Girt by B: Radio Polarimetric Observations of Kiloparsec-Scale Magnetic Fields

Thesis title: Girt by B: Radio Polarimetric Observations of Kiloparsec-Scale Magnetic Fields Thesis abstract: In this thesis I address some of the central questions regarding the growth of magnetic fields from the confines of galaxies into the large-scale pervasive fields observed today. Specifically, I have used radio polarisation data, acquired primarily with the Australia Telescope Compact Array, and large surveys to carry out four individual investigations into the evolution of magnetic fields on kiloparsec scales. I carry out an investigation into the magnetic field structure in the Magellanic Bridge, a nearby tidal remnant, which was conducted through the observation of Faraday rotation towards 167 polarised background radio sources. Comparing measured Faraday depth values of sources `on' and `off' the Bridge, I find that the two populations are different. Assuming that this difference in populations is due to a coherent field in the Magellanic Bridge, the observed Faraday depths indicate a median line-of-sight magnetic-field strength of B|| 0.3 G directed uniformly away from us. This is the first detection of a coherent magnetic field spanning the entirety of the Magellanic Bridge and I argue that this is a direct probe of a `pan-Magellanic' field. I present broadband polarisation observations of the radio galaxy NGC 612 (PKS B0131-637). By fitting complex polarisation models to the polarised spectrum of each pixel, I find that a single polarisation component can adequately describe the observed signal for most of the radio galaxy. I argue that the bulk of the Faraday rotation must be taking place very near, or at, the location of the polarised emission, yielding an estimated total magnetic field strength of 4 G. If the location of the bulk polarised emission is located cospatially with a coherent magnetic field, the implied thermal mass of the two lobes is of the order 109 M. I carry out a broadband polarisation investigation into the lobes of radio galaxy MSH 05-313 (ESO363 G-027). After correcting for the dominant Galactic Faraday component, I reveal gradients in the Faraday depth across the outer lobes. Using derived age estimates in conjunction with magnetic field strength estimates, I show that Kelvin-Helmholtz instabilities could serve as a plausible explanation for the observed Faraday depth patterns. I go on to argue that the thermal material responsible for the observed Faraday rotation is likely to be swept up material from the surrounding intergalactic medium. Finally, I combine the GAMA galaxy group catalogue with a large radio catalogue of rotation measures to cross-correlate galaxy group locations with the positions of known background polarised radio sources. I identify 64 instances where the line of sight to a polarised radio source passes through the angular projection of a galaxy group, but find no significant excess in residual rotation measure with respect to a number of different galaxy group parameters. Restricting the sample to radio-optical pairs with impact parameters less than the inner radius of a group, I calculate a 3 upper detection limit of B|| 2.0 G

Probing the Emission States of PSR J1107−5907

The emission from PSR J1107−5907 is erratic. Sometimes the radio pulse is undetectable, at other times the pulsed emission is weak, and for short durations the emission can be very bright. In order to improve our understanding of these state changes, we have identified archival data sets from the Parkes radio telescope in which the bright emission is present, and find that the emission never switches from the bright state to the weak state, but instead always transitions to the "off" state. Previous work had suggested the identification of the "off" state as an extreme manifestation of the weak state. However, the connection between the "off" and bright emission reported here suggests that the emission can be interpreted as undergoing only two emission states: a "bursting" state consisting of both bright pulses and nulls, and the weak emission state

Detection of a coherent magnetic field in the Magellanic Bridge through Faraday rotation

We present an investigation into the magnetism of the Magellanic Bridge, carried out through the observation of Faraday rotation towards 167 polarized extragalactic radio sources spanning the continuous frequency range of 1.3–3.1 GHz with the Australia Telescope Compact Array. Comparing measured Faraday depth values of sources ‘on’ and ‘off’ the Bridge, we find that the two populations are implicitly different. Assuming that this difference in populations is due to a coherent field in the Magellanic Bridge, the observed Faraday depths indicate a median line-of-sight coherent magnetic-field strength of B∥ ≃ 0.3 μG directed uniformly away from us. Motivated by the varying magnitude of Faraday depths of sources on the Bridge, we speculate that the coherent field observed in the Bridge is a consequence of the coherent magnetic fields from the Large and Small Magellanic Clouds being pulled into the tidal feature. This is the first observation of a coherent magnetic field spanning the entirety of the Magellanic Bridge and we argue that this is a direct probe of a ‘pan-Magellanic’ fieldThe Australia Telescope is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO. BMG and CRP acknowledge the support of the Australian Research Council through grant FL100100114. The Dunlap Institute is funded through an endowment established by the David Dunlap family and the University of Toronto. NMM-G acknowledges the support of the Australian Research Council through Future Fellowship FT15010002

High Resolution H I Distributions and Multi-Wavelength Analyses of Magellanic Spirals NGC 4618 and NGC 4625

We present a detailed analysis of high resolution H I observations of the Magellanic spiral galaxies NGC 4618 and NGC 4625. While the H I disk of NGC 4625 is remarkably quiescent with a nearly uniform velocity dispersion and no evidence of H I holes, there is a dynamic interplay between star formation and the distribution of neutral hydrogen in NGC 4618. We calculate the critical density for widespread star formation in each galaxy and find that star formation proceeds even where the surface density of the atomic gas is well below the critical density necessary for global star formation. There are strong spatial correlations in NGC 4618 between UV emission, 1.4 GHz radio continuum emission, and peaks in the H I column density. Despite the apparent overlap of the outer disks of the two galaxies, we find that they are kinematically distinct, indicating that NGC 4618 and NGC 4625 are not interacting. The structure of NGC 4618 and, in particular, the nature of its outer ring, are highly suggestive of an interaction, but the timing and nature of such an interaction remain unclear

Revealing the Faraday depth structure of radio galaxy NGC612 with broad-band radio polarimetric observations

We present full-polarization, broad-band observations of the radio galaxy NGC 612 (PKS B0131-637) from 1.3 to 3.1 GHz using the Australia Telescope Compact Array. The relatively large angular scale of the radio galaxy makes it a good candidate with which to investigate the polarization mechanisms responsible for the observed Faraday depth structure. By fitting complex polarization models to the polarized spectrum of each pixel, we find that a single polarization component can adequately describe the observed signal for the majority of the radio galaxy. While we cannot definitively rule out internal Faraday rotation, we argue that the bulk of the Faraday rotation is taking place in a thin skin that girts the polarized emission. Using minimum energy estimates, we find an implied total magnetic field strength of 4.2 µG.The Australia Telescope Compact Array is part of the Australia Telescope National Facility that is funded by the Australian Government for operation as a National Facility managed by CSIRO. We thank the staff at the Australia Telescope Compact array for their assistance and support with this project. We also appreciate the invaluable insight offered to us by R. W. Hunstead, R. A. Laing, and L. Rudnick. B.M.G. and C.R.P acknowledge the support of the Australian Research Council through grant no. FL100100114. The Dunlap Institute is funded through an endowment established by the David Dunlap family and the University of Toronto. N.M.M.-G. acknowledges the support of the Australian Research Council through Future Fellowship FT150100024. X.H.S. is supported by the National Natural Science Foundation of China under grant no. 11763008

Wide Bandwidth Observations of Pulsars C, D and J in 47 Tucanae

We report the first wideband observations of pulsars C, D and J in the globular cluster 47Tucanae (NGC 104) using the Ultra-Wideband Low (UWL) receiver system recently installed on the Parkes 64 m radio telescope. The wide frequency range of the UWL receiver (704-4032 MHz), along with the well-calibrated system, allowed us to obtain flux density measurements and polarization pulse profiles. The mean pulse profiles have significant linear and circular polarization, allowing for determination of the Faraday rotation measure for each pulsar. Precise measurements of the dispersion measures show a significant deviation in the value for pulsar D compared to earlier results. Searches for new pulsars in the cluster are on-going and we have determined optimal bands for such searches using the Parkes UWL receiver system

Probing the Emission States of PSR J1107−5907

The emission from PSR J1107−5907 is erratic. Sometimes the radio pulse is undetectable, at other times the pulsed emission is weak, and for short durations the emission can be very bright. In order to improve our understanding of these state changes, we have identified archival data sets from the Parkes radio telescope in which the bright emission is present, and find that the emission never switches from the bright state to the weak state, but instead always transitions to the "off" state. Previous work had suggested the identification of the "off" state as an extreme manifestation of the weak state. However, the connection between the "off" and bright emission reported here suggests that the emission can be interpreted as undergoing only two emission states: a "bursting" state consisting of both bright pulses and nulls, and the weak emission state