Regular magnetic fields in the dwarf irregular galaxy NGC 4449

TL;DR

This study reveals strong, galaxy-scale regular magnetic fields in the dwarf irregular galaxy NGC 4449, challenging existing dynamo theories due to its weak rotation and chaotic gas motions.

Contribution

It provides high-resolution radio observations showing unexpected large-scale magnetic fields in a slowly rotating, irregular galaxy, suggesting the need for revised dynamo models.

Findings

Regular magnetic fields reach 8 μG in strength.

Magnetic vectors form radial fans and spiral fragments.

Systematic Faraday rotation indicates genuine galaxy-scale fields.

Abstract

We present a high-resolution VLA study of the total power and polarized radio continuum emission at 8.46 and 4.86 GHz of the irregular galaxy NGC 4449, known for its weak rotation and non-systematic gas motions. We found strong galaxy-scale regular magnetic fields, which is surprising because of a lack of ordered rotation required for the dynamo action. The strength of the regular field reaches 8 $\mu$G and that of the total field 14 $\mu$G, comparable to that of the total magnetic field strength in radio-bright spirals. The magnetic vectors in NGC 4449 form radial ``fans'' in the central region and fragments of a spiral pattern in the galaxy's outskirts. These structures are associated with large regions of systematic Faraday rotation, implying genuine galaxy-scale magnetic fields rather than random ones compressed and stretched by gas flows. The observed pattern of polarization B-vectors is similar to dynamo-type fields in normal spirals. Nonstandard, fast dynamo concepts are required to explain the observed field strengths, though it is unknown what kind of magnetic field geometry can be produced in slowly and chaotically rotating objects. The so far neglected role of magnetic fields for the dynamics and star formation in dwarf irregulars also needs to be revised.