The Gala Revolution - How to weigh a black hole with Gaia astrometry in gravitational microlensing events

Gravitational microlensing offers a unique method to detect and characterise dark and faint compact objects independent of their luminosity. While the classical photometric-only microlensing gives very limited constraints on lens mass, distance, and relative motion, the addition of astrometric measurements breaks these degeneracies and enables direct mass determination. The forthcoming Gaia Data Release 4 (DR4) will, for the first time, provide extensive time-series astrometry alongside photometry for hundreds of microlensing events across the entire sky.These data will reveal the characteristic astrometric deflection of the source position caused by the lens, allowing measurement of the angular Einstein radius and, combined with parallax information, the lens mass and distance. This opens a new discovery space for identifying isolated stellar remnants — black holes, neutron stars, and white dwarfs — that are otherwise undetectable. In particular, Gaia DR4 will enable the first large, homogeneous census of non-accreting black holes in the Milky Way based purely on gravitational effects.In this talk, we will show how photometric and astrometric modelling of Gaia microlensing events helps identify dark remnant lenses. We will discuss expected performance and selection effects, and highlight the scientific potential for studying the population, kinematics, and formation history of compact objects. These results will demonstrate Gaia’s transformative role in probing the unseen mass distribution of our Galaxy.