Abstract
Abstract We present unprecedented high-fidelity radio images of the M87 jet. We analyzed Jansky Very Large Array broadband full-polarization radio data from 4 to 18 GHz. The observations were taken with the most extended configuration (A configuration), which allows the study of the emission of the jet up to kiloparsec scales with a linear resolution of ∼10 pc. The high sensitivity and resolution of our data allow us to resolve the jet width. We confirm a double-helix morphology of the jet material between ∼300 pc and ∼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 of 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.
Highlights
Active Galactic Nuclei (AGN) are powerful astrophysical objects, emitting at all wavelengths
Our new Very Large Array (VLA) observations offer a view of the M87 jet with an unprecedent combination of high sensitivity and high angular resolution
These knots have been proposed to trace filamentary structures, most probably a consequence of K-H instabilities. This morphology of a jet characterized by bright knots up to kpc scale is very clear in M87, mainly because of its brightness and its proximity
Summary
Accretion disk and vicinity of the BH ( 1 pc), up to the typical lengths of jets (∼1 kpc) (for a recent review on magnetic field in AGN, see Pudritz et al 2012; Gabuzda 2018, and references therein). In order to be interpreted as a solid sign of a helical configuration, the spatial gradients of Faraday Depth should be detected at several places along the jet direction, and it would be desirable to rule out the presence of external material, and/or to identify that the polarization properties are tracing the internal magnetic field This is not an easy task since it requires of very high angular resolution to resolve the jet width, very high sensitivity to detect polarized emission, and observations at several wavelengths to study polarization properties across the spectrum (Broderick & McKinney 2010; Taylor & Zavala 2010; Hovatta et al 2012). This is the most clear evidence to date for a helical configuration of the magnetic field in an AGN jet at these scales
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