This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Astrophysicists think that the supermassive black holes (SMBH) we see in the modern universe are the result of a long, drawn-out process. That process is referred to as the hierarchical black hole mergers framework.
It explains how some SMBHs can reach such staggering masses. Some of the most massive appear to have billions of solar masses. The idea is that these SMBHs start out as seeds, and those seeds are stellar-mass black holes.
Two processes create them. One is when massive stars between about 8 and 50 solar masses reach the end of their lives of fusion. They either explode as supernovae, and their cores collapse, forming black holes, or they collapse directly into black holes.
The other is when two neutron stars merge. In between stellar-mass black holes and SMBHs are intermediate-mass black holes (IMBH). These black holes range in mass from about 100 solar masses to 100,000 solar masses.
The observational evidence for IMBHs is still thin, but it's persistent. A 2024 paper focused on Omega Centauri, a globular cluster on the outskirts of the Milky Way, as the site of an IMBH. A group of fast-moving stars in the center of Omega Centauri can only be explained by the presence of an IMBH with about 8,200 solar masses, according to that work.
Gravitational waves from a black hole merger are also evidence of an IMBH. A 2020 paper presented the only direct evidence of an IMBH when the waves showed that two black holes, with 85 and 66 solar masses, merged to create an IMBH with about 142 solar masses. But what are the birthplaces of these IMBHs?
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