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Luminous stars in background galaxies straddling the lensing caustic of aforeground galaxy cluster can be individually detected due to extrememagnification factors of $\sim 10^2$--$10^3$, as recently observed in deep HSTimages. We propose a direct method to probe the presence of dark mattersubhalos in galaxy clusters by measuring the astrometric perturbation theyinduce on the image positions of magnified stars or bright clumps: lensing bysubhalos breaks the symmetry of a smooth critical curve, traced by themidpoints of close image pairs. For the giant arc at $z = 0.725$ behind thelensing cluster Abell 370 at $z = 0.375$, a promising target for detectingimage pairs of stars, we find that subhalos of masses in the range$10^6$--$10^8\,M_\odot$ with the abundance predicted in the cold dark mattertheory should typically imprint astrometric distortions at the level of$20$--$80\,{\rm mas}$. We estimate that $\sim 10\,$hr integrations with JWST at$\sim 1$--$3\,\mu{\rm m}$ may uncover several magnified stars whose imagedoublets will reveal the subhalo-induced structures of the critical curve. Thismethod can probe a dynamic range in the subhalo to cluster halo mass ratio $m/M\sim 10^{-7}$--$10^{-9}$, thereby placing new constraints on the nature of darkmatter.

Probing Dark Matter Subhalos in Galaxy Clusters Using Highly Magnified Stars