Does Planck 2015 polarization data favor high redshift reionization?

Here, 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_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 $\ell$ ~10 region of the data which show high power relative to $\ell$ ≤ 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 2Δ ln $L$ = 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 thataccommodate 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 $\ell$ ~15-20, as well as small but detectable differences at $\ell$ >30 thatcan further restrict the high redshift limit of reionization