Sleeping giant surprises Gaia scientists

The matter inside a black hole is so tightly packed that nothing can escape its immense gravitational pull, not even light. The vast majority of stellar-mass black holes that we know of are ingesting matter from a nearby stellar companion. The captured material falls onto the collapsed object at high speed, becomes extremely hot and releases 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 doesn’t generate any light and is extremely hard to spot. These black holes are called “sleepers”.

To prepare for the release of the next Gaia catalog, Data Release 4 (DR4), scientists are checking the motions of billions of stars and running sophisticated tests to see if anything is out of the ordinary. The motions of the stars can be influenced by satellites: light, such as exoplanets; heavier, like stars; or very heavy, like black holes. Specialized teams are on site at the Gaia Collaboration to investigate any “strange” cases.

One such team was deeply involved in this work when their attention fell on an old giant star in the constellation Auriga, 1,926 light-years from Earth. By analyzing the fluctuations in the star’s path in detail, they discovered a big surprise. The star was locked in orbital motion with a dormant black hole of extremely high mass, about 33 times that of the Sun.

It is the third dormant black hole discovered with Gaia and is aptly named “Gaia BH3”. Its discovery is very exciting because of the object’s mass. “This is a once-in-a-lifetime research discovery,” exclaims Pasquale Panuzzo of CNRS, Paris Observatory, France, who is the lead author of this discovery. “So far, black holes this large have only been detected in distant galaxies by the LIGO-Virgo-KAGRA collaboration, thanks to observations of gravitational waves.”

The average mass of known stellar black holes in our galaxy is about 10 times the mass of our Sun. Until now, the record for mass was held by a black hole in an X-ray binary system in the constellation Cygnus (Cyg X-1), whose mass is estimated to be about 20 times that of the Sun.

“It is impressive to see the transformative impact Gaia is having on astronomy and astrophysics,” notes Prof. Carol Mandel, ESA’s Science Director. “Its findings go far beyond the mission’s original goal, which was to create an extremely precise multidimensional map of the more than one billion stars in our Milky Way.”

Unsurpassed accuracy

The exceptional quality of the Gaia data allowed scientists to determine the mass of the black hole 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 black holes the size of Gaia BH3. Our current understanding of how massive stars evolve and die does not immediately explain how these types of black holes came to be.

Most theories predict that as massive stars age, they eject a significant portion of their material through powerful winds; eventually they are partially blown into space when they explode as supernovae. What remains of their core further collapses to become either a neutron star or a black hole, depending on its mass. Cores large enough to become black holes with 30 times the mass of our Sun are very difficult to explain.

Yet, the key to this puzzle may lie very close to Gaia BH3.

An intriguing companion

The star orbiting Gaia BH3 at about 16 times the Sun-Earth distance is quite unusual: an ancient giant star that formed in the first two billion years after the Big Bang, around the time our galaxy began to come together. It belongs to the galactic stellar halo family and moves in the opposite direction to the stars of the galactic disk. Its trajectory indicates that this star was probably part of a small galaxy or globular cluster that was swallowed up by our own galaxy more than eight billion years ago.

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

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

“There is no evidence that this star was contaminated by the material ejected from the supernova explosion of the massive star that became BH3.” This could suggest that the black hole acquired its companion only after its birth by capturing it from another system.

A delicious appetizer

The discovery of Gaia BH3 is just the beginning, and much remains to be learned about its puzzling nature. Now that scientists’ curiosity has been piqued, this black hole and its companion will undoubtedly be the subject of much in-depth research to come.

The Gaia Collaboration came across this ‘sleeping giant’ while checking preliminary data in preparation for the fourth edition of the Gaia catalogue. Since the find is so exceptional, they decided to announce it before the official release.

The next release of data from Gaia promises to be a gold mine for the study of binary systems and the discovery of more dormant black holes in our galaxy. “We are working extremely hard to improve the way we process specific data sets compared to the previous data release (DR3), so we expect to uncover many more black holes in DR4,” concludes Barry Hall of the University of Geneva, Switzerland , a member of the Gaia collaboration.

Notes for editors

“Discovery of a 33 solar-mass dormant black hole in pre-launch Gaia astrometry” 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 managed by ESA. It was approved in 2000 as the European Space Agency’s flagship mission under ESA’s Horizon 2000 Plus science programme, supported by all ESA Member States. Member States also have a key role in the scientific part of the mission as part of the Data Processing and Analysis Consortium (DPAC) responsible for converting the raw data in science products for Gaia Data Releases, in collaboration with ESA. DPAC brings together more than 450 specialists from the entire scientific community in Europe. Gaia was designed and built by Astrium (now Airbus Defense and Space), with a core team made up of Astrium France, Germany and the UK. The industrial team included 50 companies from 15 European countries as well as companies from the USA. The spacecraft was launched by Arianespace on December 19, 2013.

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

Additional information about this discovery