Using data from ESA’s Gaia mission, astronomers have discovered a nearby binary system composed of a giant star orbiting a dormant black hole of of stellar origin in 11.6 years. The estimated mass of the black hole (33 solar masses) is substantially higher than all known stellar-mass black holes in the Milky Way and is in the mass range of the extragalactic black holes detected by gravitational waves.

The location of the first three black holes discovered by ESA’s Gaia mission in the Milky Way. Image credit: ESA / Gaia / DPAC.

The binary system in question, labeled Gaia BH3, resides 1,926 light-years away from Earth in the constellation of Aquila.

Also known as Gaia DR3 4318465066420528000, LS II +14 13 and 2MASS J19391872+1455542, it consists of an old, very metal-poor giant star and a dormant stellar-mass black hole.

Gaia BH3 is the third dormant black hole found with ESA’s star-mapping satellite Gaia.

“This is the kind of discovery you make once in your research life,” said Dr. Pasquale Panuzzo, an astronomer at CNRS and Observatoire de Paris.

“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.”

The average mass of known black holes of stellar origin in our Galaxy is around 10 solar masses.

Astronomers face the pressing question of explaining the origin of black holes as large as 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 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 supernovae.

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 30 solar masses are very difficult to explain. Yet, a clue to this puzzle may lie very close to Gaia BH3.

The star orbiting Gaia BH3 at about 16 times the Sun-Earth distance is rather uncommon: an ancient giant star, that 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 disk.

Its trajectory indicates that this star was probably part of a small galaxy, or a globular cluster, engulfed by the Milky Way more than 8 billion years ago.

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.

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 primeval massive stars poor in heavy elements.

These early stars might have evolved differently from the massive stars we currently see in our Galaxy.

The composition of the companion star can also shed light on the formation mechanism of this astonishing binary system.

“What strikes me is that the chemical composition of the companion is similar to what we find in old metal-poor stars in the Galaxy,” said Dr. Elisabetta Caffau, an astronomer at CNRS and Observatoire de Paris.

“There is no evidence that this star was contaminated by the material flung out by the supernova explosion of the massive star that became BH3.”

“This could suggest that the black hole acquired its companion only after its birth, capturing it from another system.”

The team’s paper will be published in the journal Astronomy & Astrophysics.


P. Panuzzo et al. (Gaia Collaboration). 2024. Discovery of a dormant 33 solar-mass black hole in pre-release Gaia astrometry. A&A, in press; doi: 10.1051/0004-6361/202449763

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