Magnetic fields near the peripheries of galactic discs

¹ Department of Physics, Moscow University, 119992 Moscow, Russia
² Sternberg Astronomical Institute, Moscow M.V. Lomonosov State University, Universitetskij pr., 13, 119992 Moscow, Russia
³ School of Mathematics, University of Manchester, Oxford Road, Manchester, M13 9PL, UK
⁴ MPI für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: rbeck@mpifr-bonn.mpg.de

Received: 27 December 2013
Accepted: 23 June 2014

Abstract

Context. Magnetic fields are observed beyond the peripheries of optically detected galactic discs, while numerical models of their origin and the typical magnitudes are still absent. Previously, studies of galactic dynamo have avoided considering the peripheries of galactic discs because of the very limited (though gradually growing) knowledge about the local properties of the interstellar medium.

Aims. Here we investigate the possibility that magnetic fields can be generated in the outskirts of discs, taking the Milky Way as an example.

Methods. We consider a simple evolving galactic dynamo model in the “no-z” formulation, applicable to peripheral regions of galaxies, for various assumptions about the radial and vertical profiles of the ionized gas disc.

Results. The magnetic field may grow as galaxies evolve, even in the more remote parts of the galactic disc, out to radii of 15 to 30 kpc, becoming substantial after times of about 10 Gyr. This result depends weakly on the adopted distributions of the half thickness and surface density of the ionized gas component. The model is robust to changes in the amplitude of the initial field and the position of its maximum strength. The magnetic field in the remote parts of the galactic disc could be generated in situ from a seed field by local dynamo action. Another possibility is field production in the central regions of a galaxy, followed by transport to the disc’s periphery by the joint action of the dynamo and turbulent diffusivity.

Conclusions. Our results demonstrate the possibilities for the appearance and strengthening of magnetic fields at the peripheries of disc galaxies and emphasize the need for observational tests with new and anticipated radio telescopes (LOFAR, MWA, and SKA).