Abstract
We investigate constraints on cosmic reionization extracted from the Planck
cosmic microwave background (CMB) data. We combine the Planck CMB anisotropy
data in temperature with the low-multipole polarization data to fit
ŁambdaCDM models with various parameterizations of the reionization history.
We obtain a Thomson optical depth \tau=0.058 +/- 0.012 for the commonly
adopted instantaneous reionization model. This confirms, with only data from
CMB anisotropies, the low value suggested by combining Planck 2015 results with
other data sets and also reduces the uncertainties. We reconstruct the history
of the ionization fraction using either a symmetric or an asymmetric model for
the transition between the neutral and ionized phases. To determine better
constraints on the duration of the reionization process, we also make use of
measurements of the amplitude of the kinetic Sunyaev-Zeldovich (kSZ) effect
using additional information from the high resolution Atacama Cosmology
Telescope and South Pole Telescope experiments. The average redshift at which
reionization occurs is found to lie between z=7.8 and 8.8, depending on the
model of reionization adopted. Using kSZ constraints and a redshift-symmetric
reionization model, we find an upper limit to the width of the reionization
period of \Deltaz < 2.8. In all cases, we find that the Universe is ionized
at less than the 10% level at redshifts above z~10. This suggests that an early
onset of reionization is strongly disfavoured by the Planck data. We show that
this result also reduces the tension between CMB-based analyses and constraints
from other astrophysical sources.
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