A new study has found that there are two distinct populations of black holes. One forms through established models of stellar collapse, and the other forms as a result of repeated violent mergers in dense star clusters.
The largest black holes in the universe are detected through ripples in space-time and are not formed directly from collapsing stars, according to new research led by Cardiff University. Instead, these cosmic giants grow through repeated violent mergers within densely populated star clusters.
In this study, we analyzed version 4.0 of the Gravitational Wave Transient Catalog (GWTC-4) compiled by the LIGO-Virgo-KAGRA collaboration. This catalog includes 153 positively detected black hole mergers. The research team aimed to investigate whether the most massive black holes observed were actually second-generation objects. This object is formed when smaller black holes merge and re-merge within the dense core of a star cluster. In star clusters, stars are packed together up to a million times more densely than in the vicinity of the Sun.
The findings, published in Nature Astronomy, revealed two distinct populations of black holes. Lead author Dr Fabio Antonini, from Cardiff University’s School of Physics and Astronomy, said: “Gravitational wave astronomy is now doing more than just counting black hole mergers. We are learning how black holes grow, where they grow, and how they grow. “This is interesting because we can use this information to test our understanding of how stars and star clusters evolve in the Universe.”
The research team determined the following from the gravitational wave data:
A low-mass population consistent with a black hole formed by the collapse of a normal star. A high-mass population that exhibits spin characteristic of hierarchical mergers in dense star clusters.
Dr Isobel Romero-Shaw, co-author and Ernest Rutherford Research Fellow at Cardiff University, said: “What surprised us most was how clearly the high-mass black holes stood out as a separate population, unlike the lower-mass systems that typically rotated slowly. , the high-mass systems have faster rotations that are directed in seemingly random directions, which is exactly the sign you would expect if black holes were merging repeatedly in dense star clusters. ”
The study also provides the strongest evidence yet for a “mass gap,” or the range of black hole masses predicted to be absent due to pair instability phenomena. This theory suggests that very massive stars catastrophically explode rather than collapsing into black holes, resulting in a forbidden mass range for black holes that form directly from stars.
The researchers determined that this mass difference starts at about 45 times the mass of the Sun. Dr Antonini said: “We found evidence of the long-predicted pair-instability mass gap, a mass range in which stars would be expected to leave no black holes at all. Gravitational wave detectors found a black hole that appears to be in or near that gap, confirmed to be about 45 solar masses.”
Reconsidering the stellar evolution model
He added: “The key question now is: Are these black holes telling us that models of stellar evolution are wrong, or are they being created in some other way? “This phenomenon seems to reflect the dynamics of star clusters. When the solar mass exceeds about 45, the spin distribution changes in a way that is difficult to explain with normal binary stars alone, but it can be naturally explained if these black holes merge early in dense star clusters.”
Additionally, the team used this transition to investigate the key nuclear reactions involved in burning helium inside massive stars. Co-author Dr Fani Dosopoulou, a research fellow at Cardiff University, said: “In the future, gravitational wave data may help scientists study nuclear physics, because the mass limit set by the pairwise instability depends on the nuclear reactions that occur at the centers of massive stars.”
This study represents a major advance in understanding the growth and origin of the universe’s most massive black holes, highlighting the dynamic environment of star clusters as important sites of cluster formation.
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