Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of "How to Die in Space." He contributed this article to Space.com's Expert Voices: Op-Ed & Insights.
Medium-sized black holes are some of the most elusive creatures to inhabit the cosmos. Finding them and understanding them will help unravel the mysteries of the growth of supermassive black holes, and the intimate relationship between giant black holes and their host galaxies.
While the medium black holes remain elusive for now, a team of astronomers has devised a strategy for listening to their gravitational wave emissions when they crash into other objects. But we're going to have to wait a while…
Related: Black holes of the universe (images)
The mystery of the middle
Black holes in our universe come in mainly two flavors: relatively small and absolutely gargantuan. The smaller black holes form from the deaths of giant stars, and can have masses anywhere between a handful and a few dozen solar masses (where one "solar mass" is, as you might have guessed, the mass of the sun). There are billions of these black holes wandering the depths of every galaxy, including the Milky Way, and in the past few decades astronomers have managed to observe quite a fair number.
The gargantuan black holes, however, are truly beasts of a different nature. These things start at a staggering millions of solar masses, and can easily climb their way up to hundreds of billions of times the mass of the sun. They're much rarer than their diminutive cousins — each galaxy only hosts one (or if they're lucky/unlucky, two), which lurks in its central core.
And that's pretty much it. What we don't see a lot of are the medium-sized black holes, also known as intermediate mass black holes (also, also known as IMBHs). These are hypothetical black holes thought to weigh in at a few thousand times the mass of the sun. They are the ultimate astrophysical "missing link" — a bridge between the small black holes and the big ones.
And they're very, very hard to find.
Going big — but not too big
Despite years of searching, astronomers have no conclusive evidence for the existence of any IMBHs. Sure, there are hints and signs here and there — an odd orbit in the center of a cluster, a strange light signal — but nothing definitive.
One of the challenges of finding IMBHs is that we're not exactly sure how and where they form. In one scenario, IMBHs serve as a bridge between small and big black holes. It could be that all black holes start out small (well, smallish — they are still many times more massive than the sun), and over the eons they merge and feed, with a lucky few bulking up to supermassive proportions. According to this model, the IMBHs are simply a stepping stone on the road to greatness, an intermediate step in the normal evolution of giant black holes.
But it could be possible for IMBHs to have their own formation mechanisms, separate from both their smaller and bigger cousins. Perhaps in the early universe giant stars formed (stars far, far larger than any we see today) and ultimately collapsed, leading directly to black holes of thousands of solar masses.
No matter the case, astronomers think that IMBHs probably hang out in globular clusters. Globular clusters are balls of old, dying stars that orbit a galactic center, like a decaying town on the outskirts of a major city. While we don't fully understand the origins of globular clusters, it's thought that they might be the remnant cores of dead galaxies, stripped of their star-forming abilities through countless interactions with the larger galaxies.
That's why globular clusters may be the ideal place to build IMBHs: either the medium black holes directly formed here but never got the chance to merge with a supermassive black hole, or smaller black holes began merging but were stopped short due to the limited supply of food in the clusters.
This all sounds great, but right now it's hypothetical. Globular clusters are dim and far away, and searching their centers for signs of medium black holes is an incredibly hard task. So astronomers are working hard to come up with ways to detect IMBHs, and recently a team has put together a proposal involving the next generation of gravitational wave detectors.
Just like their supermassive brethren, IMBHs can occasionally swallow other galactic denizens, up to and including smaller black holes. This is relatively rare, however. Orbiting black holes are frustratingly stable — they can just dance in circles around each other for billions of years. In order to make two black holes merge, there has to be a third black hole in the system, destabilizing their orbits and triggering the beginning of the merger event.
The team of astronomers estimated how often this scenario could happen inside globular clusters, plugging in known and estimated values of star populations, small black hole populations and the theoretical existence of IMBHs. From there, they calculated the emission of gravitational waves from those events, and how frequently those gravitational waves would wash over the Earth.
The bad news: current gravitational wave detectors, like the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo, don't have the sensitivity to detect these gravitational waves.
The good news: new detectors are right around the corner.
The astronomers estimate that the Laser Interferometer Space Antenna, a planned space-based detector, could spot a few dozen IMBH mergers every year, assuming that the black holes have a mass of a few thousand times that of the sun. It might even be possible for LIGO to detect one or two a year, but only if the IMBHs are around 100 solar masses.
The more we understand IMBHs, the more we learn about the growth and evolution of black holes, their relationships to their host galaxies, and the role they play in the history of the cosmos. We just need better ears.
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