17th December 2020
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The mystery of the whereabouts of a supermassive black hole has deepened.
Despite searching with the Chandra X-ray Observatory and NASA's Hubble Space Telescope, astronomers have no evidence that a distant black hole, estimated to weigh between 3 and 100 billion times the mass of the Sun, is anywhere to be found.
This missing black hole should be in the vast galaxy at the center of the Abell 2261 galaxy cluster, which is about 2.7 billion light years from Earth. This composite image of Abell 2261 includes optical data from Hubble and the Subaru telescope showing galaxies in the cluster and in the background, as well as Chandra X-ray data showing hot gas (colored pink) pervading the cluster. The center of the image shows the large elliptical galaxy in the center of the cluster.
Almost every large galaxy in the universe contains a supermassive black hole at its center, with a mass that is a million or billion times that of the sun. Since the mass of a central black hole is usually the same as the mass of the galaxy itself, astronomers expect the galaxy at the center of Abell 2261 to contain a supermassive black hole that can rival the weight of some of the largest known black holes in the universe.
Using Chandra data from 1999 and 2004, astronomers had already searched the center of the large central galaxy of Abell 2261 for signs of a supermassive black hole. They looked for material that was overheated when it fell towards the black hole and generated x-rays, but failed to detect such a source.
With new, longer Chandra observations from 2018, a team led by Kayhan Gultekin of the University of Michigan at Ann Arbor conducted a deeper search for the black hole at the center of the galaxy. They also considered an alternate explanation in which the black hole was ejected from the center of the host galaxy. This violent event may be due to two galaxies merging into the observed galaxy, accompanied by the central black hole in each galaxy merging into one giant black hole.
When black holes merge, they create waves in space-time called gravitational waves. If the large amount of gravitational waves generated by such an event were stronger in one direction than another, the theory predicts that the new, even more massive black hole would have been sent away from the center of the galaxy in the opposite direction. This is known as a repulsive black hole.
Astronomers have not found definitive evidence of black hole recoil, and it is not known whether supermassive black holes are even close enough to create gravitational waves and merge. So far, astronomers have only verified the fusions of much smaller black holes. The detection of repulsive supermassive black holes would encourage scientists to use and develop observatories to look for gravitational waves that result from the merging of supermassive black holes.
The galaxy at the center of Abell 2261 is an excellent cluster for finding a repulsive black hole, as there are two indirect signs that a fusion between two massive black holes may have occurred. First, data from Hubble and Subaru's optical observations reveal a galactic core – the central area where the number of stars in the galaxy in a given area of the galaxy is at or near the maximum value – that is much larger than expected for a galaxy its size. The second sign is that the densest concentration of stars in the galaxy is more than 2,000 light years from the center of the galaxy, which is remarkably far away.
These features were first identified by Marc Postman of the Space Telescope Science Institute (STScI) and coworkers in their previous Hubble and Subaru images, and led them to propose the idea of a fused black hole in Abell 2261. During a merger, the supermassive black hole formed in each galaxy sinks towards the center of the newly merged galaxy. When they are bound together by gravity and their orbits begin to shrink, the black holes are expected to interact with the surrounding stars and eject them from the center of the galaxy. This would explain the great gist of Abell 2261. The off-center concentration of stars may also have been caused by a violent event, such as the merging of two supermassive black holes and the subsequent recoil of a single larger black hole.
Although there is evidence that black hole fusion occurred, neither Chandra nor Hubble data showed evidence of the black hole itself. Gultekin and most of his co-authors, led by Sarah Burke-Spolaor of West Virginia University , had previously used Hubble to search for a star cluster that might have been carried away by a recoiling black hole. They examined three clumps near the center of the galaxy and examined whether the motions of the stars in these clumps are high enough to indicate that they contain a black hole with a mass of ten billion solar masses. No clear evidence of a black hole was found in two of the clumps, and the stars in the other were too faint to draw useful conclusions.
Previously, they also examined observations made by Abell 2261 using the NSF's Karl G. Jansky Very Large Array. Radio emissions detected near the center of the galaxy indicated that there was supermassive black hole activity 50 million years ago, but does not indicate that the center of the galaxy currently contains such a black hole.
Then they turned to Chandra to look for material that was overheated and was generating x-rays as it hit the black hole. While the Chandra data showed that the densest hot gas was not in the center of the galaxy, they did not reveal any possible X-ray signatures of a growing supermassive black hole – no X-ray source was found in the center of the cluster, or in one of the star clusters, or at the location of the radio emission.
The authors concluded that there is no black hole in any of these locations or that the material is being drawn in too slowly to produce a detectable X-ray signal.
The mystery of the location of this gigantic black hole therefore continues. Although the search was unsuccessful, hope remains for astronomers searching for this supermassive black hole in the future. Once launched, the James Webb Space Telescope may be able to detect the presence of a supermassive black hole in the center of the galaxy or one of the star clusters. If Webb can't find the black hole, the best explanation is that the black hole has retreated well from the center of the galaxy.
An article describing these results has been accepted for publication in a journal of the American Astronomical Society and is also available online at https://arxiv.org/abs/2010.13980. Gultekin's co-authors are Sarah Burke-Spolaor; Tod R. Lauer (National Research Laboratory for Infrared Optical Astronomy, Tucson, Arizona); T. Joseph W. Lazio and Leonidas A. Moustakas (Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California); and Patrick Ogle and Marc Postman (Space Telescope Science Institute, Baltimore, Maryland).
NASA's Marshall Space Flight Center administers the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science from Cambridge, Massachusetts and flight operations from Burlington, Massachusetts.
Photo credit: Roentgen: NASA / CXC / Univ of Michigan / K. Gültekin; Optical: NASA / STScI and NAOJ / Subaru; Infrared: NSF / NOAO / KPNO
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