The OBELISK simulation: Galaxies contribute more than AGN to H I reionization of protoclusters

¹ Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis bd Arago, 75014 Paris, France
e-mail: m.trebitsch@rug.nl
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
³ Zentrum für Astronomie der Universität Heidelberg, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
⁴ DARK Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
⁵ Department of Physics, The University of Hong Kong, Pokfulam Road, Kowloon Tong, Hong Kong
⁶ Department of Physics and Astronomy, University College London, WC1E 6BT London, UK
⁷ Sub-department of Astrophysics, University of Oxford, Keble Road, OX1 3RH Oxford, UK
⁸ Univ Lyon, Univ Lyon 1, Ens de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon, UMR5574, 69230 Saint-Genis-Laval, France
⁹ Department of Astronomy, Yonsei University, 50 Yonsei-ro, 03722 Seodaemun-gu, Seoul, Republic of Korea
¹⁰ Korea Institute for Advanced Study (KIAS), 85 Hoegiro, 02455 Dongdaemun-gu, Seoul, Republic of Korea

Received: 10 February 2020
Accepted: 25 May 2021

Abstract

We present the OBELISK project, a cosmological radiation-hydrodynamics simulation that follows the assembly and reionization of a protocluster progenitor during the first two billion years after the big bang, down to z = 3.5. The simulation resolves haloes down to the atomic cooling limit and tracks the contribution of different sources of ionization: stars, active galactic nuclei, and collisions. The OBELISK project is specifically designed to study the coevolution of high-redshift galaxies and quasars in an environment favouring black hole growth. In this paper, we establish the relative contribution of these two sources of radiation to reionization and their respective role in establishing and maintaining the high-redshift ionizing background. Our volume is typical of an overdense region of the Universe and displays star formation rate and black hole accretion rate densities similar to those of high-redshift protoclusters. We find that hydrogen reionization happens inside-out, is completed by z ∼ 6 in our overdensity, and is predominantly driven by galaxies, while accreting black holes only play a role at z ∼ 4.