By | November 15, 2022
Two massive black holes are about to collide.  When they do, the explosion will be incalculable

Stars like our Sun are violent creatures that are constantly spewing forth radiation, gamma rays and all kinds of nasty things (though luckily Earth’s ozone layer (and the atmosphere protects us from the worst.) But when stars die, especially large ones, their wrath becomes even more merciless. Stars at the end of their life cycle that are large enough will collapse in on themselves and form one black hole. These singularities are defined by their gravitational pull, which is so incredibly strong that nothing – not even light – can escape. In other words, what happens in a black hole stays in a black hole.

But black holes are not too limited in size or number. On average, a standard black hole is about three to ten times the size of our Sun. As massive as “ordinary” black holes can get, supermassive black holes – likely formed over billions of years when black holes merge – can reach a size millions or billions that of our nearest star. And the universe could be filled with billions of supermassive black holes. In fact, there is one at the center of our galaxy, the Milky Way, called Sagittarius A*, around which all things in the galaxy revolve.

The two closest supermassive black holes ever recorded are each about 200 million and 125 million times the mass of our Sun.

Supermassive black holes certainly live up to their name, but if two were to collide and mutually envelop each other, it would create one of the most insanely large explosions in the universe – sending out reverberations in the form of gravitational waves which would ripple through the entire universe. Scientists recently announced the discovery of just such a situation: the two closest supermassive black holes on a collision course, at least as far as humans have detected. The discovery also suggests that merging black holes may be more common than previously thought.

Despite their relative frequency and unfathomable size, finding a supermassive black hole is no easy task. They do not generate light, of course; thus, scientists must infer the size and location of black holes using indirect clues, such as how they distort spacetime, their effect on nearby stars, the orbital speed nearby starsand the discovery of huge gravitational waves which occurs when black holes collide. When this happens, two become an even more massive black hole.

To find just these two supermassive black holes, a team of 29 scientists had to crunch a lot of data. They analyzed records from a dozen instruments across seven telescopes scattered around the world and in orbit, including the Hubble Space Telescope, the Keck Observatory in Hawaii and the cluster of 66 radio telescopes in a desert in Chile known as the Atacama Large Millimeter Array. They looked at UGC 4211, a galaxy tucked away in the constellation Cancer.

The gap between them “is pretty close to the limit of what we can detect, which is why this is so exciting.”

No single observation was enough to pinpoint these giant dead stars, but together the data paint a clear picture. At the center of UGC 4211 – which is technically two galaxies that crashed into each other – is an extremely bright clump of matter known as the active galactic nuclei (AGN). Astronomers believe that AGN are caused by supermassive black holes, but as they looked closer and closer to the center of UGC 4211, they found not one black hole, but two. Their research was published this month in Astrophysical Journal Letters.

Both are about the same size, and they appear to be super close together – again, the two closest supermassive black holes ever recorded – and are each about 200 million and 125 million times the mass of our Sun. Thankfully, these monstrosities are nowhere near us, located some 480 million light years from the Milky Way.

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The gap between them “is pretty close to the limit of what we can detect, and that’s why this is so exciting,” Chiara Mingarelli, one of the study authors and an associate researcher at the Flatiron Institute’s Center for Computational Astrophysics in New York City, said in a statement.

“It’s important that with all these different pictures you get the same story – that there are two black holes,” Mingarelli added, comparing this new multi-observation research to previous efforts. “That’s where other studies are [of close-proximity supermassive black holes] has fallen into the past. When people followed them up, it turned out that there was only one black hole. [This time we] have many observations, all agreed.”

Eventually, the two supermassive black holes will collide, but it won’t happen anytime soon. Although these two giant dead stars are closer together than any other supermassive black holes we have discovered, they are still about 750 light years apart. This inevitable collision probably won’t happen for another hundred million years or so.

But when it does, the wave of energy it will blast into space will be incomprehensible. First, the two former stars will orbit ever closer, eventually crashing and sending out gravitational waves larger than anything humans have ever captured before. Currently, the largest black hole merger ever detected by Earth’s gravitational wave observatories resulted in a new black hole with a mass of 142 solar masses (or 142 times the mass of our Sun). Upon merging, eight solar masses were erased from the universe and instantly converted into energy in the form of gravitational waves. When these two supermassive black holes merge, the energy release will be exponentially greater.

This research is about much more than just finding some cool space objects. It can help us better understand the evolution and life cycles of stars and can help astronomers find more black holes in the nearby universe. In turn, this information can help scientists better calibrate gravitational wave detection devices. Black holes can be mysterious and hard to find, but knowing how and where to look just got a little easier.

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