Heaviest black hole
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The Heaviest Black Hole: Insights from Recent Discoveries
GW190521: The Heaviest Black Hole Merger
Discovery and Significance
The gravitational wave event GW190521, detected by the LIGO/Virgo collaboration, represents the heaviest binary black hole merger observed to date. The remnant black hole from this merger has a mass of approximately 142 solar masses (M☉), providing the first strong evidence for the existence of intermediate-mass black holes. This discovery challenges existing models of stellar evolution, which struggle to explain such massive black holes.
Formation Theories
Several theories have been proposed to explain the formation of such a massive black hole. One hypothesis suggests that GW190521 could be the result of hierarchical mergers, where the black holes involved in the merger are themselves products of previous black hole mergers . This theory is supported by the estimated masses of the ancestral black holes, which indicate a high probability that they fall within the 50 M☉–120 M☉ mass gap, suggesting a history of multiple mergers.
Eccentricity and Spin
Another study posits that GW190521 is best explained by a highly eccentric black hole merger, with an eccentricity of approximately 0.7. This high eccentricity, along with the precession due to black hole spin orientation, points to a formation channel involving repeated gravitational captures in dense stellar environments. This scenario is consistent with the properties observed in GW190521, making it a unique case among LIGO/Virgo's discoveries.
Implications for Black Hole Formation Models
Hierarchical Mergers in AGN Disks
Active Galactic Nuclei (AGN) disks are proposed as a conducive environment for hierarchical black hole mergers. In these dense and dynamic regions, black holes can undergo multiple mergers, leading to the formation of more massive black holes over time. Studies suggest that up to 40% of AGN-assisted mergers detected by LIGO/Virgo could involve black holes with masses around 50 M☉, supporting the hierarchical merger theory.
Mass Gap and Accretion
The detection of black holes in the so-called lower mass gap (2.2 M☉–5 M☉) further complicates our understanding of black hole formation. Objects in this mass range, such as the 2.6 M☉ object detected in GW190814, are thought to originate from distinct astrophysical processes, possibly involving accretion in AGN disks. This process could also contribute to the growth of black holes to intermediate masses, bridging the gap between stellar-mass and massive black holes.
Conclusion
The discovery of GW190521 has significantly advanced our understanding of black hole formation and evolution. Theories involving hierarchical mergers and the dynamic environments of AGN disks provide plausible explanations for the existence of such massive black holes. As gravitational wave astronomy continues to evolve, future observations will likely shed more light on these enigmatic objects, further refining our models of black hole formation and growth.
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