An underlying universal pattern in galaxy halo magnetic fields

Abstract

Magnetic fields in galaxy halos are in general very difficult to observe. Most recently, the Continuum HAlos in Nearby Galaxies - an EVLA Survey (CHANG-ES) Collaboration investigated the radio halos of 35 nearby edge-on spiral galaxies in detail and detected large-scale magnetic fields in 16 of them. We used the CHANG-ES radio polarization data to create rotation measure maps for all galaxies in the sample and stack them with the aim of amplifying any underlying universal toroidal magnetic field pattern in the halo above and below the disk of the galaxy. We discovered a large-scale magnetic field in the central region of the stacked galaxy profile, which is attributable to an axial electric current that universally outflows from the center, both above and below the plane of the disk. A similar symmetry-breaking has also been observed in astrophysical jets, but never before in galaxy halos. This is an indication that galaxy halo magnetic fields are probably not generated by pure magnetohydrodynamic processes in the central regions of galaxies. One such promising physical mechanism is the Cosmic Battery operating in the innermost accretion disk around the central supermassive black hole. We anticipate that our discovery will stimulate a more general discussion on the origin of astrophysical magnetic fields.