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Tour: On the Hunt for a Missing Giant Black Hole

The mystery surrounding the whereabouts of a supermassive black hole has deepened.

Despite searching with NASA's Chandra X-ray Observatory and Hubble Space Telescope, astronomers have no evidence that a distant black hole estimated to weigh between 3 billion and one hundred billion times the mass of the Sun is anywhere to be found. This missing black hole should be in the enormous galaxy in the center of the galaxy cluster Abell 2261, which is located about 2.7 billion light years from Earth.

Nearly all large galaxies contain a supermassive black hole, with a mass that is millions or billions of times that of the sun in their centers. Since the mass of a central black hole usually tracks with the mass of the galaxy itself, astronomers expect the galaxy in the center of Abell 2261 contains a supermassive black hole that rivals the heft of some of the largest known black holes in the Universe.

Such black holes are usually found in the centers of galaxies. Using Chandra data obtained in 1999 and 2004, astronomers had already searched the center of Abell 2261's large central galaxy for signs of a supermassive black hole. They looked for material that has been superheated as it fell towards the black hole and produced X-rays, but did not detect such a source.

In 2018, a team of scientists used Chandra to get new, longer observations obtained in 2018 of Abell 2261. They also considered an alternative explanation: what if the black hole was ejected from the host galaxy's center? During a merger of two galaxies, which likely happened in the past to form Abell 2261, the central black holes in each galaxy could have merged to form one enormous black hole. This violent event would have also generated a huge amount of gravitational waves. If the gravitational waves were stronger in one direction than another, theory predicts that the new, even more massive black hole would have been sent careening away from the center of the galaxy in the opposite direction. This is called a recoiling black hole.

Astronomers have not found definitive evidence for recoiling black holes and they do not know whether supermassive black holes even get close enough to each other to produce gravitational waves and merge. The detection of recoiling supermassive black holes would embolden scientists using and developing observatories to detect gravitational waves from merging supermassive black holes.

The mystery of this titanic-sized black hole in Abell 2261 therefore continues. Although this latest search was unsuccessful, hope remains for astronomers looking for this supermassive black hole in the future. Once the James Webb Space Telescope launches, astronomers should be able to use its capabilities to join with Chandra's and others to look at Abell 2261 and others like it.

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