The strength and structure of the magnetic field in the galactic outflow of M82

Abstract

Galactic outflows driven by starbursts can modify the galactic magnetic fields and drive them away from the galactic planes. Here, we quantify how these fields may magnetize the intergalactic medium. We estimate the strength and structure of the fields in the starburst galaxy M82 using thermal polarized emission observations from SOFIA/HAWC+ and a potential field extrapolation. We modified the Davis-Chandrasekhar-Fermi method to account for the large-scale flow and the turbulent field. Results show that the observed magnetic fields arise from the combination of a large-scale ordered potential field associated with the outflow and a small-scale turbulent field associated with bow-shock-like features. Within the central 900 pc radius, the potential field accounts for 53 ± 4% of the observed turbulent magnetic energy with a median field strength of 305 ± 15 µG, while small-scale turbulent magnetic fields account for the remaining 40 ± 5% with a median field strength of 222 ± 19 µG. We estimate that the turbulent kinetic and turbulent magnetic energies are in close equipartition up to ∼ 2 kpc (measured), while the turbulent kinetic energy dominates at ∼ 7 kpc (extrapolated). We conclude that the fields are frozen into the ionized outflowing medium and driven away kinetically. This indicates that the magnetic field lines in the galactic wind of M82 are 'open,' providing a direct channel between the starburst core and the intergalactic medium. Our novel approach offers the tools needed to quantify the effects of outflows on galactic magnetic fields as well as their influence on the intergalactic medium and evolution of energetic particles.

The observable properties of cool winds from galaxies, AGN, and star clusters -- II. 3D models for the multiphase wind of M82

“…Our results therefore favour a magnetic confinement scenario, and hence suggest that M82 is characterised by a strong magnetic field with field lines tracing the trajectory of launched clouds. This scenario is consistent with the recent measurements of magnetic field strength and morphology in M82 (Yoast-Hull et al 2013;Jones et al 2019;Buckman et al 2020;Lopez-Rodriguez et al 2021), which show that the magnetic field threads from starburst core all the way to intergalactic medium with a strength of 𝐵 ∼ 300 𝜇G and 𝛽 ∼ 0.7.…”

Submillimetre observations of the two-component magnetic field in M82

“…We see the poloidal field extending to ∼ 350 pc above and below the plane, in very good agreement with the extent over which the HAWC+ 53𝜇m emission is observed. However, the 154𝜇m-inferred field geometry continues to be poloidal to significantly larger distances above the plane, and the superwind field lines have been extrapolated to extend into the intergalactic medium (Lopez-Rodriguez et al 2021). This suggests that at 850𝜇m, we are observing the poloidal magnetic field in the central starburst region.…”

“…The magnetic field in M82 has recently been observed with HAWC+ at 53𝜇m and 154𝜇m (Jones et al 2019), finding that the near-vertical field geometry in the nucleus to extend into the galactic halo, inconsistent with the magnetic bubble model of Greaves et al (2000). The magnetic field lines seen by HAWC+ have been inferred to be open -connecting the starburst core to the intergalactic medium, rather than creating a galactic fountain -in a recent preprint (Lopez-Rodriguez et al 2021).…”

Simulating radio synchrotron emission in star-forming galaxies: small-scale magnetic dynamo and the origin of the far infrared-radio correlation

“…In particular, polarized radio halos in edge-on galaxies demonstrate the presence of poloidal magnetic field lines connecting the disk to the halo and show that CR electrons escape into the circum-galactic medium via diffusion and advection (Tüllmann et al 2000;Heesen et al 2009;Miskolczi et al 2019;Stein et al 2020;Krause et al 2020). This picture of a dominant large-scale ordered poloidal field associated with the outflow is confirmed by thermal polarized emission observations from SOFIA/HAWC+ in combination with a potential field extrapolation (Lopez-Rodriguez et al 2021). Radio synchrotron emission probes CR electrons, which cannot directly provide dynamical feedback owing to their negligible energy density.…”