Astronomers studying the heart of the giant galaxy cluster Abell 2261 expected to find a supermassive black hole with an appetite for matter and energy that would match the enormous scale of its cosmic home. But although the supermassive black hole should have had a mass between 3 and 100 billion times that of the sun – very visible to most observations – no object was found. NASA scientists do not know what to do with the missing black hole and its evasive nature may be another reminder that we still know very little about these mysterious objects.
Just as stars cluster around the core of a galaxy, so do galaxies cluster, related to the gravity of an extremely dense nucleus. Most known galaxies, including the Milky Way, are based on a supermassive black hole that holds them together. Similarly, clusters of hundreds or thousands of galaxies have a core, with an even larger supermassive black hole that delimits all those galaxies in place. The galaxy closest to this core is known as the brightest galaxy in the cluster (BCG) – this is where the most important supermassive black hole in the cluster should be located.
Abell 2261’s BCG has an astonishment of a million light-years, which makes it about 10 times larger than the Milky Way. Moreover, it has a huge core 10,000 light-years away, the largest galactic core ever found.
But defying all expectations, a team of astronomers led by Kayhan Gultekin of the University of Michigan could not find any supermassive black hole in the galaxy in the middle of Abell 2261, located about 2.7 billion light-years from Earth.
Astronomers used the most sophisticated techniques and searched the sky using NASA’s Chandra X-ray Observatory and the Hubble Space Telescope, but were quite surprised when they came out empty-handed. After all, an object that could weigh up to 100 billion times the mass of the sun should not be able to hide easily. For comparison, the supermassive black hole of the Milky Way has 4 million solar masses.
As a black hole devours matter, some of the material becomes overheated as it falls into the black hole, producing X-rays in the process. However, scientists have not detected any such source. This is the second time that researchers have failed to find the supermassive black hole in the center of the great central galaxy of Abell 2261, after previous attempts using Chandra data obtained in 1999 and 2004. So there is probably no black hole – at at least not where the scientists expected.
It is possible that the black hole was evacuated from the center of the host galaxy as a result of the fusion of two galaxies to form the observed one. When two black holes merge, they produce gravitational waves that can be stronger in one direction than another, so that the newly formed supermassive black hole could have been ejected from the center of the galaxy in the opposite direction. Scientists call this a retreating black hole.
But that’s all in theory. We have no evidence that supermassive black holes can merge, let alone evidence of a retracting black hole. So far, researchers have only confirmed the mergers of much smaller black holes.
Alternatively, the black hole may not be active enough to produce visible amounts of X-rays to appear in Chandra’s observations, although this sounds plausible, given the expected scale. Finally, the black hole may simply not exist.
These hypotheses can be verified with the entry into operation of NASA’s future James Webb telescope.
The findings are to be published in the journal American Astronomical Society. In the meantime, it is available online on the pre-print server arXiv.