Black holes come in two distinct types – small and supermassive. A group in the middle has long been hypothesized to exist, and now Hubble has found some strong evidence for one of these intermediate-mass black holes in a nearby star cluster.
Stellar mass black holes are the most common ones, with masses from a few to a few dozen times that of the Sun. Then there’s a gigantic gap between them and the next type of black hole – supermassive monsters with the mass of millions or billions of Suns, which lurk at the center of galaxies.
Astronomers have long suspected that there should be medium-sized black holes in the middle of that range. But strangely, evidence of them is lacking. A few candidates have been spotted here and there, but arguably could still be attributed to other objects and phenomena.
In a new study, scientists claim to have found the best evidence yet for these mysterious missing-link black holes. The team examined 12 years’ worth of Hubble and Gaia data on a globular star cluster called Messier 4, located about 6,000 light-years from Earth, making it the closest cluster to us.
The stars in the cluster seem to be swarming around a large mass in the center, and by watching their motions over that long period of time, they can calculate the mass of that central object. This number came to about 800 solar masses, firmly in the range of an intermediate black hole.
Of course, without directly detecting the object the scientists can’t fully confirm that it is an intermediate black hole. But if it isn’t, the astronomers calculated that it would take about 40 stellar-mass black holes, crammed into a space only one-10th of a light-year wide, to create the same effect. That would be an unstable arrangement, causing them to merge or flick each other out of the cluster.
“While we cannot completely affirm that it is a central point of gravity, we can show that it is very small,” said Eduardo Vitral, lead author of the study. “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.”
The research was published in the journal Science.
Source: ESA Hubble