Just 89 million light-years away, in the galaxy NGC 7727, two supermassive black holes are destined to become one.
New measurements examining the heart of the galaxy have shown that the core is composed of a double pair of supermassive black holes. It is the closest such binary object we have found to date, well ahead of the previous record holder at 470 million light years.
What’s more, the two supermassive black holes are closer together than any other supermassive binary black hole we’ve seen, only 1,600 light-years apart. Ultimately, astronomers believe the two black holes will grow so close to each other that they will merge into one much larger beast.
The discovery not only confirms that NGC 7727 is the product of a merger of two galaxies, but also gives us a remarkably tight laboratory to study the interactions between supermassive black holes as they whirl in their orbital dance before merging.
“This is the first time we have discovered two supermassive black holes that are so close to each other that they are less than half the distance from the previous record holder,” says astronomer Karina Foggel of the Strasbourg Observatory in France.
There are several reasons why supermassive binary black holes are of interest to astronomers. Supermassive black holes are found at the centers of most galaxies, the nuclei around which everything else revolves.
When two are found together, it means that two galaxies have come together. We know this process is going on; the discovery of a supermassive binary black hole can tell us what it looks like in its final stages.
Supermassive binary black holes can also tell us something about how these colossal objects – millions or billions of times the mass of the Sun – can become so incredibly massive. Merging binary black holes is one possible pathway for this growth. Finding binary supermassive black holes will help us understand if this is the usual path for such growth, which, in turn, will allow for more accurate simulations.
NGC 7727 has been of interest for a supermassive binary black hole for a while. According to its physical characteristics, the galaxy is a product of a merger of galaxies that occurred sometime in the distant past. However, black holes are difficult to see if they are not actively accumulating material. Then the cloud of matter around the black hole glows brightly with radiation. NGC 7727 did not have the bright stream commonly associated with two active supermassive black holes.
This is where the galaxy’s proximity gave astronomers an edge. Because NGC 7727 is so close, the researchers were able to obtain data on the motion of stars in the center of the Galaxy, based on how their light changes as they rotate. This showed that the galactic center contains not one, but two supermassive black holes.
One of these supermassive black holes is relatively large, about 154 million times the mass of the Sun. Another, much smaller companion has a mass of only 6.3 million solar masses. Only one of these black holes turned out to be active: the smaller one. This explains why there was so little radiation; a much larger black hole is dormant.
According to the researchers, this suggests that there are many more supermassive binary black holes than we can currently detect.
“Our discovery suggests that there may be many more of these merging relics, and they may contain many hidden massive black holes that are still awaiting discovery,” Foggel said.
“This could increase the total number of supermassive black holes known in the local universe by 30 percent.”
Although the two black holes are destined to merge, it will take some time; the researchers said it was about 250 million years ago. But while we may not be able to observe how this happens, the binary system gives hope that there are many more mergers of supermassive black holes happening around us.
We cannot find them yet; the emitted gravitational waves will be too low for our current instruments. But new instruments, such as the space-based gravitational wave detector LISA, should have that capability, the researchers say.
And this discovery could tell us how to find binary supermassive black holes even in more distant galaxies, where we cannot get detailed data about the motion of the central stars.
“Targeting bright surviving nuclear star clusters in united galaxies could facilitate the discovery of shorter-range supermassive binary black holes in the Local Universe, even if they are not luminous active galactic nuclei,” they wrote in their paper.
“This will allow a much more detailed study of these systems, which can then serve as a blueprint for how to find them in a wider perspective in the distant universe.”
Research published in Astronomy and astrophysics…