Evidence of elusive ‘intermediate’ black hole found in cosmic neighbourhood by Hubble

While the galaxy is inundated with these “small” black holes, the universe at large is swamped with supermassive black holes that weigh millions or even billions of times the mass of our Sun. These are typically found at the centres of galaxies.

Just like in theories of evolution, there is a missing link there—an intermediate-mass black hole that is expected to weigh roughly between 100 to 100,000 times the mass of the Sun.

Hubble discovered one of the two best candidates for such an intermediate-mass black hole back in 2020, identified as 3XMM J215022.4-055108. Another candidate is HLX-1, which was identified in 2009. Both candidates reside on the outskirts of other galaxies and have masses of tens of thousands of suns. According to ESA, they may once have been at the centre of dwarf galaxies.

In a study documented in a research article published in the Monthly Notices of the Royal Astronomical Society, astronomers detected a possible intermediate-mass black hole that weighs roughly 800 solar masses. The object itself is not directly visible but the researchers calculated its mass by studying the motion of the stars caught in its gravitational field.

To do this, the astronomers looked at 12 years’ worth of Hubble observations of the globular star cluster Messier 4 (M4) and resolved pinpoint stars. They also took advantage of the European Space Agency’s Gaia spacecraft, which contributed the results with scans of over 6,000 stars that help the scientists constrain the shape and mass of the cluster.

“We have good confidence that we have a very tiny region with a lot of concentrated mass. It’s about three times smaller than the densest dark mass that we had found before in other globular clusters,” said Eduardo Vitral, lead author of the research paper, in a press statement.

According to Vitral, this object is very compact and has a very high amount of mass, which means that a black hole being there is the most plausible explanation.

If the object is not a single intermediate-mass black hole, it would have to be an estimated 40 smaller black holes crammed into a space that is only one-tenth of a light-year across. If this is the case, there is a good chance that some of them would merge or would be shot outside the system as if it was catapulted.

“While we cannot completely affirm that it is a central point of gravity, we can show that it is very small. It’s too tiny for us to be able to explain other than it being a single black hole. Alternatively, there might be a stellar mechanism we simply don’t know about, at least within current physics,” added Vitral.