THE EXTENT OF MAGNETIC FIELDS AROUND GALAXIES OUT TO z ∼ 1Based on observations made with the ESO Telescopes at the La Silla Observatories under program 082.A-0917 and 085.A-0417.

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 vertical bar RM vertical bar pass within 50 kpc of a galaxy and that the vertical bar RM vertical bar 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 vertical bar RM vertical bar 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 mu G, 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 alpha-Omega galactic dynamos can be significantly alleviated via galactic winds.