Constraining Reionization with the z ∼ 5–6 Lyα Forest Power Spectrum: The Outlook after Planck

Abstract

Abstract The latest measurements of cosmic microwave background electron-scattering optical depth reported by Planck significantly reduces the allowed space of H I reionization models, pointing toward a later ending and/or less extended phase transition than previously believed. Reionization impulsively heats the intergalactic medium (IGM) to ∼ 10 4 K , and owing to long cooling and dynamical times in the diffuse gas that are comparable to the Hubble time, memory of reionization heating is retained. Therefore, a late-ending reionization has significant implications for the structure of the z ∼ 5 – 6 Lyα forest. Using state-of-the-art hydrodynamical simulations that allow us to vary the timing of reionization and its associated heat injection, we argue that extant thermal signatures from reionization can be detected via the Lyα forest power spectrum at 5 < z < 6 . This arises because the small-scale cutoff in the power depends not only the the IGM temperature at these epochs, but is also particularly sensitive to the pressure-smoothing scale set by the IGM full thermal history. Comparing our different reionization models with existing measurements of the Lyα forest flux power spectrum at z = 5.0 – 5.4 , we find that models satisfying Planck’s τ e constraint favor a moderate amount of heat injection consistent with galaxies driving reionization, but disfavoring quasar-driven scenarios. We study the feasibility of measuring the flux power spectrum at z ≃ 6 using mock quasar spectra and conclude that a sample of ∼10 high-resolution spectra with an attainable signal-to-noise ratio will allow distinguishing between different reionization scenarios.