Probing the Magnetized Medium of Galaxies and AGNs using Wideband Radio Polarimetry

Abstract

I performed an observational study of diffuse linearly polarized synchrotron emission of a nearby galaxy and of extragalactic background sources in order to study cosmic magnetism. The speciality of my work is given by the polarimetric observations using <em>broadband</em> and multi-channel capabilities of the observing instrument. With these new broadband observations, depolarization mechanisms were used as a powerful new tool to probe the 3D structure of magnetic fields in a spiral galaxy. After an introduction to the research field of cosmic magnetism and observational techniques used for my work, I describe the new high angular resolution and broadband polarization observations of the nearby face-on oriented spiral galaxy M51 at S-band (2 – 4 GHz) using the Very Large Array (VLA). I discuss in detail the different imaging parameters used for wideband polarimetric data. I present new images of the total intensity, the polarized intensity, the magnetic field structure, and the rotation measure (RM) of M51. The observed frequency range probes the magnetic field in a so far unknown layer of the transition region between the disk and the halo in M51. Surprisingly, I found the observed RM in the disk-halo transition region to be dominated by fluctuations. This was also shown by the RM structure function at S-band. A fluctuating RM pattern indicates that the magnetic field in the disk-halo transition region is dominated by vertical (with respect to the galaxy plane) magnetic fields. I combined the new S-band polarization data with radio polarization VLA+Effelsberg data at 4.85GHz and 8.35GHz and with broadband L-band (1–2GHz) VLA data. The observed degree of polarization as a function of wavelength was compared to an analytical depolarization model developed by Shneider et al. (2014a). The model makes distinct predictions of a two-layer (disk – halo) and three-layer (far-side halo – disk – near-side halo) system. I show that a two-layer system is more likely for M51. In a second project, I investigate magnetic fields of unresolved extragalactic radio sources (EGSs). I observed 77 sources with the VLA at L-band (1– 2 GHz) which were selected to have degrees of polarization > 30 % at 1.4 GHz. Polarized emission of EGSs is believed to be produced in the jets and radio lobes of AGNs. The degrees of polarization of my sample are exceptionally high and thus could originate from extremely well-ordered magnetic fields. I found a linearly increasing degree of polarization with increasing synchrotron spectral index with a correlation coefficient of 0.7 (I ∝ ν^α, with spectral index α). I propose this to be related to the jet location traced by the observation: The radio emission of EGSs with flat spectral indices may originate from the region near the central core of the AGN where the jet is collimated and hence the magnetic field is well- ordered. Furthermore, I found all Faraday spectra to be simple with only one prominent peak (given the resolution in Faraday depth of 126 rad m⁻²). This shows that the sources probably experience only little Faraday depolarization intrinsic to the source and along the line-of-sight which is in agreement with the observed high degrees of polarization. In my thesis, I successfully probed the 3D structure of magnetic fields in the nearby spiral galaxy M51 and in a sample of extragalactic polarized background sources. I used modern tools such as wavelength-dependent depolarization mechanisms and RM-Synthesis technique and devised new ways of analyzing and interpreting broadband multi-channel polarimetric data. This thesis is one of the first projects analyzing broadband polarization data and provides important insights towards studying magnetic fields during the new era of radio astronomy provided by upcoming new instruments.