Does Planck 2015 polarization data favor high redshift reionization?

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 ${z}_{\mathrm{max}}=30$, 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 $\ensuremath{\ell}\ensuremath{\sim}10$ region of the data which show high power relative to $\ensuremath{\ell}\ensuremath{\le}8$ 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 $2\mathrm{\ensuremath{\Delta}}\mathrm{ln}\mathcal{L}=5--6$. 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 $\ensuremath{\ell}\ensuremath{\sim}15--20$, as well as small but detectable differences at $\ensuremath{\ell}>30$ that can further restrict the high redshift limit of reionization.