Complete Reionization Constraints from Planck 2015 Polarization

TL;DR

This paper analyzes Planck 2015 polarization data to constrain reionization history across redshifts 6 to 30, developing a model-independent approach that reveals a possible high-redshift ionization component.

Contribution

It introduces a principal component-based method to constrain reionization history from polarization data, allowing model-independent inferences and testing the robustness of optical depth estimates.

Findings

Data favor a high-redshift ionization component at 2σ

Excess power observed in multipole range 10-20

Method enables application to arbitrary reionization models

Abstract

We conduct an analysis of the Planck 2015 data that is complete in reionization observables from the large angle polarization $E$-mode spectrum in the redshift range $6 < z < 30$. Based on 5 principal components, all of which are constrained by the data, this single analysis can be used to infer constraints on any model for reionization in the same range; we develop an effective likelihood approach for applying these constraints to models. By allowing for an arbitrary ionization history, this technique tests the robustness of inferences on the total optical depth from the usual step-like transition assumption, which is important for the interpretation of many other cosmological parameters such as the dark energy and neutrino mass. The Planck 2015 data not only allow a high redshift $z>15$ component to the optical depth but prefer it at the $2\sigma$ level. This preference is associated with excess power in the multipole range $10 \lesssim \ell \lesssim 20$ and may indicate high redshift ionization sources or unaccounted for systematics and foregrounds in the 2015 data.