Evolution of the magnetic field structure in the jet outflows from active galaxies

Abstract

This thesis concerns studies into the magnetic fields in the jets from active galactic nuclei (AGN). Helical and toroidal magnetic fields are expected to exist in AGN jets close to the jet base and to assist in the launching and collimation of these structures, however if they can persist to the large kiloparsec scales and whether they still affect the jet on those scales is unknown. The magnetic fields of AGN jets are studied using Faraday rotation measure, which is proportional to the line of sight magnetic field strength, a transverse gradient in this value being an indicator of a toroidal magnetic field. It has been predicted by the “Cosmic Battery” model that the direction of any observed kiloparsec scale toroidal field component is such that an associated outward electrical current is present in the jet. Multiwavelength observations of 9 AGN are studied in this thesis. Of these 9 sources, 6 display significant transverse RM gradients on kiloparsec scales, of which 5 indicate toroidal fields with outward associated electrical currents, as predicted by the Cosmic Battery. Furthermore, the magnetic field environments of the jets are analysed as to how much the jets interact with their external medium. These effects can be studied using spectropolarimetry, where the behaviour of fractional polarization and polarization angle over a wide and continuous range of wavelengths can show evidence of strong depolarization and other complex magnetic field effects, some of which can be attributed to interactions with external media. An in-depth analysis of Coma A using wide band observations is presented here with accompanying Hα observations, showing regions of significant depolarization spatially correlated with observed Hα. The Hα can be used to estimate electron density, and in combination with the rotation measure values, the magnetic field can be calculated directly. It is shown that in a particular region, the depolarizing screen features a magnetic field strength upward of 36 μG, indicating the Hα gas was mixing with the radio lobe. In a similar vein, investigations into the radio galaxies 3C 382 and 3C 433 are also presented albeit without accompanying Hα data. However useful new rotation measure maps are provided for these sources, of which none have been published earlier, made using traditional χ versus λ squared fitting and rotation measure synthesis. Some of the interesting features in these sources are fitted for depolarization models to infer more complex underlying structure and were compared with previous published data at higher resolutions.