  Gravitational waves from black-hole merging events have revealed a population
of extra-galactic BHs residing in short-period binaries with masses that are
higher than expected based on most stellar evolution models - and also higher
than known stellar-origin black holes in our Galaxy. It has been proposed that
those high-mass BHs are the remnants of massive metal-poor stars. Gaia
astrometry is expected to uncover many Galactic wide-binary systems containing
dormant BHs, which may not have been detected before. The study of this
population will provide new information on the BH-mass distribution in binaries
and shed light on their formation mechanisms and progenitors. As part of the
validation efforts in preparation for the fourth Gaia data release (DR4), we
analysed the preliminary astrometric binary solutions, obtained by the Gaia
Non-Single Star pipeline, to verify their significance and to minimise
false-detection rates in high-mass-function orbital solutions. The astrometric
binary solution of one source, Gaia BH3, implies the presence of a 32.70 \pm
0.82 M\odot BH in a binary system with a period of 11.6 yr. Gaia radial
velocities independently validate the astrometric orbit. Broad-band photometric
and spectroscopic data show that the visible component is an old, very
metal-poor giant of the Galactic halo, at a distance of 590 pc. The BH in the
Gaia BH3 system is more massive than any other Galactic stellar-origin BH known
thus far. The low metallicity of the star companion supports the scenario that
metal-poor massive stars are progenitors of the high-mass BHs detected by
gravitational-wave telescopes. The Galactic orbit of the system and its
metallicity indicate that it might belong to the Sequoia halo substructure.
Alternatively, and more plausibly, it could belong to the ED-2 stream, which
likely originated from a globular cluster that had been disrupted by the Milky
Way.
