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Open AccessArticle

Evidence for Toroidal B-Field Components in AGN Jets on Kiloparsec Scales

1
Department of Physics, University College Cork, Cork, Ireland
2
Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
*
Author to whom correspondence should be addressed.
Academic Editors: Emmanouil Angelakis, Markus Boettcher and Jose L. Gómez
Galaxies 2017, 5(4), 61; https://doi.org/10.3390/galaxies5040061
Received: 23 August 2017 / Revised: 26 September 2017 / Accepted: 28 September 2017 / Published: 4 October 2017
(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)
Though helical magnetic fields are generally believed to arise when the jets of Active Galactic Nuclei (AGN) are launched, it is still unclear what role they play (and if they survive) to the largest jet scales. A helical or toroidal B-field may contribute substantially to the collimation of the jet. This B-field structure can be detected in images of the Faraday rotation measure (RM)—a measure of the change in polarisation angle of an electromagnetic wave as it passes through a magneto-ionic medium. The Faraday rotation measure is directly proportional to the line-of-sight magnetic field; therefore a monotonic gradient in the RM transverse to the jet indicates similar behaviour of the line-of-sight B-field component. This type of analysis has mostly been done on parsec scales using VLBI observations at centimetre wavelengths, while relatively few studies have probed decaparsec to kiloparsec scales. The detection of RM gradients with significances of 3 σ or more on such large scales can demonstrate the presence of a toroidal field component, which may be associated with a helical field that has persisted to these distances from the centre of the AGN. We present the results of new Faraday rotation analyses for 2 AGN on kiloparsec scales based on multiwavelength VLA observations, with robust transverse RM gradients detected in both. Furthermore, the direction of the inferred toroidal B-fields on the sky supports previous results indicating a predominance of outward currents in the jets on kiloparsec scales. View Full-Text
Keywords: active galactic nuclei; magnetic fields; radio interferometry active galactic nuclei; magnetic fields; radio interferometry
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Knuettel, S.; Gabuzda, D.; O’Sullivan, S.P. Evidence for Toroidal B-Field Components in AGN Jets on Kiloparsec Scales. Galaxies 2017, 5, 61.

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