Constraining Reionization with the $z \sim 5-6$ Lyman-$\alpha$ Forest Power Spectrum: the Outlook after Planck

Understand

The latest measurements of CMB electron scattering optical depth reported by Planck significantly reduces the allowed space of HI reionization models, pointing toward a later ending and/or less extended phase transition than previously believed.

• Reionization impulsively heats the intergalactic medium (IGM) to $\sim10^4$ K, and owing to long cooling and dynamical times in the diffuse gas, comparable to the Hubble time, memory of reionization heating is retained.
• Therefore, a late ending reionization has significant implications for the structure of the $z\sim5-6$ Lyman-$\alpha$ (ly$\alpha$) 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 Ly$\alpha$ forest power spectrum at $5< z<6$.