Scientists believe that a supermassive black hole lurks at the centre of virtually every galaxy in the universe, with a mass that is millions or billions of times that of the Sun and whose immense force of gravity is responsible for holding all the stars together. However, the heart of the Abell 2261 galaxy cluster, located about 2.7 billion light-years from Earth, appears to break the theory. There, the rules of astrophysics indicate that there should be a huge monster of between 3,000 and 100,000 million solar masses, comparable to the weight of some of the largest known. However, as much as researchers search incessantly, there is no way to find it. The latest observations with NASA’s Chandra X-ray Observatory and the Hubble Space Telescope only delve into the mystery.
Using Chandra data obtained in 1999 and 2004, astronomers had already searched the centre of Abell for 2,261 signs of a supermassive black hole. They were hunting for material that had become overheated as it fell into the black hole and produced X-rays, but they did not detect such a source.
Expelled after a merger
Now, with new and longer observations of Chandra obtained in 2018, a team led by Kayhan Gultekin of the University of Michigan conducted a deeper search for the black hole at the centre of the galaxy. They also considered an alternative explanation, in which the black hole was ejected after the merger of two galaxies, each with its own hole, to form the observed galaxy.
When black holes merge, they produce waves in space-time called gravitational waves. If the large number 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 at full speed from the centre of the galaxy in the opposite direction. This is called a receding black hole.
Astronomers have found no definitive evidence of black hole recoil, and it is not known whether supermassives get close enough to each other to produce gravitational waves and merge. So far, they have only verified the meltdowns of much smaller objects. Finding a larger receding one would encourage scientists to search for gravitational waves from merging supermassive black holes.
Scientists believe this could have occurred in the centre of Abell 2261 by two indirect signs. First, data from optical observations from Hubble and the Subaru telescope reveal a galactic core, the central region where the number of stars in the galaxy has a maximum value, much larger than expected, for a galaxy of its size. The second sign is that the densest concentration of stars in the galaxy is more than 2,000 light-years from the centre, surprisingly distant.
During a merger, the supermassive black hole in each galaxy sinks toward the centre of the newly merged galaxy. If they are held together by gravity and their orbit begins to shrink, black holes are expected to interact with surrounding stars and expel them from the centre of the galaxy. This would explain the large core of Abell 2261.
The off-centre concentration of stars may also have been caused by a violent event such as the merger of two supermassive black holes and the subsequent recoil of a single, larger black hole.
No trace in the stars
Although there are indications that a black hole merger occurred, neither the Chandra nor the Hubble data showed evidence of the black hole itself. The researchers had previously used Hubble to search for a group of stars that could have been swept away by a receding black hole. They studied three clusters near the centre of the galaxy and examined whether the motions of the stars in these clusters are high enough to suggest they contain a 10 billion solar mass black hole. No clear evidence was found for a black hole in two of the groups and the stars in the other were too faint to produce useful conclusions.
They also previously studied observations of Abell 2261 with NSF’s Karl G. Jansky Very Large Array. Radio emission detected near the centre of the galaxy suggested that the activity of a supermassive black hole had occurred there 50 million years ago, but that does not indicate that the centre of the galaxy currently contains such a black hole.
They then headed to Chandra to search for material that had overheated and produced X-rays as it fell into the black hole. While the data revealed that the densest hot gas was not in the centre of the galaxy, it was not shown either in the centre of the cluster or in any of the star clusters. The authors concluded that either there is no black hole at any of these locations, or that it is attracting material too slowly to produce a detectable X-ray signal.
The mystery of the location of this gigantic black hole continues. Although the search was unsuccessful, astronomers hope that the James Webb Space Telescope can reveal its presence. If Webb can’t find it, then the best explanation is that the black hole has moved far enough away from the centre of the galaxy.