The 33 M$_\odot$ black hole Gaia BH3 is part of the disrupted ED-2 star cluster (2404.11604v2)

Abstract: The Gaia Collaboration has recently reported the detection of a 33 M$\odot$ black hole in a wide binary system located in the Solar neighbourhood. Here we explore the relationship between this black hole, known as Gaia BH3, and the nearby ED-2 halo stellar stream. We study the orbital characteristics of the Gaia BH3 binary and present measurements of the chemical abundances of ED-2 member stars derived from high-resolution spectra obtained with the VLT. We find that the Galactic orbit of the Gaia BH3 system and its metallicity are entirely consistent with being part of the ED-2 stream. The characteristics of the stream, particularly its negligible spread in metallicity and in other chemical elements as well as its single stellar population, suggest that it originated from a disrupted star cluster of low mass. Its age is comparable to that of the globular cluster M92 that has been estimated to be as old as the Universe. This is the first black hole unambiguously associated with a disrupted star cluster. We infer a plausible mass range for the cluster to be relatively narrow, between $2\times 10^3M\odot$ and $4.2\times 10^4M_\odot$. This implies that the black hole could have formed directly from the collapse of a massive very-metal-poor star, but that the alternative scenario of binary interactions inside the cluster environment also deserves to be explored.

• The paper establishes that Gaia BH3’s orbital parameters and chemical profile closely match ED-2 stars, indicating a shared dynamical origin.
• The paper employs Gaia astrometry and high-resolution VLT spectra to reveal a negligible metallicity spread, supporting a low-mass star cluster progenitor.
• The paper discusses formation scenarios, favoring a direct collapse and merger process within ED-2 as a viable pathway for Gaia BH3’s development.

The 33 M$_\odot$ Black Hole Gaia BH3 in the Context of the ED-2 Star Cluster

This paper explores the recent discovery of a 33 M$_\odot$ black hole, designated as Gaia BH3, and its association with the ED-2 stellar stream, a disrupted star cluster in the Milky Way's halo. The paper provides an insightful analysis of the dynamical and chemical characteristics of ED-2 and establishes Gaia BH3 as an integral part of this structure. The findings presented in this paper build upon existing data from the Gaia mission, high-resolution spectroscopic observations, and contemporary understandings of stellar and black hole dynamics.

The analysis focuses on several key aspects:

1. Orbital and Kinematic Consistency: The orbital parameters of the Gaia BH3 system were found to be consistent with those of the ED-2 stellar stream members. The position of Gaia BH3 aligns closely with ED-2 stars in integral of motion (IoM) space, suggesting a common dynamical origin. The paper emphasizes the retrograde orbit of Gaia BH3, supporting its membership to the ED-2 stream rather than to larger structures like the Sequoia.
2. Chemical Abundances: The authors measured the metallicity of stars in ED-2 using high-resolution spectra from the VLT, finding a negligible spread. This metallicity closely matches that of Gaia BH3's companion star, further substantiating their shared origin. The chemical homogeneity implies the progenitor of ED-2 was a low-mass star cluster.
3. Age and Evolutionary Insights: By comparing the color-magnitude diagram (CMD) of ED-2 with that of the well-studied globular cluster M92, the paper infers that the age of the ED-2 stream is comparable to some of the oldest known star clusters. The analysis suggests that ED-2 predates galactic formations, potentially dating back over 13 Gyr.
4. Formation Scenarios for Gaia BH3: The paper discusses potential formation mechanisms for Gaia BH3, including direct collapse of a massive star and hierarchical growth through stellar mergers facilitated by dynamical interactions within a dense star cluster environment. The findings favor a formation within a star cluster due to the lack of multiple stellar populations within ED-2, typical in older stellar clusters.

This research holds significant implications:

• It brings clarity to the evolution of black holes within the context of disrupted star clusters, emphasizing their potential birth in low-mass, metal-poor environments which has bearings on our understanding of stellar dynamics and chemical evolution in galaxies.
• By positioning Gaia BH3 within the ED-2 stream, it highlights the role of star clusters as birthplaces of massive black holes and calls for more sophisticated models to explore such evolutionary pathways.
• The paper may guide future spectroscopic and astrometric campaigns aimed at mapping other similar stellar flows within the Milky Way, using Gaia's extensive catalog and complementing it with ground-based follow-up data.

In conclusion, the paper presents a rigorous analysis that enhances our understanding of black holes associated with star clusters. The methodologies employed and the clear establishment of Gaia BH3's connection to ED-2 make a meaningful contribution to the field, presenting avenues for deeper investigation into the life cycles of star clusters and their remnants. Future research following these findings could provide further insight into the environments fostering the formation of stellar mass black holes and their dynamical signatures.