The Universe is full of galaxy clusters, but Abell 2261 is in a class of its own. In the galaxy in the center of the cluster, where there should be one of the largest supermassive black holes in the universe, astronomers have been unable to find any trace of this object.
And new research has only made the absence more puzzling: if the supermassive black hole came out into space, it should have left evidence of its passage. But there is also no trace in the material surrounding the galactic center.
But that means restrictions can be placed on what the supermassive black hole does (if any, avoiding detection).
Galaxy clusters are the largest gravitationally bound structures in the universe. They are usually clusters of hundreds to thousands of galaxies joined together, with a huge abnormally bright galaxy in or near the center, known as the brightest cluster galaxy (BCG).
But even among BCGs, Abell 2261’s BCG stands out (called, in fact, A2261-BCG, located about 2.7 billion light-years away). It is a million light-years in diameter (up to 10 times the size of the Milky Way galaxy) and has a huge, inflated core 10,000 light-years in diameter, the largest galactic nucleus ever seen.
What could it be ??? (NASA; ESA; M. Postman, STScI; T. Lauer, NOAO, Tucson; CLASH team)
Based on the mass of the galaxy, which correlates with the size of the black hole, there should be an absolute beast of a black hole in the core, between 3 and 100 billion times the mass of the Sun, which it could make it one of the largest known black holes (the supermassive black hole in the Milky Way is 4 million solar masses).
But instead of containing the radiation expected from an active supermassive black hole when the surrounding material is produced and heated, the core of the A2261-BCG is filled with diffuse light fog. bright stellar. Several instruments, including the Chandra X-ray Observatory, the Very Large Array, and the Hubble Space Telescope, have found no evidence of a black hole in the center of the A2261-BCG.
Now, a team of astronomers led by Kayhan Gultekin of the University of Michigan at Ann Arbor has returned to Chandra to make a set of deeper observations, based on the hypothesis that the supermassive black hole was ejected.
It’s not such a wild idea. BCGs are expected to grow when they merge with other galaxies. When this happens, the supermassive black holes in the center of the merging galaxies would also merge, slowly turning toward each other before coming together to become a larger black hole.
We now know, thanks to gravitational wave astronomy, that the fusion of supermassive black holes causes gravitational waves to tilt through space-time. It is possible that if the gravitational waves were stronger in one direction, the gravitational pullback could suffer the fused black hole in the opposite direction.
Finding evidence of this would be amazing. First, the reversal of black hole fusion has not yet been detected, meaning it is still hypothetical. But we also don’t know if supermassive black holes can really be combined.
According to numerical simulations of supermassive black hole fusions, they cannot do so. This is because as their orbit shrinks, so does the region of space to which they can transfer energy. By the time the black holes are at a parsec difference (about 3.2 light-years), theoretically this region of space is no longer large enough to withstand orbital decay, so they remain in a stable binary orbit, potentially for billions of years. This is called the final parsec problem.
There have been several clues suggesting that such a merger could have taken place in the heart of the A2261-BCG. There is the size of the kernel, to begin with. In 2012, scientists suggested that two merging black holes could have ejected a whole bunch of stars from the core, inflating the region. This would also explain why the densest concentration of stars was 2,000 light-years from the core.
In 2017, scientists looked for a concentration of high-density stars that would have been trapped by the gravity of an object as massive as the fused supermassive black hole as it exited the galactic center. Of the three groups, two were discarded and the third was inconclusive.
(NASA / CXC, NASA / STScI, NAOJ / Subaru, NSF / NRAO / VLA)
Thus, Gultekin and his team used Chandra to delve into the center of A2261-BCG and combined it with archival data to look for a low level of supermassive black hole activity. The radio broadcast had previously shown that the last activity of the supermassive black hole in the center of the galaxy occurred about 48 million years ago, so the team was very careful to investigate this region as well.
They also examined stellar concentrations around the galactic nucleus.
What the team did find is that the density of the hot gas decreases as it approaches the center; so that the highest gas density is not in the center of the core, but around it. But none of the sites they examined showed any evidence of X-rays associated with black hole activity.
Since black holes do not emit any detectable radiation on their own and we can only detect them when they feed, there may be a black hole in the center of the A2261-BCG. If any, it is at rest or accreting too slowly to be detected by our current instruments.
The other explanation is that the black hole has been ejected far beyond what we have been looking for. More sensitive instruments in the future could help answer this fascinating question.
The research has been accepted by AAS Journals and is available at arXiv.