Finding and modelling gravitationally lensed quasars

Gravitationally lensed quasars are useful tools to probe cosmological parameters. However currently their contribution to making competitive constraints in cosmology is hindered by small number statistics. The proposed work would be to find many more of these systems in the Dark Energy Survey (DES) data and potentially other archived data. The systems can then be modelled and used to determine interesting cosmological and galactic astronomy.

The effects of atmospheric distortion on deep astronomical images from ground-based telescopes, make classification of systems difficult. In particular large datasets must be reduced before identification of the many astronomical systems contained within them. In searching for gravitationally lensed quasars, in which you expect multiple blue quasar point sources around a red galaxy lens, it can be difficult to segment the separate parts of the image (during deblending). This means that other factors, such as their morphological structure and colours, must be considered to identify the relevant systems. Due to the large number of candidate systems identified, I will consider image invariants to help classify them, as more promising candidates are followed-up on a spectroscopic telescope.

Another way to identify such systems is to look for an intrinsic variability. This is due to the variable nature of a quasar (an accreting supermassive black hole). This could be done with the DES data after more observing time.

The light travel time for separate quasar images differs and hence introduces a time delay which can be measured because of the intrinsic quasar variability (matching two light curves by shifting in time and brightness). This introduces a correlation between the two light curves. Much deep data detecting these systems can be of low resolution such that the two (or more) quasar images and galaxy cannot be resolved. I will investigate how this blended lightcurve over many epochs can be used to extract relevant time delay information and also how it can be used as a candidate selection criterion for a lensed quasar.

As lensed quasars are found, they can be fitted with galaxy and quasar light profiles which give accurate astrometry and photometry, which can then either be used to determine an expected time delay for the system, or if the time delay is known, parameters such as the present day Hubble constant or the dark energy equation of state may be constrained.

These ideas lead on to many interesting identification techniques that can be developed. Furthermore many datasets can be probed, such as the upcoming GAIA data release.