Does Planck 2015 polarization favor high redshift reionization?

TL;DR

This paper investigates whether the Planck 2015 polarization data supports high redshift reionization, finding a preference for ionization at z>15 and suggesting that models including high redshift components fit the data better.

Contribution

It introduces a principal component analysis approach to assess high redshift reionization signatures in CMB polarization data, revealing a robust preference for z>15.

Findings

Preference for ionization at z>15 with >95% confidence level.

Better fit to data when allowing high redshift reionization components.

Testable differences in polarization spectra at specific multipoles.

Abstract

We study the relationship between signatures of high redshift ionization in large-angle CMB polarization power spectra and features in the Planck 2015 data. Using a principal component (PC) ionization basis that is complete to the cosmic variance limit out to $z_{\rm max}=30,40,50$, we find a robust $>95\%$ CL preference for ionization at $z>15$ with no preference for $z>40$. This robustness originates from the $\ell \sim 10$ region of the data which show high power relative to $\ell \le 8$ and result in a poor fit to a steplike model of reionization. Instead by allowing for high redshift reionization, the PCs provide a better fit by $2\Delta \mathrm{ln}\mathcal{L} = 5-6$. Due to a degeneracy in the ionization redshift response, this improved fit is due to a single aspect of the model: the ability to accommodate $z>10$ component to the ionization as we illustrate with a two-step reionization model. For this and other models that accommodate such a component, its presence is allowed and even favored; for models that do not, their poor fit reflects statistical or systematic fluctuations. These possibilities produce very different and testable predictions at $\ell \sim 15-20$, as well as small but detectable differences at $\ell>30$ that can further restrict the high redshift limit of reionization.