Witnessing the reionization history using Cosmic Microwave Background observation from Planck

TL;DR

This paper uses Planck 2015 CMB data and prior Lyman-alpha observations to reconstruct the reionization history, revealing a preference for extended reionization and higher optical depths than standard models.

Contribution

It introduces a flexible polynomial-based framework to reconstruct the reionization history from CMB data and prior constraints, allowing for more general reionization scenarios.

Findings

CMB data marginally favors extended reionization histories.

Optical depth estimates are larger than standard steplike models.

Maximum free electron fraction at high redshift is about 0.25.

Abstract

We constrain the history of reionization using the data from Planck 2015 Cosmic Microwave Background (CMB) temperature and polarization anisotropy observations. We also use prior constraints on the reionization history at redshifts $\sim7-8$ obtained from Lyman-$\alpha$ emission observations. Using the free electron fractions at different redshifts as free parameters, we construct the complete reionization history using polynomials. Our construction provides an extremely flexible framework to search for the history of reionization as a function of redshifts. We present a conservative and an optimistic constraint on reionization that are categorized by the flexibilities of the models and datasets used to constrain them, and we report that CMB data marginally favors extended reionization histories. In both the cases, we find the mean values of optical depth to be larger ($\approx0.09$ and $0.1$) than what we find in standard steplike reionization histories ($0.079\pm0.017$). At the same time we also find that the maximum free electron fraction allowed by the data for redshifts more than 15 is $\sim0.25$ at 95.4\% confidence limit in the case of optimistic constraint.