Introducing SPHINX-MHD: The impact of primordial magnetic fields on the first galaxies, reionization, and the global 21-cm signal

Harley Katz, Sergio Martin-Alvarez, Joakim Rosdahl, Taysun Kimm, Jérémy Blaizot, Martin G. Haehnelt, Léo Michel-Dansac, Thibault Garel, Jose Oñorbe, Julien Devriendt, Adrianne Slyz, Omar Attia, Romain Teyssier

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13 Citations (Scopus)


We present the first results from SPHINX-MHD, a suite of cosmological radiation-magnetohydrodynamics simulations designed to study the impact of primordial magnetic fields (PMFs) on galaxy formation and the evolution of the intergalactic medium (IGM) during the epoch of reionization. The simulations are among the first to employ multifrequency, on-the-fly radiation transfer and constrained transport ideal MHD in a cosmological context to simultaneously model the inhomogeneous process of reionization as well as the growth of primordial magnetic fields. We run a series of (5 cMpc)3 cosmological volumes, varying both the strength of the seed magnetic field and its spectral index. We find that PMFs with a spectral index (nB) and a comoving amplitude (B0) that have nB >-0.562 log 10 (B0/1nG) - 3.35 produce electron optical depths (τe) that are inconsistent with CMB constraints due to the unrealistically early collapse of low-mass dwarf galaxies. For nB ≥ -2.9, our constraints are considerably tighter than the ∼nG constraints from Planck. PMFs that do not satisfy our constraints have little impact on the reionization history or the shape of the UV luminosity function. Likewise, detecting changes in the Ly α forest due to PMFs will be challenging because photoionization and photoheating efficiently smooth the density field. However, we find that the first absorption feature in the global 21-cm signal is a particularly sensitive indicator of the properties of the PMFs, even for those that satisfy our τe constraint. Furthermore, strong PMFs can marginally increase the escape of LyC photons by up to 25 per cent and shrink the effective radii of galaxies by ∼ 44 per cent which could increase the completeness fraction of galaxy surveys. Finally, our simulations show that surveys with a magnitude limit of MUV,1500 Å = -13 can probe the sources that provide the majority of photons for reionization out to z = 12.

Original languageEnglish
Pages (from-to)1254-1282
Number of pages29
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
Publication statusPublished - 2021 Oct 1

Bibliographical note

Funding Information:
We thank the referee for their detailed revision of the manuscript. HK thanks Richard Shaw for providing us his customized version of CAMB which allowed us to create the matter initial conditions. This work was supported by the Programme National Cosmology et Galaxies (PNCG) of INSU, CNRS with INP and IN2P3, cofunded by CEA and CNES. MGH acknowledges support from the UKRI Science and Technology Facilities Council (grant numbers ST/N000927/1 and ST/S000623/1). Support by ERC Advanced Grant 320596 'The Emergence of Structure during the Epoch of reionization' is gratefully acknowledged. SMA acknowledges support by ERC Starting Grant 638707 'Black holes and their host galaxies: co-evolution across cosmic time'. TK was supported by the National Research Foundation of Korea (NRF-2019K2A9A1A0609137711 and NRF-2020R1C1C100707911). TG acknowledges support from the European Research Council under grant agreement ERC-stg757258 (TRIPLE). The research of AS and JD is supported by Adrian Beecroft and STFC. The authors gratefully acknowledge the Gauss Centre for Supercomputing e.V. ( for funding this project by providing computing time on the GCS Supercomputer JUWELS (Jülich Supercomputing Centre 2019) at Jülich Supercomputing Centre (JSC). Computing time for this work was provided by the Partnership for Advanced Computing in Europe (PRACE) as part of the 'First luminous objects and reionization with SPHINX (cont.)' (2019215124) project. We additionally acknowledges support and computational resources from the Common Computing Facility (CCF) of the LABEX Lyon Institute of Origins (ANR-10-LABX-66).

Publisher Copyright:
© 2021 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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