Dark matter physics through small-scale lensing anisotropies

Over the last two decades, strong gravitational lensing has emerged as a promising tool for probing the nature and distribution of dark matter on sub-galactic scales. In addition to the main-lens substructure, dark matter halos along the line-of-sight between the observer and the source contribute significantly to the subtle perturbations of lensed images. These line-of-sight halos, unlike dark matter subhalos, appear stretched tangentially around the Einstein radius in the effective convergence maps, imprinting a distinct anisotropic signature. These anisotropies produce a quadrupole moment of the image-plane averaged two-point correlation function of the effective convergence field. We show that current space-based telescopes and future extremely large telescopes can detect this quadrupole signal using Fisher forecasting. In addition, we demonstrate how, in a strongly lensed system, this anisotropic signal can statistically separate the contribution of line-of-sight halo perturbations to lensing perturbations from that of main-lens subhalos. Finally, we will look at how the anisotropic two-point function changes when warm dark matter and self-interacting dark matter are present. This strategy opens the door to improving constraints on the evolution of dark matter structure on small scales with upcoming large-scale surveys