Intermediate-mass black hole

An intermediate-mass black hole (IMBH) is a special kind of black hole. Its mass ranges from one hundred to one hundred thousand times the mass of our Sun. This makes it much bigger than the black holes formed when big stars end their lives, called stellar black holes, but much smaller than the giant black holes found at the centers of galaxies, known as supermassive black holes.

Scientists have found several possible IMBHs, especially in our own galaxy, the Milky Way. They discover these hidden giants by watching how nearby gas clouds move and by studying the light from material being pulled into the black hole. These clues help astronomers piece together the existence of these mysterious objects. Studying IMBHs helps us understand how bigger black holes might form and how they influence the space around them.

Observational evidence

Scientists have found clues that suggest the existence of intermediate-mass black holes, which are bigger than black holes formed from single stars but smaller than the huge black holes found in the centers of galaxies. One important clue came from a gravitational wave signal detected on May 21, 2019. This signal showed that two black holes, with masses of 85 and 65 times that of our Sun, merged into one black hole.

Other clues come from special areas in galaxies that shine very brightly in X-ray light. These bright areas, called ultraluminous X-ray sources, might be caused by intermediate-mass black holes. Scientists also study how stars move in clusters to look for more evidence of these mysterious black holes.

Potential discoveries

In November 2004, astronomers found GCIRS 13E in the Milky Way, which they believed to be the first intermediate-mass black hole. It was thought to have a mass of 1,300 times that of the Sun and was located near Sagittarius A*.

Since then, more possible intermediate-mass black holes have been found. In 2009, HLX-1 was discovered in the galaxy ESO 243–49. In 2012, another possible intermediate-mass black hole was found by a team using the CSIRO radio telescope in Australia. In 2015, a gas cloud with unusual movement suggested the presence of a black hole with about 100,000 solar masses. In 2018, another possible black hole was found near the center of our galaxy, designated HCN-0.009-0.044, with a mass of 32,000 solar masses.

In 2019, scientists observed a gravitational wave event (GW190521) from two smaller black holes merging into one that was 142 times the mass of the Sun. In 2020, a possible intermediate-mass black hole was found in the Aquarius constellation, and in 2021, a very massive black hole candidate was found in the Andromeda Galaxy. Recent studies continue to search for more evidence of these fascinating objects.

Origin

Intermediate-mass black holes are much larger than the black holes formed when a single star collapses. They are too big to come from one star, and they don’t form in the extreme conditions found in the centers of galaxies where supermassive black holes form. Scientists think intermediate-mass black holes might form in three ways: by smaller black holes merging, by many large stars crashing together and collapsing, or by being special black holes created very early in the universe during the Big Bang. Some theories also suggest they could form from extremely massive stars, but this is less likely.