• galaxies: magnetic fields;
  • galaxies: high-redshift;
  • intergalactic medium;
  • cosmology: theory;
  • dark ages, reionization, first stars


Primordial magnetic fields generated in the early Universe are subject of considerable investigation, and observational limits on their strength are required to constrain the theory. Due to their impact on the reionization process, the strength of primordial fields can be limited using the latest data on reionization and the observed UV luminosity function of high-redshift galaxies. Given the steep faint-end slope of the luminosity function, faint galaxies contribute substantial ionizing photons, and the low-luminosity cut-off has an impact on the total budget thereof. Magnetic pressure from primordial fields affects such cut-off by preventing collapse in haloes with mass below 1010 Minline image, with B0 the comoving field strength. In this Letter, the implications of these effects are consistently incorporated in a simplified model for reionization, and the uncertainties due to the cosmological parameters, the reionization parameters and the observed UV luminosity function are addressed. We show that the observed ionization degree at z∼ 7 leads to the strongest upper limit of B0≲ 2–3 nG. Stronger limits could follow from measurements of high ionization degree at z > 7.