OBSERVATIONS AND MODELLING OF GRAVITATIONAL LENSES

Abstract

Ever since the discovery of the first gravitational lens system by Walsh, Carswell and Weymann in 1979, there has been tremendous excitement in the astronomy community about exploiting the power of gravitational lenses. Lenses act as cosmic telescopes, magnifying the emission from sources in the background. This helps astronomers study objects that otherwise would not have been detected using current telescopes. Lensing is also an excellent tool for studying cold dark matter substructure, the mass distribution of the lensing galaxies, the stellar processes in the lensing galaxies, and for constraining the Hubble constant. One of the topics of ongoing debate in astronomy is the radio emission mechanism in radio quiet quasars. The two most likely processes responsible for the emission are star formation, and AGN activity. Understanding this emission mechanism is crucial, since it would give an insight into processes of star formation and black hole accretion, thus helping us understand galaxy formation and evolution in greater detail. Even though radio quiet quasars form 90% of the quasar population, it is difficult to detect them because of their faint radio emission. With the combined power of lensing and current telescopes, lensed radio quiet quasars can be directly imaged and investigated. This thesis presents a study of 24 lensed radio quiet quasars. One of them is SDSS J0924+0219, a lensed quasar that shows the most extreme flux anomaly in the optical ever seen in lensed systems. Using VLA and ALMA observations of this object, maps of the radio, submillimetre continuum, and CO(5-4) molecular line were obtained. The intrinsic sources were reconstructed using lens modelling techniques. The radio and molecular gas emission were found to originate from the same region, thus suggesting that star formation in the host galaxy of the quasar is responsible for the radio emission. Microlensing was found to be the reason behind the flux anomaly, a result that is consistent with previous studies of this object. The other 23 radio quiet quasars are a part of a larger sample that is being investigated using VLA data to understand the nature of the radio emission in radio quiet quasars. The study shows mixed results, although more data is needed to make conclusive arguments. This thesis also presents the LOFAR Long Baseline Calibrator Survey, a survey whose aim is to find sources that could be used for calibration of data obtained using the international stations of the LOFAR telescope. A study of MG 0751+2716, a radio loud quasar observed using the international stations of LOFAR, is also presented. Using the visibility data of the observation, lens modelling was undertaken to determine the intrinsic structure of the source. By comparing these results with a previous study at 27.4 GHz, it was found that the low frequency radio source is cospatial with the source at the higher frequency.

Date of Award	1 Aug 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Neal Jackson (Supervisor) & Scott Kay (Supervisor)