Submillimetre observations of the two-component magnetic field in M82

Abstract

We observed the starburst galaxy M82 in 850𝜇m polarised light with the POL-2 polarimeter on the James Clerk Maxwell Telescope (JCMT). We interpret our observed polarisation geometry as tracing a two-component magnetic field: a poloidal component aligned with the galactic 'superwind', extending to a height ∼ 350 pc above and below the central bar; and a spiralarm-aligned, or possibly toroidal, component in the plane of the galaxy, which dominates the 850𝜇m polarised light distribution at galactocentric radii 2 kpc. Comparison of our results with recent HAWC+ measurements of the field in the dust entrained by the M82 superwind suggests that the superwind breaks out from the central starburst at ∼ 350 pc above the plane of the galaxy.