JWST Constraints on Primordial Magnetic Fields

Abstract

Primordial magnetic fields (PMFs) enhance small-scale structure formation through the Lorentz force acting on baryons, boosting the abundance of low-mass halos and their hosted galaxies. We show that the reionisation history calibrated with the UV luminosity function (UVLF) provides stringent bounds: strong PMFs induce a characteristic double reionisation at $z\approx 24$ that is incompatible with CMB measurements of the optical depth, yielding $\sqrt{⟨B^2⟩}<0.27\mathrm{nG}$ and $<0.18\mathrm{nG}$ for $n_B=-2$ and $n_B=2$ respectively at $95\%\mathrm{CL}$ using Planck priors on $\ddot{I}„$. This establishes early galaxy observables as among the most sensitive probes of PMFs in Gaussian, non-helical scenarios.

5 pages