Detection of a ~20 kpc coherent magnetic field in the outskirt of merging spirals: the Antennae galaxies

TL;DR

This study reveals a large-scale, regular magnetic field of about 20 kpc in the tidal tail of the Antennae galaxies, showing how tidal interactions can generate coherent magnetic structures in merging galaxies.

Contribution

First detection of a large-scale, regular magnetic field in the outskirts of merging spiral galaxies, demonstrating magnetic field stretching during tidal interactions.

Findings

Detected a highly polarized, regular magnetic field in the tidal tail.

The regular field reaches ~8.5 μG, formed within a few hundred million years.

Tidal interactions can produce large-scale magnetic structures despite turbulent dominance.

Abstract

We present a study of the magnetic field properties of NGC 4038/9 (the `Antennae' galaxies), the closest example of a late stage merger of two spiral galaxies. Wideband polarimetric observations were performed using the Karl G. Jansky Very Large Array between 2 and 4 GHz. Rotation measure synthesis and Faraday depolarization analysis was performed to probe the magnetic field strength and structure at spatial resolution of $\sim1$ kpc. Highly polarized emission from the southern tidal tail is detected with intrinsic fractional polarization close to the theoretical maximum ($0.62\pm0.18$), estimated by fitting the Faraday depolarization with a volume that is both synchrotron emitting and Faraday rotating containing random magnetic fields. Magnetic fields are well aligned along the tidal tail and the Faraday depths shows large-scale smooth variations preserving its sign. This suggests the field in the plane of the sky to be regular up to $\sim20$ kpc, which is the largest detected regular field structure on galactic scales. The equipartition field strength of $\sim8.5~\mu$G of the regular field in the tidal tail is reached within a few 100 Myr, likely generated by stretching of the galactic disc field by a factor of 4--9 during the tidal interaction. The regular field strength is greater than the turbulent fields in the tidal tail. Our study comprehensively demonstrates, although the magnetic fields within the merging bodies are dominated by strong turbulent magnetic fields of $\sim20~\mu$G in strength, tidal interactions can produce large-scale regular field structure in the outskirts.