Cosmic reionization after Planck

TL;DR

This paper combines Planck data and high-redshift observations to constrain cosmic reionization history, indicating a rapid completion around redshift 6-8 with a consistent escape fraction of about 10%.

Contribution

It provides new constraints on reionization history and escape fraction by integrating Planck optical depth data with high-redshift galaxy observations.

Findings

Reionization completed around redshift 5.8 to 8.5.

Escape fraction of ionizing photons is approximately 10%.

Reionization history favors a single stellar population scenario.

Abstract

Cosmic reionization holds the key to understand structure formation in the Universe, and can inform us about the properties of the first sources, as their star formation efficiency and escape fraction of ionizing photons. By combining the recent release of Planck electron scattering optical depth data with observations of high-redshift quasar absorption spectra, we obtain strong constraints on viable reionization histories. We show that inclusion of Planck data favors a reionization scenario with a single stellar population. The mean $x_{\rm HI}$ drops from $\sim0.8$ at $z=10.6$ to $\sim10^{-4}$ at $z=5.8$ and reionization is completed around $5.8\lesssim z\lesssim8.5$ (2-$\sigma$), thus indicating a significant reduction in contributions to reionization from high redshift sources. We can put independent constraints on the escape fraction $f_{\rm esc}$ of ionizing photons by incorporating the high-redshift galaxy luminosity function data into our analysis. We find a non-evolving $f_{\rm esc}$ of $\sim10\%$ in the redshift range $z=6-9$.