Even if you don’t consider yourself forgetful, everyone has misplaced their keys, phone, or wallet at some point and not thought any less of themselves for it. You’re going to feel more than a little sheepish, however, if the object you cannot currently locate is one of the biggest objects in the universe, say, a supermassive black hole.
Abell 2261 is a giant of a galaxy cluster, and at its center should be a supermassive black hole – one of the largest in the universe. However, astronomers searching for it have found no evidence of it.
Almost every large galaxy in the universe has a supermassive black hole at its center. Galaxy clusters are made up of hundreds of thousands of galaxies bound together by gravity, with one extremely bright galaxy near their center, called the brightest cluster galaxy (BCG).
Abell 2261, located 2.7 billion light-years from Earth, is a BCG. Even as BCGs go, it’s big. It’s about 1 million light-years across – around 10 times that of the Milky Way – and has the largest galactic core we know of. Because the size of supermassive black holes is related to the mass of a galaxy, astronomers can estimate how big a black hole likely is, and they had expected a giant at its center.
According to NASA, astronomers expected Abell 2261 to contain a black hole that weighed between 3 billion and 100 billion solar masses, rivaling some of the largest ever discovered. To not find one is, to put it lightly, a mystery.
Researchers had already searched through data from the Chandra X-ray Observatory from 1999 and 2004, looking for the radiation that is evidence of superheated material surrounding a black hole, but found none. Now with new Chandra data from 2018, astronomers led by Kayhan Gultekin from the University of Michigan conducted a new search, and have successfully deepened the mystery.
In a paper accepted for publication in the journal American Astronomical Society, Gultekin and his team explored the idea that the black hole was actually ejected from the galaxy's center. This violent event could have been the result of two galaxies merging, with the black hole at the center of each merging to form one large one.
When black holes merge, they produce gravitational waves; disturbances or ripples in spacetime. If the gravitational waves detected were stronger in one direction, it's thought the new huge black hole gets sent hurtling away from the galactic center in the opposite direction, which is known as a recoiling black hole. This is the theory at least, no evidence has been found of a recoiling black hole, and it's not known if supermassive black holes even get close enough to merge.
However, there are two signs that a merger between supermassive black holes may have taken place here. The first is that Abell 2261 has a galactic core much larger than expected for a galaxy of its size. The second is that the densest concentration of stars is over 2,000 light-years away from the galactic center. Researchers used Hubble data to see if they could locate clumps of stars that could have been moved by a recoiling black hole, but found none. They then turned to the more recent Chandra data to try and detect radiation associated with a black hole, and also found none.
This doesn't mean there isn't a black hole there. We can usually only detect them when they're feeding, as they give off no radiation of their own, so it's possible it's eating material too slowly for us to be able to pick up with our current instruments. If the James Webb Space Telescope meets its launch deadline of October 2021, it may be able to detect the mysterious object. If the JWST can't find it, then it's likely the supermassive black hole got kicked out even further than thought. Perhaps next year, this mystery will be solved.