Sleeping giant surprises Gaia scientists

The matter in a black hole is so dense that nothing can escape its immense gravity, not even light. The vast majority of stellar-mass black holes that we know of gobble up matter from a nearby companion star. The captured material falls on the collapsing object at high speed, becomes extremely hot and emits X-rays. These systems belong to a family of celestial objects called X-ray binaries.

When a black hole doesn't have a companion close enough to steal matter from it, it generates no light and is extremely difficult to detect. These black holes are called “inert” black holes.

To prepare for Gaia's next catalog release, Data Release 4 (DR4), scientists are examining the motions of billions of stars and performing complex tests to see if anything is out of the ordinary. The motions of stars can be affected by companions: bright stars, such as exoplanets; Heavier like the stars. Or very heavy, like black holes. There are dedicated teams in the Gaia Collaboration to investigate any “strange” cases.

One such team was deeply involved in this work, when they came across an ancient giant star in the constellation Orion, 1,926 light-years from Earth. Through detailed analysis of the fluctuations in the star's path, they found a big surprise. The star was trapped in orbital motion with an inert black hole of exceptionally high mass, about 33 times the mass of the Sun.

This is the third dormant black hole found with Gaia, and it has been named “Gaia BH3.” Its discovery is very exciting because of the mass of the object. “This is the kind of discovery you make once in your research life,” says Pasquale Panozzo of the Center National de la Recherche Scientifique, Observatory of Paris, in France, who is the lead author on this result. “Until now, black holes of this size have only been discovered in distant galaxies through the LIGO-Virgo-KAGRA collaboration, thanks to gravitational wave observations.”

The average mass of known black holes of stellar origin in our Galaxy is about 10 times the mass of our Sun. Until now, the weight record was held by a black hole in the X-ray binary in the constellation Cygnus (Cyg X-1), which has an estimated mass of about 20 times that of the Sun.

“It is impressive to see the transformative impact Gaia is having on astronomy and astrophysics,” says Professor Carol Mundell, ESA's Director of Science. “Its discoveries reach far beyond the mission’s original goal, which was to create an extremely accurate, multi-dimensional map of more than a billion stars throughout the Milky Way.”

Unparalleled accuracy

The remarkable quality of the Gaia data enabled scientists to determine the black hole's mass with unparalleled precision and provide the most direct evidence that black holes exist in this mass range.

Astronomers face the pressing question of explaining the origin of large black holes like Gaia BH3. Our current understanding of how massive stars evolve and die does not immediately explain how these types of black holes appeared.

Most theories predict that, as massive stars age, they shed much of their matter through strong winds; Eventually, they are partially blown out into space when they explode as supernovas. What's left of its core shrinks into either a neutron star or a black hole, depending on its mass. Cores large enough to become black holes 30 times the mass of our Sun are very difficult to explain.

However, the key to solving this mystery may lie very close to Gaia BH3.

Interesting companion

The star orbiting Gaia BH3 at about 16 times the distance between the Sun and Earth is fairly uncommon: an ancient giant star, which formed in the first two billion years after the Big Bang, around the time our galaxy began to come together. It belongs to the family of galactic halo stars and moves in the opposite direction to the stars of the galactic disk. Its path suggests that this star may have been part of a small galaxy, or globular cluster, that swept into our galaxy more than eight billion years ago.

The companion star contains very few elements heavier than hydrogen and helium, suggesting that the massive star that became Gaia BH3 may also be very poor in heavy elements. This is wonderful. It supports, for the first time, the theory that the high-mass black holes observed by gravitational wave experiments resulted from the collapse of massive primordial stars poor in heavy elements. These early stars may have evolved differently from the massive stars we currently see in our galaxy.

Companion star formation could also shed light on the formation mechanism of this amazing binary system. “What amazes me is that the chemical composition of the companion is similar to what we find in ancient, metal-poor stars in the Galaxy,” explains Elisabetta Cavao of the CNRS, Paris Observatory, also a member of the Gaia collaboration.

“There is no evidence that this star was contaminated with material ejected by the supernova explosion of the massive star that became BH3.” This may indicate that the black hole did not acquire its companion until after its birth, as it acquired it from another system.

Delicious appetizers

The discovery of Gaia BH3 is just the beginning and much remains to be investigated about its puzzling nature. Now that scientists have been intrigued, this black hole and its companions will undoubtedly be the subject of many in-depth studies to come.

The Gaia team stumbled upon this “sleeping giant” while examining raw data in preparation for the fourth edition of the Gaia catalog. Because the result is so exceptional, they decided to announce it before the official release.

The next release of Gaia data promises to be a goldmine for studying binary systems and discovering more dormant black holes in our galaxy. “We have worked very hard to improve the way we process specific data sets compared to the previous data release (DR3), so we expect to discover more black holes in DR4,” concludes Perry Hall of the University of Geneva in Switzerland. Member of the Gaia Collaboration.

Notes to editors

“A dormant black hole with a mass of 33 solar masses was discovered in the pre-release Gaia astronomical measurement” by the Gaia Collaboration, P. Panuzzo, et al. is published today in the journal Astronomy and astrophysics (A&A).

Gaia is a European mission, built and operated by the European Space Agency. It was approved in 2000 as a core ESA mission within ESA's Horizon 2000 Plus science programme, with support from all ESA Member States. Member States also play a key role in the scientific part of the mission as part of the Data Processing and Analysis Consortium (DPAC) responsible for transforming raw data In the science products of Gaia data releases, in collaboration with the European Space Agency. DPAC brings together more than 450 specialists from across the scientific community in Europe. Gaia was designed and built by Astrium (now known as Airbus Defense and Space), with a core team consisting of Astrium France, Germany and the United Kingdom. The industrial team included 50 companies from 15 European countries, in addition to companies from the United States. The spacecraft was launched by Arianespace on December 19, 2013.

A list of the names of participating researchers and the role of ESA Member States is available to the media here [PDF].

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