Interlopers speak out: studying the dark universe using small-scale lensing anisotropies

Understand

Strongly lensed systems are powerful probes of the distribution of dark matter on small scales.

• In this paper, we show that line-of-sight haloes between the source and the observers give rise to a distinct anisotropic signature in the two-point function of the effective lensing deflection field.
• We show in particular that the non-linear coupling between line-of-sight haloes and the main-lens plane imprints a characteristic quadrupole moment on this two-point function whose amplitude reflects the abundance of such haloes within the strongly lensed field.
• We discuss how, by taking ratios of different multipole moments, such observables could be made robust under the mass-sheet transform.