Planck intermediate results: XLVII. Planck constraints on reionization history

Research output: Contribution to journalArticle


  • Planck Collaboration

Research units

  • University of Cambridge
  • International School for Advanced Studies
  • University of Oviedo
  • University of California at Berkeley
  • Sorbonne Université-UPMC, UMR 7095, Institut d'Astrophysique de Paris, 98bis boulevard Arago, 75014, Paris, France
  • Sub-Department of Astrophysics, University of Oxford, Keble Road, Oxford, OX1 3RH, UK
  • Astrophysics & Cosmology Research Unit, School of Mathematics, Statistics & Computer Science, University of KwaZulu-Natal, Westville Campus, Private Bag X54001, 4000, Durban, South Africa
  • Jodrell Bank Centre for Astrophysics, Alan Turing Building, School of Physics and Astronomy, TheUniversity of Manchester, Oxford Road, Manchester, M13 9PL, UK
  • Laboratoire de Physique Subatomique et Cosmologie, Université Grenoble-Alpes, CNRS/IN2P3, 53, rue des Martyrs, 38026, Grenoble Cedex, France
  • Institut d'Astrophysique Spatiale, CNRS, Univ. Paris-Sud, Université Paris-Saclay, Bât. 121, 91405, Orsay Cedex, France
  • INFN, Sezione di Bologna, viale Berti Pichat 6/2, 40127, Bologna, Italy
  • CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028, Toulouse Cedex 4, France
  • Instituto de Física de Cantabria (CSIC-Universidad de Cantabria), Avda. de los Castros s/n, 39005, Santander, Spain
  • Istituto Nazionale di Fisica Nucleare, Sezione di Padova, via Marzolo 8, 35131, Padova, Italy
  • UPMC Univ. Paris 06, UMR 7095, 98bis boulevard Arago, 75014, Paris, France
  • INAF/IASF Milano, via E. Bassini 15, 20133, Milano, Italy
  • Infrared Processing and Analysis Center, California Institute of Technology Pasadena, CA 91125, USA
  • University of Toronto
  • APC, AstroParticule et Cosmologie, Université Paris Diderot, CNRS/IN2P3, CEA/lrfu, Observatoire de Paris, Sorbonne Paris Cité, 10 rue Alice Domon et Léonie Duquet, 75205, Paris Cedex 13, France
  • Institut d'Astrophysique de Paris, CNRS (UMR 7095), 98bis boulevard Arago, 75014, Paris, France
  • University of Sussex
  • Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA, 91109, USA
  • LAL, Université Paris-Sud, CNRS/IN2P3, 91405, Orsay, France
  • LERMA, CNRS, Observatoire de Paris, 61 Avenue de l'Observatoire, 75000, Paris, France
  • Kavli Institute for Cosmology Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
  • INAF/IASF Bologna, via Gobetti , 101, Bologna, Italy
  • University of Manchester
  • Dipartimento di Fisica, Università La Sapienza, P. le A. Moro 2, 00133, Roma, Italy
  • Imperial College London
  • European Space Agency, ESAC, Planck Science Office, Camino bajo del Castillo, s/n, Urbanización Villafranca del Castillo, Villanueva de la Cañada, 28691, Madrid, Spain
  • University of Helsinki


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 ΛCDM models with various parameterizations of the reionization history. We obtain a Thomson optical depth τ = 0.058 ± 0.012 for the commonly adopted instantaneous reionization model. This confirms, with data solely 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 Δz <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.


Original languageEnglish
Article numberA108
Number of pages19
JournalAstronomy & Astrophysics
Publication statusPublished - Dec 2016
MoE publication typeA1 Journal article-refereed

    Research areas

  • cosmic background radiation, dark ages, reionization, first stars, polarization

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ID: 10393374