IMBHs

Intermediate-mass black holes (IMBHs) are black holes with masses between roughly 100 and 100,000 solar masses, placing them between stellar-mass black holes and supermassive black holes found in galactic centers. They are a proposed population that could illuminate the growth of the most massive black holes and the dynamics of dense stellar systems.

Possible formation channels include the direct collapse of very massive Population III stars in the early universe, runaway mergers of massive stars in dense star clusters, or remnants of Population III stars that did not produce bright supernovae. Accretion and successive mergers over time could also grow stellar-mass black holes toward the intermediate range.

Direct, unambiguous evidence for IMBHs remains elusive. Indirect hints come from ultraluminous X-ray sources (ULXs) whose luminosities exceed the Eddington limit for typical stellar-mass black holes, suggesting possible accretion onto an IMBH, though alternative explanations such as super-Eddington accretion or beaming are also proposed. A few dynamical studies of globular clusters have reported central dark masses compatible with IMBHs, but such claims are debated and may reflect measurement limitations or mass segregation. Notable candidates include HLX-1 in ESO 243-49 and other ULXs; none has achieved definitive confirmation. Gravitational-wave detections of mergers involving IMBHs would provide strong evidence in the future.

In cosmic evolution, IMBHs are hypothesized to act as seeds for the growth of supermassive black holes and to influence the dynamical evolution of star clusters. Their existence and frequency affect models of black-hole growth and galaxy evolution. Ongoing and future observations, including gravitational-wave astronomy with space-based detectors, aim to identify IMBHs and constrain their demographics.