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Southern radio gravitational lens survey and observations

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Lovell, James Edward John (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 (S2.7 > 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)
Copyright Holders: The Author
Copyright Information:

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).

Additional Information:

Presents two related investigations, the first a systematic radio survey in search of strong (S2.7 > 0.34 Jy) gravitational lenses in a complete sample of flat-spectrum radio sources, the second a detailed study of the known, southern gravitational lens/Einstein ring radio source PKS 1830-211. Thesis (Ph.D.)--University of Tasmania, 1997. Includes bibliographical references. Presents two related investigations, the first a systematic radio survey in search of strong (S2.7 > 0.34 Jy) gravitational lenses in a complete sample of flat-spectrum radio sources, the second a detailed study of the known, southern gravitational lens/Einstein ring radio source PKS 1830-211

Date Deposited: 19 Dec 2014 02:36
Last Modified: 17 Jul 2017 08:02
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