Statistical approaches to the selection of strong gravitationally lensed spectral line sources in the Square Kilometre Array era

Abstract

Neutral atomic hydrogen (HI) and hydroxyl (OH) are important gas components within the context of galaxy evolution. HI is a key transitory phase in the baryon cycle and, hence, has a crucial role in star formation. Extragalactic OH is typically found in highly luminous infrared emission, as well as dense molecular hydrogen gas regions, associated with extreme star formation activity and gas-rich major galaxy mergers. Currently, both HI and OH emission are only directly detected at low redshifts (z < 0.2). Strong gravitational lensing, together with the upcoming large spectral line surveys of the SKA1-Mid, will offer opportunities to study HI and OH sources out to intermediate and high redshifts, which is not possible with current radio telescopes. A large number of HI and OH sources are predicted to be detected with these surveys, of which only a fraction will be lensed. This thesis investigates a statistical approach to select these lensed sources in the upcoming surveys. The approach is based on the distortion that the magnification bias has on the source number counts at high HI masses or high OH luminosities and uses this effect to find a flux density selection threshold above which gravitationally lensed sources are preferentially selected. This thesis finds that the surface density of lensed HI galaxies and lensed OH megamasers are low and that the flux density selection thresholds are faint, requiring deep and wide observations in order to select lensed HI galaxies and OH megamasers in this way, in the absence of any other information. This thesis also starts to investigate whether it will be possible to remove these lensed contaminants using ancillary optical/near infrared data from the Legacy Survey of Space and Time. These results indicate that a large fraction of these sources could be removed in this way. Future work will extend this investigation. The results of this thesis will provide valuable input to the increase of the scientific yield of the SKA1-Mid surveys by combining high sensitivity observations with the natural amplification of strong gravitational lensing to directly observe both HI and OH at cosmological epochs well beyond the peak of cosmic star formation and AGN activity.