What is an intermediate-mass black hole?

It is a black hole with a mass between roughly 100 and 100,000 solar masses — heavier than any that can form from a single star, lighter than the supermassive black holes at galactic centres.

Why are intermediate-mass black holes hard to detect?

They are too light to dominate the dynamics of a galaxy centre and usually too quiet to shine as bright X-ray sources. Most candidates come from dense star clusters or from gravitational-wave detections of mergers.

A gravitational-wave event detected in 2019 in which black holes of about 85 and 66 solar masses merged, producing a remnant of roughly 142 solar masses — the clearest direct evidence for an intermediate-mass black hole.

Do intermediate-mass black holes become supermassive ones?

That is the leading idea. They are thought to be the seeds that grow, through accretion and repeated mergers, into the supermassive black holes found at the centres of galaxies.

Что такое Intermediate-Mass Black Hole?

An intermediate-mass black hole holds between about 100 and 100,000 solar masses — far heavier than anything a single star can produce, far lighter than the giants at galactic centres. For decades the class was a conspicuous gap in the census, and confirming it has been one of the harder problems in modern astrophysics.

The gap matters because of what it implies about origins. Supermassive black holes were already billions of solar masses less than a billion years after the Big Bang, which is difficult to explain by steady growth from stellar-mass seeds alone. Intermediate-mass black holes are the natural stepping stone: seeds formed in the dense cores of early star clusters, growing by mergers and accretion into the monsters we see today.

The strongest single piece of evidence arrived in 2019. LIGO and Virgo detected GW190521, the merger of two black holes of roughly 85 and 66 solar masses, leaving behind a remnant of about 142 solar masses — squarely in the intermediate range, and firmly measured. Other candidates come from ultraluminous X-ray sources such as HLX-1, and from the motions of stars in dense globular clusters like Omega Centauri.

They are hard to find precisely because of their size. They are too small to dominate a galaxy's centre and too large to be spotted by their effect on a single companion star, so they tend to sit quietly in crowded clusters, betraying themselves only when they eat something or merge. The model shows a mid-sized hole with a cooler, wider accretion disk and the first hint of a polar jet.