Submillimetre observations of the two-component magnetic field in M82

TL;DR

This study uses submillimetre polarimetric observations to reveal a two-component magnetic field structure in M82, comprising a poloidal component associated with the superwind and a spiral or toroidal component in the galactic plane.

Contribution

It provides the first detailed polarimetric mapping of M82's magnetic field, identifying a two-component structure and linking it to the galaxy's superwind activity.

Findings

Detection of a poloidal magnetic field component aligned with the superwind.

Identification of a spiral or toroidal magnetic field component in the galactic plane.

Evidence that the superwind breaks out at about 350 pc above the galaxy plane.

Abstract

We observed the starburst galaxy M82 in 850$\mu$m 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 $\sim 350$ pc above and below the central bar; and a spiral-arm-aligned, or possibly toroidal, component in the plane of the galaxy, which dominates the 850$\mu$m polarised light distribution at galactocentric radii $\gtrsim 2$ 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 $\sim 350$ pc above the plane of the galaxy.