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Open AccessArticle

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

1
Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
2
School of Mathematics, Statistics and Physics, Newcastle University, Newcastle-upon-Tyne NE13 7RU, UK
3
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
4
Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Avenue, New York, NY 10010, USA
5
Department of Physics and Astronomy, Rutgers, The State University of New Jersey, 136 Frelinghuysen Rd, Piscataway, NJ 08854, USA
6
ASML, De Run 6501, 5504 DR Veldhoven, The Netherlands
*
Author to whom correspondence should be addressed.
Galaxies 2019, 7(4), 89; https://doi.org/10.3390/galaxies7040089
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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