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Open AccessArticle

An In-Depth Investigation of Faraday Depth Spectrum Using Synthetic Observations of Turbulent MHD Simulations

Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
School of Mathematics, Statistics and Physics, Newcastle University, Newcastle-upon-Tyne NE13 7RU, UK
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Avenue, New York, NY 10010, USA
Department of Physics and Astronomy, Rutgers, The State University of New Jersey, 136 Frelinghuysen Rd, Piscataway, NJ 08854, USA
ASML, De Run 6501, 5504 DR Veldhoven, The Netherlands
Author to whom correspondence should be addressed.
Galaxies 2019, 7(4), 89;
Received: 1 October 2019 / Revised: 18 November 2019 / Accepted: 18 November 2019 / Published: 23 November 2019
(This article belongs to the Special Issue New Perspectives on Galactic Magnetism)
In this paper, we present a detailed analysis of the Faraday depth (FD) spectrum and its clean components obtained through the application of the commonly used technique of Faraday rotation measure synthesis to analyze spectro-polarimetric data. To directly compare the Faraday depth spectrum with physical properties of a magneto-ionic medium, we generated synthetic broad-bandwidth spectro-polarimetric observations from magnetohydrodynamic (MHD) simulations of a transonic, isothermal, compressible turbulent medium. We find that correlated magnetic field structures give rise to a combination of spiky, localized peaks at certain FD values, and broad structures in the FD spectrum. Although most of these spiky FD structures appear narrow, giving an impression of a Faraday thin medium, we show that they arise from strong synchrotron emissivity at that FD. Strong emissivity at a FD can arise because of both strong spatially local polarized synchrotron emissivity at a FD or accumulation of weaker emissions along the distance through a medium that have Faraday depths within half the width of the rotation measure spread function. Such a complex Faraday depth spectrum is a natural consequence of MHD turbulence when the lines of sight pass through a few turbulent cells. This therefore complicates the convention of attributing narrow FD peaks to the presence of a Faraday-rotating medium along the line of sight. Our work shows that it is difficult to extract the FD along a line of sight from the Faraday depth spectrum using standard methods for a turbulent medium in which synchrotron emission and Faraday rotation occur simultaneously. View Full-Text
Keywords: galactic magnetic fields; polarimetry; magneto-hydrodynamics simulations galactic magnetic fields; polarimetry; magneto-hydrodynamics simulations
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Basu, A.; Fletcher, A.; Mao, S.A.; Burkhart, B.; Beck, R.; Schnitzeler, D. An In-Depth Investigation of Faraday Depth Spectrum Using Synthetic Observations of Turbulent MHD Simulations. Galaxies 2019, 7, 89.

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