Southern radio gravitational lens survey and observations

Lovell, JEJ ORCID: 0000-0001-5273-9817 1997 , 'Southern radio gravitational lens survey and observations', PhD thesis, University of Tasmania.

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Abstract

This thesis presents two related investigations, the first is a systematic radio survey in search of strong (S$$_{2.7}$$ > 0.34 Jy) gravitational lenses in a complete sample of flat-spectrum radio sources, the second is a detailed study of the known, southern gravitational lens/Einstein ring radio source PKS 1830-211.
The motivation for these investigations was the discovery in 1991 that PKS 1830-211 is a strong gravitational lens. Because of its intensity, very detailed and sensitive observations can be made, and some of them form the second part of this thesis. The discovery of such a strong gravitational lens immediately raises the question of whether there are other strong lenses to be found.
Gravitational lenses provide a powerful astrophysical tool for measuring the properties of our universe. To date the majority of the known gravitational lenses have been found in the northern sky due to the concentration of surveys in that hemisphere. This thesis describes a search of the southern part of the sky for strong gravitational lenses and involved a study of a complete sample of the 461 strongest southern flat-spectrum radio sources in the Parkes Catalogue.
Flat-spectrum sources were chosen because they typically possess a single high brightness temperature nucleus of milliarcsecond size. Such sources, if lensed, will show multiply imaged nuclei with separations that are large compared to their sizes. All objects in the sample were observed with the Australia Telescope Compact Array (ATCA) by making at least six short observations (or "cuts") at 4.8 and 8.6 GHz, separated by approximately two hours. In this way, all objects were imaged over ~8 days of telescope time, the dual wavelength observations allowing spectral index information to be determined for any structural components of the sources. Data simulations showed that the survey was capable of resolving any compact double source with component separations and flux densities greater than 1.5 arcsec and 6 mJy respectively.
The data were edited and calibrated within AIPS and imaged using Difmap. All correlated visibilities were examined and those that revealed signs of structure were imaged to search for signs of gravitational lensing. The only previously known lens in the sample, PKS 1830-211, was successfully re-discovered and the survey was demonstrated to be sensitive to extended ring-like structure by the detection of the planetary nebula PKS 1350-662. Two new gravitational lens candidates were found within the sensitivity of the survey, they are PKS 0252-549 and PKS 2321-375. As a check that no lens candidates had been overlooked and in order to obtain statistical information on the survey, a data processing script was written for Difmap to image the entire survey dataset. The data analysis revealed that over 60% of the sources contained at least 95% of their total flux density in an unresolved core. All survey sources were classified (many for the first time) according to their structural and spectral characteristics, the point source component of this catalogue being especially useful in establishing a grid of potential reference sources for future ATCA calibration observations.
Many of the sources in the sample had catalogued radio positions with large errors (up to 30 arcsec). The ATCA survey provided positions with accuracy better than 1 arcsec for all objects with compact structure and allowed optical identifications to be made from the COSMOS/UKST Southern Sky Catalogue. This observational database is a significant contribution to the completion of radio identifications in the Parkes 1/2 Jy Complete Sample (Drinkwater et al., 1996).
The second part of this thesis describes a detailed study of the properties of PKS 1830-211, the only known southern radio lens. This source is the strongest by almost an order of magnitude (Rao and Subrahmanyan, 1988; Jauncey et al., 1991) and lies in a crowded and heavily obscured field close to the Galactic Centre. So far all efforts to identify optical or infra-red counterparts either for the lensing galaxy or the lensed source have been unsuccessful (Djorgovski et al., 1992; Jauncey et al., 1993). In particular, the failure of optical measurements to furnish any redshifts has driven the search for these critical parameters into the radio spectrum. This thesis describes the detection of a new absorption feature towards the lens, due to Hi at a redshift of 0.19. This discovery, together with that of a second absorption system at $$z$$ = 0.89 (Wiklind and Combes, 1996a) indicates that PKS 1830-211 may be a compound gravitational lens.
Total flux density measurements of PKS 1830-211 have been made with the University of Tasmania's 26 m radiotelescope at 2.3 and 8.4 GHz since 1990. These observations have revealed dramatic variations in flux density on timescales of months. More recently, the ATCA has been used to monitor the total flux density at four wavelengths as well as the flux densities of the two individual compact components at 8.6 GHz. Large flux density variations in the two components were detected and analysed, leading to new constraints on the lensing time delay and the relative magnification ratio of the two compact components.

Item Type: Thesis - PhD Lovell, JEJ Copyright 1997 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s). Thesis (PhD)--University of Tasmania, 1997. Includes bibliographical references. View statistics for this item