Parsec-scale magnetic fields in Arp 220

TL;DR

This study reports the first VLBI detections of Zeeman splitting in another galaxy, measuring magnetic fields in Arp 220's nuclear regions, validating previous single-dish results, and demonstrating VLBI's potential for small-scale magnetic field studies.

Contribution

First VLBI detection of Zeeman splitting in an external galaxy, confirming single-dish measurements and showcasing high-resolution magnetic field analysis in extragalactic systems.

Findings

Magnetic fields of 1-5 mG detected in Arp 220's nuclear clouds.

Magnetic energy densities are comparable to gravitational energies.

Observed variability in maser lines likely due to intrinsic changes, not scintillation.

Abstract

We present the first very-long-baseline interferometry (VLBI) detections of Zeeman splitting in another galaxy. We used Arecibo Observatory, the Green Bank Telescope, and the Very Long Baseline Array to perform dual-polarization observations of OH maser lines in the merging galaxy Arp 220. We measured magnetic fields of $\sim$1-5 mG associated with three roughly parsec-sized clouds in the nuclear regions of Arp 220. Our measured magnetic fields have comparable strengths and the same direction as features at the same velocity identified in previous Zeeman observations with Arecibo alone. The agreement between single dish and VLBI results provides critical validation of previous Zeeman splitting observations of OH megamasers that used a single large dish. The measured magnetic field strengths indicate that magnetic energy densities are comparable to gravitational energy in OH maser clouds. We also compare our total intensity results to previously published VLBI observations of OH megamasers in Arp 220. We find evidence for changes in both structure and amplitude of the OH maser lines that are most easily explained by variability intrinsic to the masing region, rather than variability produced by interstellar scintillation. Our results demonstrate the potential for using high-sensitivity VLBI to study magnetic fields on small spatial scales in extragalactic systems.