Three-dimensional structure of the magnetic field in the disk of the Milky Way

¹ Department of Physics and Astronomy, University of Calgary, Calgary, Alberta, T2N 1N4, Canada
e-mail: aordog@ucalgary.ca
² Dominion Radio Astrophysical Observatory, Herzberg Programs in Astronomy and Astrophysics, National Research Council Canada, PO Box 248, Penticton, BC V2A 6J9, Canada

Received: 3 March 2017
Accepted: 23 April 2017

Abstract

Aims. We present rotation measures (RM) of the diffuse Galactic synchrotron emission from the Canadian Galactic Plane Survey (CGPS) and compare them to RMs of extragalactic sources in order to study the large-scale reversal in the Galactic magnetic field (GMF).

Methods. Using Stokes Q, U and I measurements of the Galactic disk collected with the Synthesis Telescope at the Dominion Radio Astrophysical Observatory, we calculate RMs over an extended region of the sky, focusing on the low longitude range of the CGPS (ℓ = 52° to ℓ = 72°).

Results. We note the similarity in the structures traced by the compact sources and the extended emission and highlight the presence of a gradient in the RM map across an approximately diagonal line, which we identify with the well-known field reversal of the Sagittarius-Carina arm. We suggest that the orientation of this reversal is a geometric effect resulting from our location within a GMF structure arising from current sheets that are not perpendicular to the Galactic plane, as is required for a strictly radial field reversal, but that have at least some component parallel to the disk. Examples of models that fit this description are the three-dimensional dynamo-based model of Gressel et al. (2013, A&A, 560, A93) and a Galactic scale Parker spiral (Akasofu & Hakamada 1982, ApJ, 253, 552), although the latter may be problematic in terms of Galactic dynamics.

Conclusions. We emphasize the importance of constructing three-dimensional models of the GMF to account for structures like the diagonal RM gradient observed in this dataset.