Fast Radio Bursts as Probes of Magnetic Fields in Galaxies at z < 0.5

We present a sample of nine fast radio bursts (FRBs) from which we derive magnetic field strengths of the host galaxies represented by normal, z < 0.5 star-forming galaxies with stellar masses M _* ≈ 10⁸-10^10.5 M _⊙. We find no correlation between the FRB rotation measure (RM) and redshift, which indicates that the RM values are due mostly to the FRB host contribution. This assertion is further supported by a significant positive correlation (Spearman test probability P _S < 0.05) found between the RM and the estimated host dispersion measure (DM_host; with Spearman rank correlation coefficient r _S = +0.75). For these nine galaxies, we estimate their magnetic field strengths projected along the sight line ∣B _∥∣, finding a low median value of 0.5 μG. This implies the magnetic fields of our sample of hosts are weaker than those characteristic of the solar neighborhood (≈6 μG), but relatively consistent with a lower limit on the observed range of ≈2-10 μG for star-forming disk galaxies, especially as we consider reversals in the B-field, and that we are only probing B_∥. We compare to RMs from simulated galaxies of the Auriga project—magneto-hydrodynamic cosmological zoom simulations—and find that the simulations predict the observed values to within a 95% confidence interval. Upcoming FRB surveys will provide hundreds of new FRBs with high-precision localizations, RMs, and imaging follow-up to support further investigation into the magnetic fields of a diverse population of z < 1 galaxies.