Astro Seminar: Gaia BH3: a 33 solar mass dormant BH discovered in Gaia preliminary astrometry

The validation activities in preparation of the Gaia DR4 resulted in the discovery, in the preliminary astrometry, of a binary system, hosting a dormant black hole with an exceptional mass. Black holes of these masses have been detected only in BH mergers in distant galaxies via gravitational waves. The fact that the stellar companion is an old very metal-poor giant of the Galactic halo supports the scenario that the progenitors of the high-mass BHs detected by gravitational-wave telescopes are metal-poor massive stars. The Galactic orbit of the system and its metallicity indicate that it might belong to the ED-2 stream, likely a globular cluster accreted by the Milky Way, giving hints on the formation mechanism of this binary system. The discovery is an example of the impressive impact of the Gaia mission on all branches of stellar astrophysics. We will describe the discovery of Gaia BH3, its implications, and report on results obtained by the community on this astonishing object.