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

Research output: Contribution to journalArticlepeer-review

28 Scopus citations


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 (US)
Pages (from-to)1254-1282
Number of pages29
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
StatePublished - Oct 1 2021
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • H ii regions
  • dark ages, reionization, first stars
  • galaxies: ISM
  • galaxies: high-redshift
  • galaxies: star formation


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