Astronomers have discovered the most massive stellar-mass black hole known in the Milky Way galaxy, a sleeping giant designated as Gaia BH3. The black hole, which is 33 times more massive than our sun, was found using the European Space Agency's Gaia space telescope, which tracks the motion of billions of stars in our galaxy. The discovery, published in the journal Astronomy & Astrophysics, represents a significant milestone in the study of black holes and their formation.
Gaia BH3 is located just 2,000 light-years from Earth, making it the second-closest black hole to our planet ever discovered. The closest black hole, Gaia BH1, is 1,560 light-years away and has a mass around 9.6 times that of the sun. While Gaia BH3 is the most massive stellar-mass black hole found in the Milky Way, it is still dwarfed by the supermassive black hole at the heart of our galaxy, Sagittarius A* (Sgr A*), which has a mass 4.2 million times that of the sun.
Stellar-mass black holes are created when a large star runs out of fuel and collapses. These black holes are typically difficult to detect when they are dormant, as they do not emit or reflect light. However, the Gaia Black Hole Task Force was able to identify Gaia BH3 by observing an odd wobble in the motion of its companion star, an old giant star in the constellation Aquila.
"Finding Gaia BH3 is like the moment in the film 'The Matrix' where Neo starts to 'see' the matrix," said George Seabrook, a scientist at Mullard Space Science Laboratory at University College London and a member of Gaia's Black Hole Task Force. "In our case, 'the matrix' is our galaxy's population of dormant stellar black holes, which were hidden from us before Gaia detected them."
The discovery of Gaia BH3 not only sheds light on the population of dormant stellar black holes in the Milky Way but also provides valuable insights into the formation of high-mass black holes. The companion star orbiting Gaia BH3 is metal-poor, suggesting that the star that collapsed to create the black hole also lacked heavier elements. This finding supports the theory that metal-poor stars can maintain enough mass to birth high-mass black holes, a question that has long puzzled astronomers.
"Gaia's measurements provide the first undisputable proof that [stellar-mass] black holes this heavy do exist," said Tsevi Mazeh, a scientist and Gaia collaboration member at Tel Aviv University. "At 30 times that of our sun, the object's mass is typical of the estimates we have for the masses of the very distant black holes observed by gravitational wave experiments."
The discovery of Gaia BH3 also offers clues to the history of the Milky Way galaxy. The companion star's trajectory, which moves in the opposite direction of many stars in the galactic disk, suggests that it was part of a small galaxy that merged with the Milky Way more than 8 billion years ago.
As the Gaia space telescope continues its mission to map the stars in our galaxy, astronomers expect to uncover more dormant stellar black holes in the upcoming data releases. These discoveries will further enhance our understanding of the formation and evolution of black holes and their role in shaping the universe.
"Gaia's next data release is expected to contain many more, which should help us to 'see' more of 'the matrix' and to understand how dormant stellar black holes form," Seabroke concluded.
The discovery of Gaia BH3 is a testament to the transformational impact of the Gaia mission on astronomy and astrophysics. As Carole Mundell, the European Space Agency's director of science, stated, "It's impressive to see the transformational impact Gaia is having on astronomy and astrophysics. Its discoveries are reaching far beyond the original purpose of the mission, which is to create an extraordinarily precise multi-dimensional map of more than a billion stars throughout our Milky Way."
