The extent of magnetic fields around galaxies out to z ~ 1

Radio quasar sightlines with strong MgII absorption lines display statistically enhanced Faraday Rotation Measures (RM) 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 likely galaxies 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}, towards 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 reconciliate with only a disk model for the magnetic field but are consistent with highly magnetized winds associated with MgII systems. We infer strong magnetic fields of a few tens of mikro Gauss, consistent with values required by the lack of evolution of the FIR-radio correlation at high redshifts. Finally, these findings lends support to the idea that the small scale helicity bottleneck of alpha-Omega galactic dynamos can be significantly alleviated via galatic winds.
Comment: Accepted for publication in ApJL, 6 pages, 3 figures