7 research outputs found
Evidence for a toroidal magnetic-field component in 5C 4.114 on kiloparsec scales
No full textContext. A monotonic, statistically significant gradient in the observed Faraday rotation measure (RM) across the jet of an active galactic nucleus (AGN) reflects a corresponding gradient in the electron density and/or line-of-sight magnetic (B) field in the region of Faraday rotation. For this reason, such gradients may indicate the presence of a toroidal B field component, possibly associated with a helical jet B field. Although transverse RM gradients have been reported across a number of parsec-scale AGN jets, the same is not true on kiloparsec scales, suggesting that other (e.g. random) magnetic-field components usually dominate on these larger scales. Aims. We wished to identify clear candidates for monotonic, transverse RM gradients across AGN jet and lobe structures on scales larger than those probed thus far, and estimate their statistical significances. Methods. We identified an extended, monotonic transverse Faraday-rotation gradient across the northern lobe of a previously published Very Large Array (kiloparsec-scale) RM image of 5C 4.114. We reanalyzed these VLA data in order to determine the significance of this RM gradient. Results. The RM gradient across the northern kiloparsec-scale lobe structure of 5C 4.114 has a statistical significance of about 4σ. There is also a somewhat less prominent monotonic transverse Faraday-rotation gradient across the southern jet/lobe (narrower range of distances from the core, significance ≠3σ). Other parts of the Faraday-rotation distribution observed across the source are patchy and show no obvious order. Conclusions. This suggests that we are observing a random RM component associated with the foreground material in the cluster in which the radio source is located and through which it is viewed, superposed on a more ordered RM component that arises in the immediate vicinity of the AGN jets. We interpret the transverse RM gradient as reflecting the systematic variations of the line-of-sight component of a helical or toroidal B field associated with the jets of 5C 4.114. These results suggest that the helical field that arises due to the joint action of the rotation of the central black hole and its accretion disc and the jet outflow can survive to distances of thousands of parsec from the central engine
Evolution of the magnetic field structure in the jet outflows from active galaxies
No full textThis 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
Evidence for Toroidal B-Field Components in AGN Jets on Kiloparsec Scales
No full textThough helical magnetic fields are generally believed to arise when the jets of Active Galactic Nuclei (AGN) are launched, it is still unclear what role they play (and if they survive) to the largest jet scales. A helical or toroidal B-field may contribute substantially to the collimation of the jet. This B-field structure can be detected in images of the Faraday rotation measure (RM)—a measure of the change in polarisation angle of an electromagnetic wave as it passes through a magneto-ionic medium. The Faraday rotation measure is directly proportional to the line-of-sight magnetic field; therefore a monotonic gradient in the RM transverse to the jet indicates similar behaviour of the line-of-sight B-field component. This type of analysis has mostly been done on parsec scales using VLBI observations at centimetre wavelengths, while relatively few studies have probed decaparsec to kiloparsec scales. The detection of RM gradients with significances of 3 σ or more on such large scales can demonstrate the presence of a toroidal field component, which may be associated with a helical field that has persisted to these distances from the centre of the AGN. We present the results of new Faraday rotation analyses for 2 AGN on kiloparsec scales based on multiwavelength VLA observations, with robust transverse RM gradients detected in both. Furthermore, the direction of the inferred toroidal B-fields on the sky supports previous results indicating a predominance of outward currents in the jets on kiloparsec scales
The magnetic field strength of the Faraday screen surrounding the radio galaxy Coma A
No full textStudying the interaction between AGN jets and lobes and their surrounding environment is important in order to understand how they transfer energy to their environment as well as determining the intrinsic physical properties of the sources themselves. This paper presents broad-band VLA polarization and Faraday rotation observations of the radio galaxy Coma A (3C 277.3) from 1 to 4 GHz, including archival VLA observations at 4.9 and 15 GHz. Through broad-band polarization model-fitting, we find that an external Faraday screen with a turbulent magnetic field provides an appropriate description to most of the data. By combining the polarization and Faraday rotation results with previous H α observations, we identified the H α-emitting gas as the Faraday screen responsible for the observed Faraday depolarization. We were able to derive the magnetic field strength in the H α-emitting gas, finding typical field strengths of ∼1 μG, which is consistent with studies of the intragroup medium local to other radio galaxies. However, we find a highly depolarized region of the southern lobe coincident with a H α filament that has a field strength comparable to the equipartition field strength in the radio lobe (i.e. ≳36 μG). This implies that the H α filament is internal to the radio emitting plasma. Such clear examples of internal Faraday depolarization are rare, thus providing another key insight into the evolution of radio galaxies and their ability to provide significant feedback on the local gas that would otherwise cool and form stars
Dominance of outflowing electric currents on decaparsec to kiloparsec scales in extragalactic jets
No full textContext. Helical magnetic fields embedded in the jets of active galactic nuclei (AGNs) are required by the broad range of theoretical models that advocate for electromagnetic launching of the jets. In most models, the direction of the magnetic field is random, but if the axial field is generated by a Cosmic Battery generated by current in the direction of rotation in the accretion disk, there is a correlation between the directions of the spin of the AGN accretion disk and of the axial field, which leads to a specific direction for the axial electric current, azimuthal magnetic field, and the resulting observed transverse Faraday–rotation (FR) gradient across the jet, due to the systematic change in the line-of-sight magnetic field.
Aims. We consider new observational evidence for the presence of a nested helical magnetic-field structure such as would be brought about by the operation of the Cosmic Battery, and make predictions about the expected behavior of transverse FR gradients observed on decaparsec and kiloparsec scales.
Methods. We have jointly considered 27 detections of transverse FR gradients on parsec scales, four reports of reversals in the directions of observed transverse FR gradients observed on parsec–decaparsec scales, and five detections of transverse FR gradients on decaparsec–kiloparsec scales, one reported here for the first time. We also consider seven tentative additional examples of transverse FR gradients on kiloparsec scales, based on an initial visual inspection of published Very Large Array FR maps of 85 extragalactic radio sources, for three of which we have carried out quantitative analyses in order to quantitatively estimate the significances of the gradients.
Results. The data considered indicate a predominance of transverse FR gradients in the clockwise direction on the sky (i.e., net axial current flowing inward in the jet) on parsec scales and in the counter-clockwise direction on the sky (i.e., net axial current flowing outward) on scales greater than about 10 pc, consistent with the expectations for the Cosmic Battery. The predominance of counter-clockwise FR gradients on larger scales has been established at the 3σ confidence level.
Conclusions. The collected results provide evidence for a reversal in the direction of the net azimuthal magnetic field determining the ordered component of the observed FR images, with distance from the jet base. This can be understood if the dominant azimuthal field on parsec scales corresponds to an axial electric current flowing inward along the jet, whereas the (weaker) dominant azimuthal field on kiloparsec scales corresponds to a outward-flowing current in the outer sheath of the jet and/or an extended disk wind. This is precisely the current/magnetic field structure that should be generated by the Cosmic Battery
