The universe's rarest black holes may be 'wandering' through our galaxy — but scientists don't know how to detect them

This illustration shows a glowing stream of material from a star as it is being devoured by a supermassive black hole in a tidal disruption flare. — An illustration of a black hole tearing a star to shreds. (Image credit: NASA/JPL-Caltech)

The Milky Way has millions of small black holes and one giant supermassive black hole at its center. But does the galaxy have any medium-sized black holes? New research suggests the answer is yes: Perhaps a dozen may inhabit the Milky Way, but they are wandering freely through space and are fiendishly difficult to detect.

For decades, researchers have wondered about the prevalence of intermediate-mass black holes (IMBHs). Certainly, every galaxy is capable of producing an enormous number — roughly a handful every century — of small black holes with masses of up to 100 or so times that of the sun. And it appears that when galaxies like the Milky Way first arrived on the cosmic scene, they already had companion supermassive black holes in their hearts. Our own supermassive black hole, Sagittarius A*, has a mass of 4.5 million suns.

But what about the IMBHs? Theoretically, they should have masses of 10,000 to 100,000 solar masses. Finding IMBHs — or disproving their existence — has enormous implications for our understanding of black hole growth and evolution. But so far, there have been only faint, sketchy hints of IMBHs residing in dwarf galaxies, and no direct evidence that they live in a galaxy like the Milky Way.

In April, a team of researchers at the University of Zurich in Switzerland explored whether our current simulations of the universe could conclusively predict if the Milky Way hosts a population of IMBHs. Their paper has been accepted for publication in the journal Monthly Notices of the Royal Astronomical Society.

Cannibal galaxies

Galaxies do not grow up alone. Instead, they develop through the cannibalization of their neighbors, by incorporating their stars — and any black holes — within their volumes. The Milky Way has consumed over a dozen dwarf galaxies, and probably many more, in its long history. Presumably, some of those dwarf galaxies held IMBHs. But the common assumption was that large black holes tend to slink down the centers of their host galaxies, where they go on to merge with the central supermassive black hole.

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Through their models, the researchers saw a different story unfold. They used a simulation of the evolution of a Milky Way-like galaxy and found that it can contain somewhere between five and 18 "wandering" IMBHs, which are not located near the central core but are left to roam within the disk of the galaxy. The exact number of IMBHs depends on whether they are born near the core of a soon-to-be-consumed dwarf galaxy or in its outskirts.

Although the researchers were heartened to find that the Milky Way should host a population of IMBHs, they urged caution in interpreting their results. They could not conclusively state what masses these black holes should have or where they would ultimately reside. So, while the new research strongly hints that IMBHs are out there, we do not yet know where to look.

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Paul M. Sutter is a research professor in astrophysics at SUNY Stony Brook University and the Flatiron Institute in New York City. He regularly appears on TV and podcasts, including "Ask a Spaceman." He is the author of two books, "Your Place in the Universe" and "How to Die in Space," and is a regular contributor to Space.com, Live Science, and more. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy.

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