There's small black holes and supermassive black holes, but where are all the "inbetweener" black holes?

This question has been foxing astrophysicists for years; the apparent dearth of medium-sized or "intimediate" black holes -- between 100 to 1 million solar masses -- doesn't make logical sense. But when it comes to black holes, you can often check logic at the door.

ANALYSIS: Intermediate Black Hole Implicated in Star's Death

One would assume that to make a supermassive black hole, there must be some growth mechanism that causes small black holes, say around 100 solar masses, to pack on the pounds and grow to the gravitational behemoths that occupy the centers of most known galaxies.

Black holes at the lower end of the mass spectrum are stellar-mass black holes and, as their name suggests, they were formed by the collapse of massive stars and the result of supernoavae. The most massive black holes that are found in the cores of galaxies -- often reaching tens of millions to billions of solar masses -- are less well understood and astronomers are currently trying to understand how they grew to be so massive.

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.

NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) has joined Europe's XMM-Newton satellite in an effort to study a recently-identified ULX in the Circinus spiral galaxy some 13 million light-years distant (pictured top). Combining these X-ray observations with archival data from NASA's Chandra, Swift and Spitzer space telescopes plus the Japanese Suzaku satellite, this has become one of the most intensely-scrutinized ULXs ever.

ANALYSIS: The Mystery 'Middle Child' of the Black Hole Family

In a paper published in the Astrophysical Journal, this collaboration deduced that the Circinus ULX is around 100 solar masses -- but it may not be an intermediate-mass black hole at all. It could actually just be a large stellar-mass black hole that has an exotic "feeding" mechanism that generates intense X-ray emissions.

In another study also accepted for publication in the Astrophysical Journal, two ULXs in NGC 1313, a spiral galaxy 13 million light-years away, were examined. Those too, after being studied by NuSTAR, appear to also be large stellar-mass black holes and not the much sought-after intermediate-mass black holes. So what's going on?

"It's possible that these objects are ultraluminous because they are accreting material at a high rate and not because of their size," said Matteo Bachetti of the Institut de Recherche en Astrophysique et Planétologie. "If intermediate-mass black holes are out there, they are doing a good job of hiding from us."

Despite the hope that ULXs may provide some answers to the apparent lack of medium black holes in the Universe, so far the evidence is suggesting otherwise.