Thanks to the Gaia, mission, the European Space Agency’s (ESA) most ambitious project to detail the stellar mapping of our galaxy, a massive stellar black hole, Gaia BH3, has been discovered in the Milky Way.  This type of black hole has been found before in distant galaxies by gravitational-wave observations, and is now identified for the first time in our galaxy. A team led by Jordi Portell, deputy director of the ICCUB Technology Unit (ICCUB-Tech), located in the Barcelona Science Park, has participated in this discovery.

This exceptional discovery confirms some theories and needs revision as well.  It is a dormant black hole, is the second closest to Earth — at a distance of 590 pc (or 1926 light-years) — is about thirty-three solar masses and forms a large binary system with its companion star. This is an exciting result for the astronomical community, which raises the question of how many such black holes there are in space, or what mass ranges of black holes the Gaia mission will be able to discover.

This finding, published in the prestigious journal Astronomy & Astrophysics, involved a team of astronomers and engineers from the Department of Quantum Physics and Astrophysics, the Institute of Cosmos Sciences (ICCUB) of the University of Barcelona and the Institute of Space Studies of Catalonia (IEEC), who have been part of the Gaia mission, the most ambitious project of the European Space Agency (ESA) to study the history and structure of the Milky Way.

How can a dormant black hole be detected?

If the black hole is dormant, doesn’t that make it hard to spot it? 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. In particular, after the release of the third Gaia data release — the Gaia Data Release (DR3) — the first dormant black holes in our galaxy could be identified: Gaia BH1 and Gaia BH2.

“It’s a real unicorn! It’s like nothing we have ever seen. This is the kind of discovery you make once in your research life. So far, black holes this big have only ever been detected in distant galaxies by the LIGO–Virgo–KAGRA collaboration, thanks to observations of gravitational waves,” says expert Pasquale Panuzzo, from the Paris Observatory of the Centre National de la Recherche Scientifique (CNRS) in France, and lead author of the paper.

“It’s impressive to see the transformational impact Gaia is having on astronomy and astrophysics,” notes 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.”

The most massive black hole of stellar origin in our Galaxy

But what makes this finding so amazing? Most of all because of its high mass. With thirty-three solar masses, Gaia BH3 is not only the most massive black hole of stellar origin known in our galaxy, but it is also in line with results obtained by gravitational-wave observatories such as LIGO/VIRGO/KAGRA. These facilities found a population of black holes with masses that contradict models of stellar evolution through the observation of gravitational waves from black hole mergers. The Gaia finding confirms that massive black holes of stellar origin also exist in our own Milky Way.

Most black holes of stellar origin in our galaxy have a mass of about ten solar masses, and its record value until now was held by the Cyg X-1 black hole, with an estimated mass of about twenty times that of the Sun. Gaia BH3 goes much further and is the record for our galaxy. Its mass is pinned down with unparalleled accuracy as well (32.7 +/- 0.82 M_solar), putting it firmly in the 30 solar mass range.

The second closest black hole to Earth

This black hole, which lies at a distance of 1926 light-years, is currently the second closest to Earth. Why is this black hole only visible now? The longer time span of observations that will form the basis of Gaia Data Release 4 (DR4) is crucial for answering this question.

The orbit of the stellar companion around their common centre of mass is estimated to be 11.6 years. This means that, with 5.5 years of data being processed for the next DR4, Gaia can map half of its orbit. This is enough to distinguish the additional oscillation in the position and motion of the companion star. It is expected that, with a longer time period of Gaia observations, more and more wide binaries can be identified. Lots of results are therefore to be expected from Gaia’s data releases.

An intriguing companion

The star orbiting Gaia BH3 at about 16 times the Sun–Earth distance is rather uncommon: an ancient giant star, which formed in the first two billion years after the Big Bang, at the time our galaxy started to assemble. It belongs to the family of the Galactic stellar halo and is moving in the opposite direction to the stars of the Galactic disc. Its trajectory indicates that this star was probably part of a small galaxy, or a globular cluster, engulfed by our own galaxy more than eight billion years ago.

For now, the formation process of this binary system with a black hole poses many questions. This new black hole challenges our understanding of how massive stars develop and evolve.

To date, Gaia data have only revealed the tip of the iceberg. Longer time spans of Gaia future data releases will undoubtedly reveal other binary systems containing black holes, but also exoplanets and other exotic binary systems. The Gaia Data Release (DR4) will be based on 5.5 years of observations, almost double the time period of the third data release, with about 3 years of observations. Currently, the full lifetime of Gaia is expected to be about 10.5 years.

»Link to the news: ICCUB website [+]

» Reference article: Gaia Collaboration; Panuzzo, P. et al. “Discovery of a dormant 33 solar-mass black hole in pre-release Gaia astrometry”. Astronomy & Astrophysics, abril de 2024. Doi: 10.1051/0004-6361/202449763