Einstein Lensing Rings to Observe the Non-baryonic matter Distribution – ELROND

Weak gravitational lensing is a key probe of the distribution of dark matter in the Universe; it is one of the main focuses of stage-IV galaxy surveys such as Euclid and the Legacy Survey of Space and Time. The standard weak-lensing measure – cosmic shear – consists in observing the apparent alignment of galaxies that results from the coherent deflection of light by the large-scale structure of the Universe. This technique is nevertheless limited by our ignorance of the intrinsic shape and orientation of galaxies. The ELROND project proposes to use strong-lensing Einstein rings as “standard shapes” to measure cosmic shear without the systematic errors that inevitably occur when using galaxies as sources. It will pave the way to a new independent measurement of the cosmological S8 parameter and offer novel probes of the distribution of dark matter on small scales.

The fundamental idea behind ELROND is well motivated by prior theoretical analyses (Birrer et al. 2017, Fleury et al. 2021b) and its feasibility has already been demonstrated on mock images (Hogg et al. 2023). The project has three mostly independent objectives. Firstly, we shall assess the cosmological potential of Einstein rings as weak-lensing probes by forecasting the associated gain in precision and accuracy of S8 measurements by stage-IV surveys. Secondly, we shall confirm the measurability of the line-of-sight shear on real strong-lensing images, and apply it to the James Webb Space Telescope COSMOS-Web survey. Thirdly, we shall explore the detectability of line-of-sight perturbations to Einstein rings beyond shear, such as flexion, and study their ability to unveil small-scale properties of the dark-matter distribution.