Just 89 million light-years away, in the galaxy NGC 7727, two supermassive black holes are destined to become one.
New measurements that probe the heart of the galaxy have found that the nucleus consists of a binary supermassive black hole pair. It's the closest such binary we have found to date, beating out the previous record-holder of 470 million light-years by a significant margin.
Moreover, the two supermassive black holes are closer to each other than any other supermassive black hole binary we've seen, separated by a distance of just 1,600 light-years. Eventually, astronomers believe, the two black holes will grow so close together that they'll merge, becoming one much larger beast.
The discovery not only confirms that NGC 7727 is the product of a merger between two galaxies, but gives us a wonderfully close laboratory for probing the interactions between supermassive black holes as they whirl about in their orbital pre-merger dance.
"It is the first time we find two supermassive black holes that are this close to each other, less than half the separation of the previous record holder," says astronomer Karina Voggel of the Strasbourg Observatory in France.
There are multiple reasons why supermassive black hole binaries are of interest to astronomers. Supermassive black holes are found at the centers of most galaxies, the nuclei around which everything else whirls.
When two are found together, it indicates that two galaxies have come together. We know this process occurs; finding a supermassive black hole binary can tell us what it looks like in the final stages.
Supermassive black hole binaries can also tell us something about how these colossal objects – millions to billions of times the mass of the Sun – can get so incredibly massive. Binary black hole mergers are one way this growth can occur. Finding binary supermassive black holes will help us understand if it's a common pathway for this growth, which in turn will allow for more accurate modeling.
NGC 7727 had been an object of interest for a binary supermassive black hole for some time. Its physical characteristics suggest that the galaxy is the product of a galactic merger, sometime in its distant past. However, black holes are hard to see unless they're actively accreting material. Then, the cloud of material around the black hole shines brightly with radiation. NGC 7727 did not have the bright flow usually 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 motion data for stars in the galactic center, based on the way their light changes as they whirl around. This revealed that the galactic center contains not one, but two supermassive black holes.
One of those supermassive black holes is relatively large, clocking in at around 154 million times the mass of the Sun. The other, much smaller, companion is only 6.3 million solar masses. Only one of these black holes was found to be active: the smaller one. This explains why there was so little radiation; the much larger black hole is quiescent.
This suggests, the researchers said, that there could be many more supermassive black hole binaries out there than we can currently detect.
"Our finding implies that there might be many more of these relics of galaxy mergers out there and they may contain many hidden massive black holes that still wait to be found," Voggel said.
"It 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's going to take some time; around 250 million years or so, the researchers said. But, while we won't be able to observe it happening, the binary system offers hope that there are many more supermassive black hole mergers taking place around us.
We can't detect them just yet; the gravitational waves emitted would be too low frequency for our current instruments. But new instruments, such as the space-based gravitational wave detector LISA, should be able to, the researchers said.
And the discovery could tell us how to find binary supermassive black holes, even in more distant galaxies, where we can't obtain detailed motion data for the central stars.
"Targeting bright surviving nuclear star clusters in merged galaxies can facilitate the discovery of dual supermassive black holes at smaller separations in the Local Universe even when they are not luminous active galactic nuclei," they wrote in their paper.
"This will allow much more detailed studies of these systems that then can serve as blueprints of how to find them more broadly in the distant Universe."
The research has been published in Astronomy & Astrophysics.