Quasars, Radio Galaxies and Gravitational Lenses in the High Radio Frequency Universe

Abstract

I present the interferometry visibility catalogue for > 94 % of sources in the Australia Telescope 20 GHz (AT20G) survey covering all de- clinations of the Southern hemisphere. The catalogue was obtained utilizing the additional dataset from the sixth antenna (with long ∼ 4500 m baselines) of the Australia Telescope Compact Array (ATCA) in the main AT20G survey follow-up observations at 18.8 GHz. It provides angular size information at subarcsec (∼ 0.1 arcsec) resolution. Applications of the visibility catalogue in different fields - study of radio spectra of AGNs and AGN powered radio sources; the effect of spectral curvature at high radio frequencies on redshift cut-off of compact AGNs and search for gravitational lenses in the southern hemisphere - are presented in detail.The visibility amplitudes were calculated using a non-standard method using scalar averaging of the visibility amplitudes. The missing visibilities, due to observational logistics, do not introduce bias and, hence, the catalogue has the same completeness and reliability level corresponding to the AT20G survey. Most sources have visibilities measured at two widely separated hour angles and are presented as independent numbers, providing limited morphological information about the sources.The visibility corresponding to 0.14 arcsec was used to separate the sources into compact and extended extragalactic sources. 77% of the AT20G sources are found to be compact AGNs. Cross matches with lower frequency radio surveys the 1.4 GHz NRAO VLA Sky Survey (NVSS) and the 0.8 GHz Sydney University Molonglo Sky Survey (SUMSS) combined with the AT20G observations at 5 and 8 GHz are used to study the spectra of the sources from lower frequencies to 20 GHz. The limitations of spectral index at higher radio frequencies as a diagnostic for separating compact and extended sources is discussed. A visibility-spectra diagnostic plot that is capable of cleanly separating different classes of extragalactic sources is introduced. Discussion of properties of sources from cross matches with optical, ROSAT X- ray and Fermi gamma-ray catalogues is presented. Galactic free-free thermal planetary nebulae that were easily detected in this visibility-spectra plot are listed in the appendix.A physical basis for the use of spectral index to select compact and extended extragalactic sources at lower frequencies is provided, with recommendations for the use of ν−0.5 as the limit in future studies to maintain consistency. A moustache plot for compact AGNs showing spectral curvature at higher rest frame frequencies is presented. Discussions supporting the validity of studies concentrating on redshift cut-off of flat-spectrum compact AGNs at lower frequencies is presented. The results showing redshift cut-off is affected by spectral steepening at higher observed frequencies is also presented.A search for gravitational lensing in the southern hemisphere using the visibility data and follow-up imaging observations at 7 mm and with the Gemini South 8 m optical telescope are presented. The motivation, the strategy pushing the boundaries of the available instrument, the data reduction steps and the results are presented. Awaiting further data processing, one source is identified as most likely case of gravitational lens system and three are listed as candidates, result consistent with the successful northern hemisphere Cosmic Lens All Sky Survey (CLASS) results