Structures of the magnetoionic media around the Fanaroff-Riley Class I radio galaxies 3C 31 and Hydra A

Abstract

We use high‐quality Very Large Array (VLA) images of the Fanaroff–Riley Class I radio galaxy 3C 31 at six frequencies in the range 1365–8440 MHz to explore the spatial scale and origin of the rotation measure (RM) fluctuations on the line of sight to the radio source. We analyse the distribution of the degree of polarization to show that the large depolarization asymmetry between the north and south sides of the source seen in earlier work largely disappears as the resolution is increased. We show that the depolarization seen at low resolution results primarily from unresolved gradients in a Faraday screen in front of the synchrotron‐emitting plasma. We establish that the residual degree of polarization in the short‐wavelength limit should follow a Burn law and we fit such a law to our data to estimate the residual depolarization at high resolution. We discuss how to interpret the structure function of RM fluctuations in the presence of a finite observing beam and how to address the effects of incomplete sampling of RM distribution using a Monte Carlo approach. We infer that the observed RM variations over selected areas of 3C 31, and the small residual depolarization found at high resolution, are consistent with a power spectrum of magnetic fluctuations in front of 3C 31 whose power‐law slope changes significantly on the scales sampled by our data. The power spectrum P(f) can only have the form expected for Kolmogorov turbulence [P(f) ∝f−11/3] on scales ≲5 kpc. On larger scales, we find . We briefly discuss the physical interpretation of these results. We also compare the global variations of RM across 3C 31 with the results of three‐dimensional simulations of the magnetic‐field fluctuations in the surrounding magnetoionic medium. We infer that the RM variation across 3C 31 is qualitatively as expected from relativistic‐jet models of the brightness asymmetry wherein the apparently brighter jet is on the near side of the nucleus and is seen through less magnetoionic material than the fainter jet. We show that our data are inconsistent with observing 3C 31 through a spherically symmetric magnetoionic medium, but that they are consistent with a field distribution that favours the plane perpendicular to the jet axis – probably because the radio source has evacuated a large cavity in the surrounding medium. We also apply our analysis techniques to the case of Hydra A, where the shape and the size of the cavities produced by the source in the surrounding medium are known from X‐ray data. We emphasize that it is essential to account for the potential exclusion of magnetoionic material from a large volume containing the radio source when using the RM variations to derive statistical properties of the fluctuations in the foreground magnetic field.

Sardinia Radio Telescope observations of Abell 194

“…For a partially resolved foreground, with Faraday rotation in a short-wavelength limit, a depolarization of the signal due to the unresolved RM structures in the external screen can be approximated, to first order, with the Burn-law (Burn 1966, see also Laing et al 2008 for a more recent derivation):…”

The Coma cluster magnetic field from Faraday rotation measures

“…The intracluster magnetic field power spectrum has been investigated in several works (Vogt & Ensslin 2003Murgia et al 2004;Govoni et al 2006;Guidetti et al 2008;Laing et al 2008). In addition, a radial decline of the magnetic field strength is expected from magneto-hydrodynamical simulations performed with different codes (Dolag et al 1999(Dolag et al , 2005Brueggen et al 2005;Dubois et al 2008;Collins et al 2010;Dolag & Stasyszyn 2009;Donnert et al 2009a), as a result of the compression of thermal plasma during the cluster gravitational collapse.…”

The intracluster magnetic field power spectrum in Abell 665

“…Turbulence in the intracluster medium has also been studied in the radio band through Faraday rotation measures. Recent results show that the magnetic field power spectrum can be estimated if very detailed rotation-measure images of cluster radio galaxies are available (Enßlin & Vogt 2003;Murgia et al 2004;Vogt & Enßlin 2005;Govoni et al 2006;Guidetti et al 2008;Laing et al 2008). Typically, rotation measure images of cluster radio galaxies permit investigating the fluctuations of the intracluster magnetic field below a spatial scale of about 50 ÷ 100 kpc.…”