Strong lensing studies with the 3.6-m Devasthal Optical Telescope: opportunities and challenges

Strongly-lensed quasars and active galactic nuclei (AGN), namely active galaxies that appear multiply imaged due to a foreground lensing galaxy, are exceptional astrophysical tools that may be used to probe the expansion rate of the Universe, study the evolution of galaxies and of their dark matter content over cosmic time, and zoom in into the structure of AGNs. We are entering a new era in observational astrophysics, no longer limited by the number of accessible targets, but by our ability to carry out follow-up observations. We list several science cases where the DOT and other Belgo-Indian telescopes may provide crucial high-quality data: (1) Photometric monitoring of strong lenses for cosmography (2) Confirmation of new strongly-lensed candidates in the era of big surveys (3) Advanced study of the structure of AGN using the technique of gravitational microlensing. For each of these topics, we provide a brief scientific overview of the technique and scientific question, and quantify the observational requirements for successful programs. <P />