Witnessing the reionization history using Cosmic Microwave Background observation from Planck

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 ~7-8 obtained from Lyman- 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 exible 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 exibilities 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 nd the mean values of optical depth to be larger ( 0:09 and 0:1) than what we nd in standard step like reionization histories (0:079 0:017). At the same time we also nd that the maximum free electron fraction allowed by the data for redshifts more than 15 is 0:25 at 95.4% con dence limit in the case of optimistic constraint.