This exceptional discovery confirms some theories and needs revision as well. 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

In the validation of the preliminary data processed by Gaia Data Release (DR4), and given the preliminary results for the non-single star pipeline, this galactic source required further checks to see if the detected data were correct or corrupt. At first, the Data Processing and Analysis Consortium (DPAC) team considered that these results could not be real. After many internal verifications, all the data suggested that it was a genuine detection, a scientific finding that is worth publishing before the release of the Gaia Data Release (DR4) to allow further follow-up of the discovery by the scientific community.

“While in the previous data release (Gaia DR3) we identified quite a few spurious black hole candidates that were traced back to data calibration issues, the quality of the latest data reduction has improved so much that we expect to publish quite a number of genuine black holes in Gaia DR4!” says Berry Holl of the Geneva Observatory, a member of the Gaia Collaboration.

“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 mass distribution for the black hole population derived from gravitational wave observations shows a clear peak around thirty solar masses”, says Tsevi Mazeh of Tel Aviv University, a member of the Gaia Collaboration. “It is very interesting to see now that Gaia BH3 is right at this peak with its 33 solar masses. This provides strong scientific support for the existence of this peak”, he adds.

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.

“In the visible wavelength range and the infrared, the light from the visible companion star outshines anything that might come from Gaia BH3 itself — or else the black hole would have been discovered much earlier, and without Gaia” says Uli Bastian, member of the Gaia Collaboration.

Because of its exceptional nature, and to rule out the possibility that the solution is spurious, a confirmation of the result with several ground-based observatories was performed. The UVES spectrum for this system was obtained from the ESO archive, and follow-up observations were performed with the HERMES spectrograph in Spain and the SOPHIE spectrograph in France. The radial velocities obtained with these ground observatories complement Gaia’s radial velocities, which confirms the orbital solution derived from Gaia's data.

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. Most theories predict that, as they age, massive stars shed a sizable part of their material through powerful winds; ultimately, they are partly blown into space when they explode as supernovas. What remains of their core further contracts to become either a neutron star or a black hole, depending on its mass. Cores large enough to end up as black holes of thirty times the mass of our Sun are very difficult to explain.
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The companion star has very few elements heavier than hydrogen and helium, indicating that the massive star that became Gaia BH3 could also have been very poor in heavy elements. This is remarkable sinceit supports, for the first time, the theory that the high-mass black holes observed by gravitational wave experiments were produced by the collapse of primeval massive stars poor in heavy elements. These early stars might have evolved differently from the massive stars we currently see in our galaxy.

There are also many questions about where this 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. The black hole may come from a merger event of a small galaxy or a globular cluster merged with our Milky Way. It is expected that further studies will provide more insight on how Gaia BH3 ended up in the Milky Way.

“A growing number of black holes being found in the Milky Way with different methods, including the microlensing one reported in 2022 by OGLE and HST, brings us closer to obtaining a broader picture of the population of these objects in the Galaxy and may shed light on the nature of dark matter if an excess of these black holes is detected nearby.” says Łukasz Wyrzykowski, from Warsaw University in Poland and member of the Gaia Collaboration.

“From an observational point of view, discovering Gaia BH3 is not that hard, and specialized astronomical instruments will be able to detect its signatures as well. The difficulty is that you need to know which of the millions of stars to point your telescope at. This is where the power of a uniform all-sky survey like Gaia comes into play. Since Gaia observes all celestial sources that are bright enough to be seen by its detectors, we were able to find the needle in the haystack,” says Johannes Sahlmann, member of the Gaia Science Operations Team at the European Space Astronomy Centre in Spain.

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.

Videos (credits: ESA/Gaia/DPAC- CC BY-SA 3.0 IGO)