THE EXTENT OF MAGNETIC FIELDS AROUND GALAXIES OUT TO z ∼ 1

Radio quasar sightlines with strong Mg ii absorption lines display statistically enhanced Faraday rotation measures (RMs), indicating the presence of additional magneto-active plasma with respect to sightlines free of such absorption. In this Letter, we use multi-color optical imaging to identify the galaxies likely hosting the magneto-active plasma, and to constrain the location of the latter with respect to the putative parent halo. We find that all of the sightlines with high |RM| pass within 50 kpc of a galaxy and that the |RM| distribution for low impact parameters, D < 50 kpc, is significantly different than for larger impact parameters. In addition, we find a decrease in the ratio of the polarization at 21 cm and 1.5 cm, p 21 / p 1.5 , toward lower D . These two effects are most likely related, strengthen the association of excess |RM| with intervening galaxies, and suggest that intervening galaxies operate as inhomogeneous Faraday screens. These results are difficult to reconcile with only a disk model for the magnetic field, but are consistent with highly magnetized winds associated with Mg ii systems. We infer strong magnetic fields of a few tens of μG, consistent with the values required by the lack of evolution of the FIR-radio correlation at high redshifts. Finally, these findings lend support to the idea that the small-scale helicity bottleneck of α–Ω galactic dynamos can be significantly alleviated via galactic winds.