increases the neutral hydrogen fraction. Using snapshots in the range 8 ≥ z ≥ 5, we compare the predicted Lyα forest (LAF) flux power spectrum with and without the assumption of ionization equilibrium. Small scales (k > 0.1 rad s km−1) exhibit reduced power from 7 ≤ z ≤ 5.5 in the ionization equilibrium case, while larger scales are unaffected. This occurs for the same reasons: ionization equilibrium artificially suppresses the neutral fraction in self-shielded gas and boosts ionizations in voids, suppressing small-scale fluctuations in the ionization field. When the volume-averaged neutral fraction drops below 10−4, the signature of nonequilibrium ionizations on the LAF disappears. Comparing with recent observations indicates that these nonequilibrium effects are not yet observable in the LAF flux power spectrum.