2026ApJ...997..106C

Gravitational lensing by galaxy clusters provides a powerful probe of the spatial distribution of dark matter and its microphysical properties. Strong and weak lensing constraints on the density profiles of subhalos and their truncation radii offer key diagnostics for distinguishing between collisionless cold dark matter (CDM) and self-interacting dark matter (SIDM). Notably, in the strongly collisional SIDM regime, subhalo core collapse and enhanced mass loss from ram-pressure stripping predict steeper central density slopes and more compact truncation radii-features that are directly testable with current lensing data. We analyze subhalo truncation in eight lensing clusters (A2218, 383, 963, 209, 2390, and MACS J0416.1, J1206.2, J1149.6) that span the redshift range $\left\langle {z}_{{\rm{spec}}}\right\rangle \simeq 0.17$-0.54 with virial masses M₂₀₀ ≃ 0.41-2.2 × 10¹⁵M_⊙ to constrain SIDM versus CDM. Our results indicate that the outer spatial extents of subhalos are statistically consistent with CDM, corroborated by redshift- and mass-matched analogs from the IllustrisTNG simulations. We conclude that the tidal radii of cluster galaxy subhalos serve as an important and complementary diagnostic of the nature of dark matter in these violent, dense environments.