Found: the most massive stellar black hole in the Milky Way

Astronomers have identified the most massive stellar black hole — that is, a black hole formed from the collapse of a star — ever discovered in the Milky Way. The dormant black hole was spotted in data from the European Space Agency’s (ESA) Gaia mission, which seeks to to chart a three-dimensional map of the galaxy, because it imposes an odd ‘wobbling’ motion on the companion star orbiting it.

Data from the European Southern Observatory’s (ESO) Very Large Telescope (VLT) and other ground-based observatories were used to verify the mass of the black hole, putting it at 33 times that of the Sun; in comparison, the stellar black holes previously identified in the Milky Way are on average about 10 times as massive as the Sun. This black hole is also extremely close to us — at a mere 1926 light-years away in the constellation Aquila, it is the second-closest known black hole to Earth.

Dubbed Gaia BH3, or BH3 for short, the black hole was found while astronomers were reviewing Gaia observations in preparation for an upcoming data release. According to Gaia collaboration member Pasquale Panuzzo, from the National Centre for Scientific Research (CNRS) at the Observatoire de Paris - PSL, “No one was expecting to find a high-mass black hole lurking nearby, undetected so far. This is the kind of discovery you make once in your research life.”

Most known black holes are detected through the X-rays they emit when material from their stellar companion is ‘eaten’. With dormant black holes, little or no radiation is emitted by the source, so the black hole can only really be seen because of the gravitational effect it exerts on its companion star. Dormant black holes had never been detected before the Gaia mission, with the first dormant black holes in our galaxy (Gaia BH1 and Gaia BH2) only identified after the third Gaia data release.

Astronomers have previously found massive black holes outside our galaxy, using gravitational-wave observations, and have theorised that they may form from the collapse of stars with very few elements heavier than hydrogen and helium in their chemical composition. These so-called metal-poor stars are thought to lose less mass over their lifetimes and hence have more material left over to produce high-mass black holes after their death. But evidence directly linking metal-poor stars to high-mass black holes has been lacking until now.

Stars in pairs tend to have similar compositions, meaning that BH3’s companion holds important clues about the star that collapsed to form the black hole. Data from the VLT’s Ultraviolet and Visual Echelle Spectrograph (UVES) instrument showed that the companion was a very metal-poor star, indicating that the star that collapsed to form BH3 was also metal-poor — just as predicted.

There are also questions about where the black hole came from. Although it is currently in the plane of the Milky Way, its motion puts it in a retrograde orbit with a large inclination to the plane of the Milky Way, so it may come from a merger event of a small galaxy or a globular cluster merged with the Milky Way. It is expected that further studies will provide further insight into this.

The black hole’s discovery has been detailed in the journal Astronomy & Astrophysics, based on preliminary data, so that other astronomers can start studying it without waiting for the next Gaia data release in late 2025. Further observations of this system could reveal more about its history and about the black hole itself; the GRAVITY instrument on the VLT Interferometer, for example, could help astronomers find out whether this black hole is pulling in matter from its surroundings and better understand this exciting object.

“It’s impressive to see the transformational impact Gaia is having on astronomy and astrophysics,” said Carole Mundell, ESA Director of Science. “Its discoveries are reaching far beyond the original purpose of the mission, which is to create an extraordinarily precise multidimensional map of more than a billion stars throughout our Milky Way.”

Image caption: This artist’s impression shows the orbits of both the companion star and the black hole around their common centre of mass. Image credit: ESO/L Calçada/Space Engine (spaceengine.org)