Discovery of a dormant 33 solar-mass black hole in pre-release Gaia astrometry

Vol. 685
7. Stellar structure and evolution

Discovery of a dormant 33 solar-mass black hole in pre-release Gaia astrometry

Detections of gravitational waves from merging black holes demonstrate that pairs of black holes more massive than 30 solar masses are relatively common in the nearby universe. Yet, the most massive black hole known to date in our Galaxy is 20 solar mass Cyg X1, with all others below or around 10 solar masses. As part of the validation effort for the fourth Gaia data release (DR4), the Gaia consortium identified a 33 solar mass black hole, Gaia BH3, through the astrometric apparent motion of its companion. Radial velocity measurements, from both Gaia and ground-based observatories, confirm the orbit. This detection fills the gap between the masses of previously known Galactic black holes and detections of mergers through gravitational waves. Analysis of an UVES spectrum of the companion shows that its metallicity is [Fe/H]=-2.6, consistent with the hypothesis that a low stellar metallicity helps form more massive black holes. This exciting discovery is no doubt only a harbinger of the detections to be expected in DR4, which will help characterize Galactic black holes as a population.