Constraints on modified gravity using Einstein rings: prospects for the LSST era

 

 

 

 

Galaxy-scale strong lensing systems are key for testing modifications of general relativity at the few kpc scales. In particular, combining the kinematics of the central galaxy with the lens modeling allows one to set constraints on the so-called slip parameter eta, which is the ratio of the two scalar potentials for general metric theories. Most constraints derived so far on eta are based on a heterogeneous collection of lenses reaching on the order of a hundred systems. The upcoming Vera Rubin Telescope Legacy Survey of Space and Time (LSST) is expected to uncover about 10,000 strong lensing systems, tremendously increasing the sample of systems useful to determine eta. As LSST is a purely photometric survey, this science will require follow-up observations, both to spectroscopically confirm the redshifts of the lens and the source, as well as to measure the lens velocity dispersions. As a preparation for LSST, we have compiled a large sample of Strong Lensing systems (both confirmed and candidates) and generated a database of their images in the current wide-field surveys, such as DES, KiDS, HSC, Legcy Survey, CFHTLens, RCSLens and CS82, yielding thousands of systems. This database is useful to train algorithms to find, classify, and model these systems and to select a fraction for follow-up. We are performing a systematic program to gather spectroscopic information on selected systems with different instrument configurations on the CASLEO, SOAR and Gemini telescopes. The yield of this program and the implications for the determination of eta will guide future follow-ups of more homogeneous, numerous and higher redshift samples derived from LSST. Finally, we carry out simulations of a fully autometed procedure to model the strong lensing systems using Machine Learning and find that it allows one to determine eta with no significant systematics. We conclude that a systematic program for finding and modeling galaxy-galaxy strong lensing systems in LSST, combined with a selected spectroscopic follow-up may yield promising constraints on some modifications of general relativity.