The validation activities in preparation of the Gaia DR4 resulted in
the discovery of a binary system, hosting a dormant stellar 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 belongs 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.
Antonella Vallenari
