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

Abstract

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. In order 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 the majority 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 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. of 30medium for a large number of sources in a much reduced amount of observing time. Broad-band spectro-polarimetry has played a crucial role in unveiling the properties of magnetic fields in nearby galaxies [1][2][3], in high redshift galaxies [4,5], in active galactic nuclei [AGN; 6-8] and the intergalactic medium [9]. In order to physically interpret such data, the technique of Faraday rotation measure (RM) synthesis [10,11] and direct fitting of the Stokes Q and U spectra of a polarized source with models of the magneto-ionic media, known as Stokes Q, U fitting [6,12,13] have been developed. It is often not straightforward to interpret the results from these techniques and connect them to the physical properties of the magnetized plasmas being investigated [14].Currently, several large-scale spectro-polarimetric campaigns are underway, mostly in the 1 to 5 GHz frequency range. On one hand, dedicated surveys with interferometers are being conducted to study the broad-band polarization properties of millions of extragalactic sources. For example, the Karl G. Jansky Very Large Array Sky Survey [VLASS; 15,16], the Polarization Sky Survey of the Universe's Magnetism [POSSUM; 17], the QU Observations at Cm wavelength with Km baselines using ATCA 1 (QUOCKA), the MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE) Survey [18], and the recent LOFAR Two-meter Sky Survey [L...

Properties of Polarized Synchrotron Emission from Fluctuation Dynamo Action—II. Effects of Turbulence Driving in the ICM and Beam Smoothing

“…In this section, we present in brief the setup used for performing direct numerical MHD simulations, and the methodology of computing the synthetic observations using the simulation data. For further detail on these methods, we refer an interested reader to Sur [15], Basu et al [31] and Sur et al [5].…”

“…The output of the MHD simulations, converted to physical dimensions, were used as input for the COSMIC package [31] to compute broad-bandwidth synthetic maps of the total and polarized synchrotron emission. For our analysis, we chose the xand y-axes to be in the plane of the sky, and the z-axis to be parallel to the line of sight (LOS).…”

“…1 The choice of the value of the flux density normalization would not affect the fractional polarization or Faraday depolarization, which are the main focus of this study, unless the type of turbulence driving itself is widely different for galaxy clusters at the extreme ends of luminosity and/or mass function. Details of numerical calculations performed in COSMIC are presented in Basu et al [31] and Sur et al [5]. Note that, for simplicity, we have not included any noise and/or systematic errors introduced while observing with a telescope.…”

Magnetism Science with the Square Kilometre Array

“…Furthermore, measurements of magnetic helicity can help to constrain dynamo theory [124]. Although the turbulent magnetic field component is expected to be ubiquitous in galaxies [83], it is only directly measurable in detail in the Milky Way (see, e.g., [125]), possibly also through interpretation of RM spectra (but see [126]). Understanding turbulent interstellar magnetic fields will allow us to gauge their influence on star formation, gas heating and dynamics, cosmic ray propagation, and other physical processes in the interstellar medium.…”

“…Recent developments in this area include the FIRESTARTER algorithm [238], which takes into account the spectral indices of each of the fitted polarized components, and the use of convolutional neural networks (CNN) to classify Faraday depth spectra, to distinguish simple sightlines that exhibit only one RM component from more complex sightlines [239]. Interpretation of Faraday spectra in the case of turbulence can be complicated [126] and will require additional consideration.…”