Galaxy halos were already magnetised a billion years after the Big Bang

By cross-matching Faraday rotation measures from the SPICE-RACS survey with DESI quasar spectra, the team identified sightlines passing through foreground galaxies traced by magnesium absorption. Those sightlines show a 4.5-sigma excess in rotation measure dispersion compared to clean control sightlines, a difference that persists after careful removal of Milky Way contamination. The excess implies circumgalactic magnetic field strengths of roughly 0.4–0.8 microgauss across projected radii of 20–150 kpc at a median redshift of z~1.14. This is the largest statistical sample used for this measurement and sets direct observational limits on when and how magnetic fields were amplified in galaxy halos.