Are There Galaxies Governed by Two Black Holes?


It’s common knowledge that most galaxies host a black hole at their center. We’ve known that for some years now. However, researchers are always looking to push the boundaries of the weird and wonderful ever further. And at this moment in time, an international team of astronomers are trying to identify periodic gamma-ray emissions, in the hope that they will point the way toward some of the more “unconventional” galaxies that might harbor two supermassive black holes at their centers.

Is that being greedy?

Well, maybe not.

On 19 June 2020, The Astrophysical Journal produced a paper from the aforementioned team, entitled, “Systematic search for gamma-ray periodicity in active galactic nuclei detected by the Fermi Large Area Telescope.”

“In general, supermassive black holes are characterized by masses of more than a million masses of that of our sun,” said Pablo Peñil, lead author of the study and a Ph.D. student at Universidad Complutense de Madrid in Spain. “Some of these supermassive black holes – known as active galactic nuclei (AGN) – have been found to accelerate particles to near the speed of light in collimated beams called jets. The emission from these jets is detected throughout the entire electromagnetic spectrum, but most of their energy is released in the form of gamma rays.”

Remember, gamma rays are one of the most extreme forms of light we know of, and can only be detected by specialized equipment, like the Large Area Telescope onboard NASA’s Fermi Gamma-ray Space Telescope. And the reason they can do that is because AGN are characterized by abrupt and unpredictable variations in brightness.

“Identifying regular patterns in their gamma-ray emission is like looking at the stormy sea and searching for the tiny regular set of waves caused by, say, the passage of a small boat,” Peñil said. “It becomes very challenging very quickly.”

Challenging it may be, but the team has already accomplished the first step of identifying those galaxies that emit periodic AGN bursts, and are now trying to solve the riddle of what’s causing them. And the answer could be remarkable.

“The next step will be the preparation of observational campaigns with other telescopes to closely follow up on these galaxies and hopefully unravel the reasons behind these compelling observations,” said co-author Marco Ajello, an associate professor in the College of Science’s department of physics and astronomy at Clemson University. “We have a few possibilities in mind—from lighthouse effects produced by the jets to modulations in the flow of matter to the black hole—but one very interesting solution would be that periodicity is produced by a pair of supermassive black holes rotating around each other. Understanding the relation of these black holes with their environment will be essential for a complete picture of galaxy formation.”

That’s harder than it sounds. Regardless, the team has been able to identify quite a few gamma-ray signals cycles over the last few years.

“Our study represents the most complete work to date on the search for periodicity in gamma rays, a study that will be instrumental in deriving insights about the origin of this peculiar behavior,” said co-author Alberto Domínguez, Peñil’s Ph.D. supervisor in Madrid and also a former postdoctoral researcher in Ajello’s group at Clemson. “We have used nine years of continuous LAT all-sky observations. Among the more than two thousand AGN analyzed, only about a dozen stand out for this intriguing cyclical emission.”

While a dozen doesn’t sound a lot, it’s a remarkable achievement, as it will constitute an important leap forward in our understanding of the underlying physical processes in these special galaxies.

“Previously only two blazars were known to show periodic changes in their gamma-ray brightness. Thanks to our study, we can confidently say that this behavior is present in 11 other sources,” said co-author Sara Buson, a professor at University of Würzburg in Germany. “In addition, our study found 13 other galaxies with hints of cyclical emission. But to confidently confirm this, we need to wait for Fermi-LAT to collect even more data.”

See what I mean about pushing the boundaries of the weird and wonderful?

It was astounding enough when we eventually realized that the most destructive force in known creation is responsible for the stability of each galaxy. But to think there might be places out there that host two such entities? Wow! And do you know what? I have a feeling we’re only just dipping our collective toes in a very, very big pond.


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