Measuring the stellar-to-halo mass relation at ∼1010 solar masses, using forthcoming space-based imaging of galaxy–galaxy strong lenses

The stellar-to-halo mass relation (SHMR) is central to understanding the co-evolution of galaxies and their host dark matter haloes, yet it remains weakly constrained for dwarf galaxies owing to their faintness, especially beyond the Local Group. Strong gravitational lensing offers a unique probe of the SHMR at subgalactic scales and cosmological distances, as the masses of subhaloes within the main lens can be inferred from the perturbations they imprint on lensed images. Anticipating the discovery of ~ 10⁵ galaxy–galaxy strong lenses by forthcoming facilities such as Euclid, we perform an end-to-end simulation to forecast Euclid’s constraints on the SHMR at the halo mass scale of ~ 10¹⁰\, M_. We generate mock Euclid Visible Camera images of lens systems hosting a fiducial 3× 10¹⁰\, M_ subhalo and vary its properties to assess the robustness of mass inference. We find that Euclid’s angular resolution cannot break the intrinsic mass–concentration degeneracy of subhaloes, nor deblend the light of satellite galaxies (when present) associated with them, leading to biased inferred halo masses. These limitations can be overcome with high-resolution follow-up imaging from facilities such as the Hubble Space Telescope, enabling accurate halo-mass measurements.We forecast that a statistical sample of ~ 100 such systems, combining lensing-derived halo masses with stellar masses from photometric spectral energy distribution fitting, can constrain the SHMR at dwarf-galaxy scales with a precision of ~ 0.05 dex in halo mass and ~ 0.03 dex in stellar mass, enabling powerful tests of galaxy formation theories.