Transport of cosmic rays in the nearby starburst galaxy NGC 253

TL;DR

This study investigates cosmic ray transport in NGC 253, revealing a constant bulk speed indicative of a disk wind, and characterizes the galaxy's magnetic field structure through radio polarimetry.

Contribution

It provides the first measurement of cosmic-ray bulk speed in NGC 253 and models the galaxy's magnetic field as a combination of disk and halo components.

Findings

Cosmic-ray bulk speed is approximately 300 km/s, constant across the disk.

The magnetic field has a spiral disk component and an X-shaped halo component.

Evidence of a disk wind facilitating cosmic-ray and magnetic field transport.

Abstract

Radio halos require the coexistence of extra-planar cosmic rays and magnetic fields. Because cosmic rays are injected and accelerated by processes related to star formation in the disk, they have to be transported from the disk into the halo. A vertical large-scale magnetic field can significantly enhance this transport. We observed NGC 253 using radio continuum polarimetry with the Effelsberg and VLA telescopes. The radio halo of NGC 253 has a dumbbell shape with the smallest vertical extension near the center. With an estimate for the electron lifetime, we measured the cosmic-ray bulk speed as 300+/-30 km/s which is constant over the extent of the disk. This shows the presence of a "disk wind" in NGC 253. We propose that the large-scale magnetic field is the superposition of a disk (r,phi) and a halo (r,z) component. The disk field is an inward-pointing spiral with even parity. The conical (even) halo field appears in projection as an X-shaped structure, as observed in other edge-on galaxies. Interaction by compression in the walls of the superbubbles may explain the observed alignment between the halo field and the lobes of hot Halpha- and soft X-ray emitting gas. The disk wind is a good candidate for the transport of small-scale helical fields, required for efficient dynamo action, and as a source for the neutral hydrogen observed in the halo.