First Results on Dark Matter Substructure from Astrometric Weak Lensing

Low-mass structures of dark matter (DM) are expected to be entirely devoid of light-emitting regions and baryons. Precisely because of this lack of baryonic feedback, small-scale substructures of the Milky Way are a relatively pristine testing ground for discovering aspects of DM microphysics and primordial fluctuations on subgalactic scales. In this Letter, we report results from the first search for Galactic DM subhalos with time-domain astrometric weak gravitational lensing. The analysis is based on a matched-filter template of local lensing corrections to the proper motion of stars in the Magellanic Clouds. We describe a data analysis pipeline detailing sample selection, background subtraction, and the handling of outliers and other systematics. For tentative candidate lenses, we identify a signature based on an anomalous parallax template that can unequivocally confirm the presence of a DM lens, opening up prospects for robust discovery potential with full time-series data. We present our constraints on substructure fraction f_{l}≲5 at 90% C.L. (and f_{l}≲2 at 50% C.L.) for compact lenses with radii r_{l}<1  pc, with best sensitivity reached for lens masses M_{l} around 10^{7}-10^{8}  M_{⊙}. Parametric improvements are expected with future astrometric datasets; by end of mission, Gaia could reach f_{l}≲10^{-3} for these massive point-like objects and be sensitive to lighter and/or more extended subhalos for O(1) substructure fractions.