Vol. 635
3. Cosmology

Reionization optical depth determination from Planck HFI data with ten percent accuracy

While it is true that the reionization optical depth determination has been greatly improved over the last two decades, tau is still the less constrained parameter of the lambdaCDM model. Uncertainties and systematics produce discrepant results for different large-scale CMB datasets (primarily Planck and WMAPS) and various methods. In this paper, the authors present an advanced approach to the Planck High Frequency Instrument (HFI) data using an improved version of the HFI map-making code, greatly reducing the residual large-scale contamination affecting the data. With large-scale polarization-only data, they measured tau = 0.0566 +0.0053 -0.0062 at 68% C.L., reducing the Planck 2018 legacy release uncertainty by ~40%. Within the lambdaCDM model, applied in combination with the Planck large scale temperature likelihood and the high-temperature and polarization likelihood, they measured tau = 0.059ą0.006 at 68% C.L. which corresponds to a mid-point reionization redshift of z re = 8.14ą0.61. This estimation of the reionization optical depth with 10% accuracy is the strongest constraint obtained to date.