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 could not find any trace of such an object.
And a new search 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 no sign in the material surrounding the galactic center.
But that means constraints can be placed on what makes the supermassive black hole – if it is there, avoiding detection -.
Galaxy clusters are the largest known gravity-related structures in the universe. Usually, these are groups of hundreds to thousands of galaxies that are linked together, with a huge galaxy, abnormally bright at the center or near the center, known as the brightest galaxy in the cluster (BCG).
But even among BCGs, Abell 2261’s BCG (actually called A2261-BCG and located about 2.7 billion light-years away) stands out. It is about a million light-years wide – up to 10 times the size of the Milky Way galaxy – and has a huge, swollen core 10,000 light-years wide, 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 at the core, between 3 and 100 billion times the mass of the Sun, which could make it one of the largest. the largest known black holes (the supermassive black hole of the Milky Way is 4 million solar masses).
But rather than containing the radiation you would expect from an active supermassive black hole, while stirring and overheating the material around it, the A2261-BCG core is filled with a diffuse fog of bright stellar light. Various instruments, including the Chandra X-ray Observatory, the Very Large Matrix, and the Hubble Space Telescope, failed to find any indication 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 in Ann Arbor has returned to Chandra for a set of deeper observations, based on the assumption that the supermassive black hole has been ejected.
It’s not such a wild idea. BCGs are expected to increase when they merge with other galaxies. When this happens, the supermassive black holes in the center of those merging galaxies would merge, slowly spiraling toward each other before joining to become a larger black hole.
We know, now, due to the astronomy of gravitational waves, that the fusions of supermassive black holes send gravitational waves into space-time. It is possible that if the gravitational waves are stronger in one direction, then the gravitational recoil could hit the black hole joined in the opposite direction.
Finding evidence in this regard would be amazing. First, the decline in black hole fusion has not yet been detected, which means it is still hypothetical. But we also don’t know if supermassive black holes can actually merge with each other.
According to numerical simulations of supermassive black hole fusions, they cannot. That’s 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 (about 3.2 light-years), theoretically this region of space is no longer large enough to support subsequent orbital decay, so they remain in a stable binary orbit, possibly for billions of years. This is called the last parsec problem.
There were several indications that such a merger could have taken place in the center of A2261-BCG. There is the size of the core, for starters. In 2012, scientists suggested that two merging black holes could have thrown a whole bunch of stars out of the core, swelling the region. This would also explain why the densest concentration of stars is 2,000 light-years from the core.
In 2017, scientists looked for a concentration of high-density stars that would have been caught by the gravity of an object as massive as a supermassive black hole fused as it exited the galactic center. Of the three clusters, two were excluded and the third was inconclusive.
(NASA / CXC, NASA / STScI, NAOJ / Subaru, NSF / NRAO / VLA)
So Gultekin and his team used Chandra for a closer look at the A2261-BCG center and combined it with archive data to look for a low level of activity of supermassive black holes. Radio broadcasts previously showed that the last activity of the supermassive black hole in the center of the galaxy took place about 48 million years ago, so the team was very careful to investigate that region as well.
They also looked at the stellar concentrations around the galactic nucleus.
What the team found is that the density of the hot gas decreases as the center approaches; so the highest density of gas is not in the middle of the core, but around it. But none of the sites they examined presented evidence of X-rays associated with black hole activity.
Because black holes do not emit detectable radiation on their own, and we can usually only detect them when they feed, there may be a black hole in the center of the A2261-BCG. If there is, it is either a period of rest or a matter that accumulates too slowly to be detected by our current instruments.
The other explanation is that the black hole was hit much farther than I was looking for. More sensitive tools in the future could help answer this fascinating question.
The research was accepted by AAS Journals and is available on arXiv.