Structure of the magnetoionic medium around the FR Class I radio galaxy 3C 449

摘要

The goal of this work is to constrain the strength and structure of themagnetic field associated with the environment of the radio source 3C 449,using observations of Faraday rotation, which we model with a structurefunction technique and by comparison with numerical simulations. We assume thatthe magnetic field is a Gaussian, isotropic random variable and that it isembedded in the hot intra-group plasma surrounding the radio source. For thispurpose, we present detailed rotation measure images for the polarized radiosource 3C 449, previously observed with the Very Large Array at sevenfrequencies between 1.365 and 8.385 GHz. We quantify the statistics of themagnetic-field fluctuations by deriving rotation measure structure functions,which we fit using models derived from theoretical power spectra. We quantifythe errors due to sampling by making multiple two-dimensional realizations ofthe best-fitting power spectrum.We also use depolarization measurements toestimate the minimum scale of the field variations. We then makethree-dimensional models with a gas density distribution derived from X-rayobservations and a random magnetic field with this power spectrum. Under theseassumptions we find that both rotation measure and depolarization data areconsistent with a broken power-law magnetic-field power spectrum, with a breakat about 11 kpc and slopes of 2.98 and 2.07 at smaller and larger scalesrespectively. The maximum and minimum scales of the fluctuations are around 65and 0.2 kpc, respectively. The average magnetic field strength at the clustercentre is 3.5 +/-1.2 micro-G, decreasing linearly with the gas density withinabout 16 kpc of the nucleus.