2018ApJ...867...24D

Luminous stars in background galaxies straddling the lensing caustic of a foreground galaxy cluster can be individually detected due to extreme magnification factors of ∼10²-10³, as recently observed in deep HST images. We propose a direct method to probe the presence of dark matter subhalos in galaxy clusters by measuring the astrometric perturbation they induce on the image positions of magnified stars or bright clumps: lensing by subhalos breaks the symmetry of a smooth critical curve, traced by the midpoints of close image pairs. For the giant arc at z = 0.725 behind the lensing cluster Abell 370 at z = 0.375, a promising target for detecting image pairs of stars, we find that subhalos of masses in the range of 10⁶-10⁸ M_☉ with the abundance predicted in the cold dark matter theory should typically imprint astrometric distortions at the level of 20-80 mas. We estimate that ∼10 hr integrations with JWST at ∼1-3 µm may uncover several magnified stars whose image doublets will reveal the subhalo-induced structures of the critical curve. This method can probe a dynamic range in the subhalo-to-cluster halo mass ratio of m/M ∼ 10^–7-10^–9, thereby placing new constraints on the nature of dark matter.