But the scarcity of intermediate-mass black holes poses a quandary: Is there some black hole growth mechanism that is stranger than we can possibly imagine? Or are current observatories simply not sensitive to the emissions from these middleweight objects?
ANALYSIS: 'Seeds' of Supermassive Black Holes Discovered
"Exactly how intermediate-sized black holes would form remains an open issue," said Dominic Walton of the California Institute of Technology (Caltech), Pasadena. "Some theories suggest they could form in rich, dense clusters of stars through repeated mergers, but there are a lot of questions left to be answered."
In an effort to get to know the nature of intermediate mass black holes, a collaboration of international observatories "went to town" on two ultraluminous X-ray sources (or ULXs) that were thought to contain black holes in the 100 to 10,000 solar mass range.
ULXs are likely composed of a star and a nearby black hole. The black hole does what it does best, sucking material from the unfortunate binary partner, generating radiation in the process. These compact sources of X-rays have led astronomers to believe that the feeding black holes in ULXs fall into the intermediate-mass category.