The European Gaia space telescope, dedicated to mapping the Milky Way, has discovered a black hole with a record mass, 33 times that of the Sun: something never before seen in our galaxy, indicates a study published Tuesday.
The black hole, called Gaia BH3, located 2,000 light years from Earth, in the constellation Eagle, belongs to the family of stellar black holes which result from the collapse of massive stars at the end of their life. They are incomparably smaller than the supermassive black holes housed in the hearts of galaxies, the formation scenario of which is not known.
A discovery made “by chance”
It was “by chance” that Gaia BH3 was discovered, says Pasquale Panuzzo, CNRS researcher at the Paris-PSL Observatory, main author of the work published in Astronomy & Astrophysics Letters.
Scientists from the Gaia consortium were clearing the latest data from the probe, with a view to publishing the next catalog in 2025, when they came across a particular binary star system. “We saw a star a little smaller than the Sun (approximately 75% of its mass) and brighter, which revolved around an invisible companion”, identifiable by the disturbances it causes it to undergo, says Pasquale Panuzzo, responsible for deputy of Gaia's spectroscopic processing.
The space telescope gives the very precise position of the stars in the sky, so astronomers were able to characterize the orbits and measure the mass of the star's invisible companion: 33 times that of the Sun.
Further observations from ground-based telescopes confirmed that it was indeed a black hole, with a mass much greater than that of the black holes of stellar origin already known in the Milky Way - between 10 and 20 solar masses.
Such behemoths have already been detected in distant galaxies, via gravitational waves. But “never in ours,” says Dr. Panuzzo.
A “dormant” black hole
Gaia BH3 is a “dormant” black hole: it is too far from its companion star to remove its material and therefore does not emit any X-ray radiation, which makes its detection extremely difficult. The Gaia telescope succeeded in finding the first two inactive black holes (Gaia BH1 and Gaia BH2) in the Milky Way, but they have standard masses.
Unlike the Sun, the small star of the binary system of BH3 is “very poor in elements heavier than hydrogen and helium”, explains the Paris Observatory in a press release.
“According to the theory, only these metal-poor stars can form such a massive black hole,” notes Dr. Panuzzo. The study therefore suggests that the “progenitor” of the black hole was a massive star that was also poor in metals.
The system's star, 12 billion years old, "ages very slowly", while the one that formed the black hole "only lived 3 million years", he describes. “These metal-poor stars were very present in the early days of the galaxy. Their study gives us information on its formation,” adds the scientist.
Another curiosity of the stellar couple: in the disk of the Milky Way, it rotates in the opposite direction to the other stars. “Perhaps because the black hole would have formed in another smaller galaxy, which would have been eaten at the beginning of the life of the Milky Way,” he suggests.
The ESA (European Space Agency) Gaia probe, which has been operating 1.5 million kilometers from Earth for 10 years, delivered in 2022 a 3D map of the positions and movements of more than 1.8 billion of stars.