Discovery of a dormant 33 solar-mass black hole in pre-release Gaia astrometry (2404.10486v2)

Abstract: Gravitational waves from black-hole merging events have revealed a population of extra-galactic BHs residing in short-period binaries with masses that are higher than expected based on most stellar evolution models - and also higher than known stellar-origin black holes in our Galaxy. It has been proposed that those high-mass BHs are the remnants of massive metal-poor stars. Gaia astrometry is expected to uncover many Galactic wide-binary systems containing dormant BHs, which may not have been detected before. The study of this population will provide new information on the BH-mass distribution in binaries and shed light on their formation mechanisms and progenitors. As part of the validation efforts in preparation for the fourth Gaia data release (DR4), we analysed the preliminary astrometric binary solutions, obtained by the Gaia Non-Single Star pipeline, to verify their significance and to minimise false-detection rates in high-mass-function orbital solutions. The astrometric binary solution of one source, Gaia BH3, implies the presence of a 32.70 \pm 0.82 M\odot BH in a binary system with a period of 11.6 yr. Gaia radial velocities independently validate the astrometric orbit. Broad-band photometric and spectroscopic data show that the visible component is an old, very metal-poor giant of the Galactic halo, at a distance of 590 pc. The BH in the Gaia BH3 system is more massive than any other Galactic stellar-origin BH known thus far. The low metallicity of the star companion supports the scenario that metal-poor massive stars are progenitors of the high-mass BHs detected by gravitational-wave telescopes. The Galactic orbit of the system and its metallicity indicate that it might belong to the Sequoia halo substructure. Alternatively, and more plausibly, it could belong to the ED-2 stream, which likely originated from a globular cluster that had been disrupted by the Milky Way.

• The paper identifies Gaia BH3, a dormant 33 solar-mass black hole in a binary system located 590 parsecs away, confirmed through astrometric and radial velocity analysis.
• The research employs Gaia DR4 pre-release data and refined non-single star pipelines to reduce false positives and validate binary orbital solutions.
• The findings challenge conventional stellar evolution models by suggesting that low-metallicity star remnants can produce high-mass black holes and hint at new Galactic halo substructures.

Discovery of a Dormant 33 Solar-Mass Black Hole in Pre-Release Gaia Astrometry

The paper discusses the identification of a dormant black hole (BH) with an estimated mass of 33 solar masses in the pre-release data from the Gaia astrometry mission. This finding presents significant implications for our understanding of black hole populations and star evolution within our galaxy.

Methodology and Findings

The discovery was made using approximation techniques from the Gaia mission, focusing on astrometric binaries. Gaia's astrometric data, intended for Gaia Data Release 4, was analyzed to extract binary star solutions from the Non-Single Star (NSS) pipeline, aiming to verify their validity and reduce false positives, especially in systems with high mass-function orbits. This specific black hole, referred to as Gaia BH3, was identified as part of these efforts and found to possess distinct characteristics:

1. Astrometric and Radial Analysis: The BH3 is part of a binary system located approximately 590 parsecs from Earth, with a star characterized as an old, very metal-poor giant. Data indicate a binary orbit of 11.6 years confirmed independently by Gaia's radial velocities. The BH's mass is calculated at 32.70 ± 0.82 solar masses.
2. Stellar Properties: The companion star in Gaia BH3 is neither a variable nor found in major X-ray observations, helping conclude the existence of the binary system without significant optical excessive contributions from the BH.
3. Galactic Implications: The Gaia BH3 system presents a mass far greater than other known Galactic BHs and suggests the star belongs to the Galactic halo, potentially part of the Sequoia halo substructure or the ED-2 stream. Such associations underline the hypothesis regarding low-metallicity progenitors leading to higher mass BHs.

Discussion and Implications

This paper sheds light on critical aspects of stellar evolution and BH formation:

• High-Mass Black Holes: The properties of Gaia BH3 support theories suggesting that high-mass black holes are remnants of massive, low-metallicity stars that experience relatively less mass loss compared to higher-metallicity counterparts.
• Galactic Halo Substructures: The orbit and metallicity of Gaia BH3 imply it might belong to a historical Galactic substructure, potentially providing insights into the Galaxy's accretion history.
• Binary Formation Avoidance of Common Envelopes: Conventional theories involving common-envelope phases leading to closer binaries appear challenged by such wide-binary formations without direct interaction with their compact companion, suggesting alternative dynamical acquisition processes.
• Follow-up Prospects: Gaia BH3 represents an easily accessible target for ongoing observations due to its proximity and magnitude, facilitating deeper investigations on mass accumulation and binary interaction mechanisms.

Conclusion

Gaia BH3 serves as an extreme instance of quiescent stellar-mass black holes, underscoring the diversity and complexity of BH systems previously uncertain in our Galactic neighborhood. It highlights the potential for the Gaia mission and its anticipated DR4 data to uncover the universe of dormant black holes bound in binary systems, ultimately enhancing knowledge on binary evolution and black hole demographics. This paper prompts a reconsideration of specific parameters in stellar evolution models and influencing factors on binary system formation, urging the broader astrophysical community to explore further observational and theoretical frameworks.