A blazing galaxy half a billion light-years away could offer us a real-time, front-row seat to a supermassive black hole collision on human timescales, possibly within a mere century.
A new analysis of the peculiar light from blazar galaxy Mrk 501 suggests the presence of not one, but two supermassive black holes, each driving its own high-speed jet of matter. It's not a conclusive detection, but according to a study led by astronomer Silke Britzen of the Max Planck Institute for Radio Astronomy in Germany, it's currently the most compelling explanation for the galaxy's strange behavior.
If confirmed, it could mean that one of the white whales of cosmology is right on our doorstep: The first observation of a merger between supermassive black holes millions to billions of times the mass of the Sun.
"So far, no double jet system in a blazar core has been detected via direct imaging," Britzen and colleagues write. "Thus, the present work reports the first detection of a double jet system, leading to the inference of a binary of supermassive black holes within the core of this blazar."
Supermassive black holes are thought to lurk at the heart of every major galaxy, the cosmic core around which the rest of the galaxy revolves. These behemoths can reach tremendous masses, and they present several perplexing problems.
One of the most pressing is how in the heck they get that big. Stellar-mass black holes – those that are tens of solar masses – form from the collapsed cores of dying massive stars. We know these can come together to form larger ones, with the largest known tipping the scales at around 225 solar masses.
The formation and evolution pathways to black holes with masses millions of times greater are more mysterious. Part of that is because we lack the tools to detect gravitational waves from a single supermassive black hole merger, which would be the best tool for understanding how they grow via mergers.
But supermassive black holes aren't as secretive as their much smaller relatives. These colossi often devour vast amounts of material that swirls around them in a disk, heating up as it does so and blazing with light.
Then, some of the material falling onto the black hole gets diverted along magnetic field lines outside the event horizon. This material is accelerated toward the black hole's poles, where it launches into space with tremendous force as a jet of plasma, glowing in radio light. Both blazing disk and high-speed jets can be detected by our telescopes – the tell-tale signature of a rapacious supermassive black hole.
We know that galaxies collide to become bigger – there are many examples of ongoing galactic collisions throughout the Universe. And the supermassive black holes in their centers are drawn together. There are several examples of post-merger galaxies with two or more supermassive black holes at the core, locked in a spiraling orbit that is expected to eventually bring them together.
Mrk 501, located around 464 million light-years away, is a galaxy that astronomers have suspected of harboring a binary supermassive black hole. However, Mrk 501 is a blazar, a galaxy with an active supermassive black hole with a relativistic jet pointed almost directly at Earth. It's tremendously bright across the electromagnetic spectrum, making detailed analyses of its core somewhat tricky.
Britzen and her colleagues turned to ultra-high-resolution radio telescopes to track changes in the center of Mrk 501 across multiple radio wavelengths. Their observations spanned around 23 years, allowing them to track bright features in the jet over time.
The researchers used these changes to reconstruct how material is moving around close to the galaxy's central engine – and that's when something strange emerged. The pattern seemed to suggest a second, fainter jet that appears to loop counterclockwise around the radio core.
"Evaluating the data felt like being on a ship," Britzen says. "The entire jet system is in motion. A system of two black holes can explain this: The orbital plane sways."
The team then modeled the motion they observed and concluded that the behavior is more easily explained by the presence of a second supermassive black hole. They found two periods in the fluctuating light. One was seven years, and the researchers found it consistent with a wobble in the jet system, like a teetering spinning top.
The other was around just 121 days; that, they said, could be consistent with the orbital period of the two black holes, separated by a distance of 250 to 540 times the distance between Earth and the Sun. For objects as tremendously large as supermassive black holes, that's extremely close.
It's also a fraction of a fraction of a parsec, which is about 3.2 light-years. This is interesting because of something called the final parsec problem.
According to models, as supermassive black holes orbit each other, they transfer their orbital energy to the stars and gas around them, causing their mutual orbit to become smaller and smaller. As the distance between them shrinks, the amount of stuff that can steal their momentum shrinks too.
By the time they're about one parsec apart, their galactic neighborhood can no longer support further orbital decay, so the orbit of the black holes may stall for what could be a very long period of time – longer than the current lifespan of the Universe.
If Mrk 501 is indeed host to a binary supermassive black hole, the pair's orbital separation is at most just 0.0026 parsecs – suggesting that such binaries can find a way to close a gap physics tells us is very difficult to surmount.
Supermassive black holes, man. Wild stuff.
Anyway, because this as-yet unconfirmed binary would be so close together, the time before they collide could be very short indeed – less than 100 years, the researchers say. So MRK 501 is worth keeping an eye on, especially with pulsar timing arrays that could detect the low-frequency gravitational waves they emit.
Related: Astronomers May Have Seen Colliding Black Holes Trigger a Blaze of Light
"If gravitational waves are detected," says astronomer Héctor Olivares of Radboud University in the Netherlands, "we may even see their frequency steadily rise as the two giants spiral toward collision, offering a rare chance to watch a supermassive black hole merger unfold."
The paper has been accepted for publication in the Monthly Notices of the Royal Astronomical Society.