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

TL;DR

This study investigates the magnetic fields around galaxies up to redshift 1 by analyzing Faraday Rotation Measures and optical imaging, revealing strong magnetic fields likely associated with galactic winds rather than simple disk models.

Contribution

It provides new evidence linking high RM values to intervening galaxies within 50 kpc, supporting the role of galactic winds in magnetic field amplification.

Findings

High |RM| correlates with galaxies within 50 kpc.

Magnetic fields of a few tens of microGauss are inferred.

Galactic winds likely contribute to magnetic field structure.

Abstract

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.