Investigating the Effects of Finite Resolution on Observed Transverse Jet Profiles
Both the emission properties and evolution of Active Galactic Nuclei (AGN) radio jets are dependent on the magnetic fields that thread them. Faraday Rotation gradients are a very important way of investigating these magnetic fields, and can provide information on the orientation and structure of the magnetic field in the immediate vicinity of the jet; for example, a toroidal or helical field component should give rise to a systematic gradient in the observed Faraday rotation across the jet, as well as characteristic intensity and polarization profiles. However, real observed radio images have finite resolution, usually expressed via convolution with a Gaussian beam whose size corresponds to the central lobe of the point source response function. This will tend to blur transverse structure in the jet profile, raising the question of how well resolved a jet must be in the transverse direction in order to reliably detect transverse structure associated with a helical jet magnetic field. We present results of simulated intensity, polarization and Faraday rotation images designed to directly and empirically investigate the effect of finite resolution on observed transverse jet structures.
Physics Department, University College Cork, Cork, Ireland
At radio wavelengths the jets of active galaxies emit synchrotron radiation, which is characterised by appreciable linear polarization, with the plane of the polarization perpendicular to the plane of the jet magnetic field in the optically thin region. The polarization structure provides information about the structure of the magnetic fields threading these jets, which influence the evolution and emission properties of the jets, as well as their stability. In addition, an understanding of the magnetic field structure is important if we wish to correctly infer the intrinsic jet structure and physical processes occurring in the jet from the observed radio images. Yet despite much observational effort the nature of the magnetic field structures of AGN remain incompletely understood. Several types of observational results suggest that the magnetic field threading the jet may have a significant helical component on parsec scales.
First, helical magnetic fields in a jet with perfect circular symmetry yield projections of the magnetic fields onto the plane of the sky which are either parallel or perpendicular to the jet axis, as is frequently observed [1,2]. Second, many parsec-scale jets show obvious antisymmetries in total intensity and polarization profiles that are reminiscent of those revealed in the helical field simulations of Laing [2,3]. Third, the presence of transverse Faraday rotation gradients across a number of VLBI jets can also be explained by helical fields in the immediate vicinity of the jets .
These observational results are all affected by the resolution of the observations used to detect them. In order to use these as identifiers for potential helical magnetic fields threading the jets of AGN, it is very important to understand the effects of finite resolution on observed transverse jet profiles.