The universe is swimming in black holes, from stellar mass to supermassive behemoths. But, there’s one class that remains elusive: the “middle child” class. These are called “intermediate-mass black holes (IMBH).” How numerous are they, how do they form, and where are they? To answer those questions, astronomers simulated possible formation scenarios.
Intermediate-mass black holes lie somewhere in the mass range between stellar mass and supermassive. They should be between 100 and 100,000 solar masses. For comparison, stellar-mass black holes range up to several tens of solar masses. Supermassive black holes found at the centers of galaxies can be monsters up to a billion times the mass of the Sun.
If intermediate-mass black holes exist, do they imply a hierarchical model of black hole formation? That is, do little ones (formed by the collapse of supermassive stars) merge to form bigger ones? If so, these IMBH would be a stepping stone between stellar mass black holes and the supermassive monsters. If that idea holds up, then do the IMBH collide to eventually form the seeds of supermassive black holes? Astronomers need more observational data about IMBH to answer all these questions.
Stellar-mass black holes are seen everywhere. They form when supermassive stars collapse. The supermassive black holes seen at the hearts of galaxies most likely form through the accretion of matter, as well as by mergers with other black holes. The existence of intermediate-mass black holes seems to be a no-brainer, but they’re a challenge to observe. And, that’s why astronomers term them “elusive”.
It’s not that there aren’t any out there. Observers found candidate IMBH in our own Milky Way. They also seem to be in dim active galactic nuclei that appear to have accreting black holes. In addition, some ultra-luminous X-ray sources (ULXs) could also have these “medium-sized” black holes. However, those require further observation. The Sloan Digital Sky Survey has also uncovered a handful of possible IMBH candidates that are bright in X-rays. That’s important since X-ray emissions are one hallmark of activity around a black hole. One of the more interesting detections involved gravitational waves given off by the collision of two massive stellar black holes. The result was a black hole with a mass of about 150 solar masses—right in the range to be an IMBH. Others have been found, too, resulting in yet another avenue of study.
So, we know they’re out there. The question remains, where and how do these IMBH form? It’s not an easy one to answer. An international team led by Arca Sedda of the Gran Sasso Science Institute (Italy) set out to simulate the possible formation mechanism for these objects. “Intermediate-mass black holes are difficult to observe”, he explained. “The current observational limits do not allow us to say anything about the population of intermediate-mass black holes with masses between 1,000 and 10,000 solar masses, and they also represent a headache for scientists regarding the possible mechanisms that lead to their formation”.
So, Sedda and his team looked at stellar clusters as a possible birthplace of IMBH. “We have carried out new computer models that can simulate the formation of these mysterious objects,” said Sedda, referring to the DRAGON-II simulation database. This is a collection of 19 computer models that represent dense clusters with up to a million stars each. Using these in further simulations, the team found that IMBHs can form in star clusters. It happens through an intricate combination of three factors: mergers between stars much larger than our Sun, accretion of stellar material onto stellar black holes, and, finally, mergers between stellar black holes.
Globular cluster Mayall II is considered a possible candidate for hosting an intermediate-mass black hole. Courtesy STScI.
“The latter is a process that results in the possibility to “see” these phenomena through the detection of gravitational waves,” Sedda explained. The team also came up with hypotheticals about what happens after intermediate black holes are born. It appears that they get thrown out of their own clusters through complex gravitational interactions. It’s also possible they experience something called “relativistic recoil” (think of it getting a “kick” out of its birthplace). That keeps them from gaining more mass. “Our models show that although IMBH seeds form naturally from energetic stellar interactions in star clusters, they are unlikely to become heavier than a few hundred solar masses unless the parent cluster is extremely dense or massive”, said Sedda.
Figuring out an origin story for these black holes still doesn’t answer the question of whether they’re a missing link between stellar and supermassive black holes. Sedda points out that the processes that keep them to a certain mass range probably don’t make it easy for such objects to get even bigger. But a lot remains unknown.
“We need two ingredients for a better clarification: one or more processes capable of forming black holes within the mass range of IMBHs, and the possibility of retaining such IMBHs in the host environment,” he explained. “Our study places stringent constraints on the first ingredient, giving us a clear overview of which processes may contribute to the formation of IMBHs. Considering more massive clusters containing more binaries (systems composed of two stars orbiting each other) in the future could be the key to obtaining the second ingredient as well. But this will require enormous efforts from a technological and computational point of view”.
The Cradle of Black Holes
The DRAGON-II simulations – II. Formation mechanisms, mass, and spin of intermediate-mass black holes in star clusters with up to 1 million stars