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Doctor of Philosophy / This thesis presents a census of radio sources selected from the NRAO (National Radio Astronomy Observatory) VLA (Very Large Array) Sky Survey (NVSS) and Sydney University Molonglo Sky Survey (SUMSS) catalogues which have also been observed in the first data release of the 6 degree Field Galaxy Survey (6dFGS), a galaxy redshift survey of the local universe. Radio detections were found for 4,506 galaxies in the 6dFGS near-infrared-selected primary sample, a radio detection rate of 16%. A further 1,196 radio sources were observed by 6dF which were missing from the 6dFGS primary sample either because their host galaxies were too blue in colour or they appeared stellar on optical plates. The full sample comprises the largest and most homogeneous set of spectra and redshifts of radio sources in the local universe ever obtained. Results from the study of these objects form an accurate benchmark from which their cosmic evolution may be understood. 6dF spectra of galaxies have been used to determine the physical cause of radio emission from each object as either star formation or an active galactic nucleus powered by a super-massive black hole. These two classes of radio source have been characterised via a determination of the local radio luminosity function at 1.4 GHz; plotting the variation in their space density with luminosity. The star-formation density of the universe at the present epoch has been determined, the value of which which turns out to be in excellent agreement with previously published values. Fractional luminosity functions have also been determined showing that more massive galaxies have higher star-formation rates and are more likely to host a radio-loud AGN. The large-scale structure of star-forming galaxies and radio-loud AGN in the local universe has been studied by determining their clustering properties via the two-point correlation function. Radio-loud AGN are found to cluster more strongly than star-forming galaxies confirming that these objects are biased tracers of the underlying matter distribution. Both star-forming galaxies and AGNs cluster similarly to the underlying host galaxy population in which they reside. This thesis also describes the 843 MHz SUMSS catalogue, made by fitting elliptical Gaussians to sources in images. The catalogue contains radio sources to a limiting peak brightness of 6 mJy/beam at declination less than -50 degrees and 10 mJy/beam at declination greater than -50 degrees. Image artefacts have been classified using a novel technique involving a decision tree, which correctly identifies and rejects spurious sources in over 96% of cases and has ensured the catalogue is more than 95% complete and 90% reliable over most of its flux density range.

This thesis presents several observational studies based on radio variability and interstellar scintillation of extragalactic flat-spectrum radio sources. Such sources are commonly called blazars, a term used to describe the phenomenon observed when the jet of a radio-loud Active Galactic Nucleus (AGN) is directed towards the observer. These sources provide unique laboratories for studying the physics of relativistic jets. Observations of selected samples of blazars, made with the Australia Telescope Compact Array (ATCA) and the Australia Telescope Long Baseline Array are presented here. Statistics for long-term (months--years) and short-term (intraday) variability in both total and linearly polarized flux density at several frequencies are presented. The sensitivity and flux density measurement accuracy of the ATCA make it particularly useful for observations of intraday variability (IDV). Resolving the question of what is the mechanism for radio IDV was of great importance at the time this thesis was being undertaken, since if intrinsic, IDV implies extremely high brightness temperatures, far in excess of the Inverse Compton limit for incoherent synchrotron radiation. Most source models are fundamentally based on the assumption that the radiation from radio to optical, and sometimes soft X-ray, energies is produced by the incoherent synchrotron mechanism, so any result which challenges this has serious implications. There is now strong evidence that interstellar scintillation (ISS) is the principal cause of radio IDV, which substantially lowers the implied source brightness temperatures from those calculated assuming intrinsic variability. Some of the results presented in this thesis have made an important contribution to the paradigm shift from IDV to ISS, by showing unequivocally that the rapid IDV observed in PKS 1257-326 is a result of scintillation due to a nearby scattering screen in the ionised interstellar medium (ISM) of our Galaxy. This unusual source, serendipitously discovered during the course of my PhD, has also proved extremely valuable in showing that ISS can be used as a probe of microarcsecond-scale source structure and also of the local Galactic ISM. Such high angular resolution is not currently achievable even with space interferometer baselines. / Thesis (Ph.D.)--Physics, 2003.