Probing extended dark objects through astrometric microlensing from Gaia

Abstract

We present the first comprehensive study of astrometric microlensing by extended dark objects, focusing on two theoretically motivated models — Q-ball and boson star. We demonstrate that these extended objects generate distinctive signatures that depart markedly from point-mass lenses like primordial black holes. The smoking-gun signature for these exotic extended objects is the emergence of caustics, which form when the lens radius is below a critical threshold. Crossing these caustics induces discontinuous jumps in the images-centroid trajectory, a distinctive feature of these extended dark objects. We show these patterns are sensitive to the internal mass profile, with boson stars generating larger, more prominent caustic structures than Q-balls — enabling the models to be distinguished. Using the Gaia DR3 stellar catalogue, we forecast a high-yield discovery potential, up to ∼ 6000 detectable astrometric microlensing events for a 10-year mission, peaking for M ∼ 1–10 M⊙ and R ≲ 102 AU. In the absence of anomalous detections, Gaia can set powerful 90% confidence level constraints on the fractional abundance of these exotic objects, reaching fDM ≤ 10-3 in the peak region which covers masses from 10-1–107 M⊙ and radii R < 106 AU. Crucially, these projected astrometric microlensing constraints are significantly stronger than existing photometric microlensing limits in the 1–10 M⊙ mass range. This work establishes astrometric microlensing with Gaia as a powerful, complementary, and near-future probe with the potential to discover extended dark objects.