Reading M87's DNA: A Double Helix revealing a large scale Helical Magnetic Field

TL;DR

This paper presents high-resolution radio images of the M87 jet, revealing a double-helix morphology and evidence of a large-scale helical magnetic field, likely maintained by Kelvin-Helmholtz instabilities, up to 1 kpc from the black hole.

Contribution

The study provides the first detailed polarization analysis confirming a large-scale helical magnetic field in M87's jet using broadband radio data.

Findings

Confirmed double-helix morphology of the jet

Detected polarization and Faraday depth gradients consistent with a helical magnetic field

Supported the presence of internal Faraday depolarization effects

Abstract

We present unprecedented high fidelity radio images of the M87 jet. We analyzed Jansky Very Large Array (VLA) broadband, full polarization, radio data from 4 to 18 GHz. The observations were taken with the most extended configuration (A configuration), which allow the study of the emission of the jet up to kpc scales with a linear resolution $\sim$10 pc. The high sensitivity and resolution of our data allow to resolve the jet width. We confirm a double-helix morphology of the jet material between $\sim$300 pc and $\sim$1 kpc. We found a gradient of the polarization degree with a minimum at the projected axis and maxima at the jet edges, and a gradient in the Faraday depth with opposite signs at the jet edges. We also found that the behavior of the polarization properties along the wide range of frequencies is consistent with internal Faraday depolarization. All these characteristics strongly support the presence of a helical magnetic field in the M87 jet up to 1 kpc from the central black hole although the jet is most likely particle dominated at these large scales. Therefore, we propose a plausible scenario in which the helical configuration of the magnetic field has been maintained to large scales thanks to the presence of Kelvin-Helmholtz instabilities.