The Universe is full of galaxy clusters, but Abell 2261 is in a class of its own. In the galaxy at the center of the cluster, where one of the largest supermassive black holes in the Universe should be, astronomers have been unable to find any trace of such an object.
And a new search has only made its absence more puzzling: If the supermassive black hole was chased into space, it should have left evidence of its passage. But there is also no sign of it in the material surrounding the galactic center.
But this means there can be limitations on what the supermassive black hole – if it exists, evades detection – does.
Galaxy clusters are the largest known gravitational structures in the Universe. Typically, they are groups of hundreds to thousands of galaxies bound together, with one huge, abnormally bright galaxy at or near the center, known as the brightest cluster system (BCG).
But even among BCGs, Abell’s BCG 2261 stands out (which is actually called A2261-BCG and is about 2.7 billion light years away). About a million light-years across – up to 10 times the size of the Milky Way – it has a huge, swollen core 10,000 light-years across, the largest galactic core ever seen.
Which could it be ??? (NASA; ESA; M. Postman, STScI; T. Lauer, NOAO, Tucson; CLASH Team)
Based on the galaxy’s mass, which correlates with the size of a black hole, there should be an absolute beast of a black hole at its core, between 3 and 100 billion times the mass of the Sun, making it one of the largest known black holes (the Milky Way’s supermassive black hole is 4 million solar masses).
But instead of containing the radiation you’d expect from an active supermassive black hole as it heats and overheats the material around it, the core of A2261-BCG is filled with a diffuse mist of bright starlight. Several instruments, including the Chandra X-ray Observatory, the Very Large Array, and the Hubble Space Telescope, failed to find any indication of a black hole in the center of A2261-BCG.
Now, a team of astronomers led by Kayhan Gultekin of the University of Michigan in Ann Arbor has gone back to Chandra for a series of deeper observations based on the hypothesis that the supermassive black hole was thrown out.
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 at the center of those merging galaxies will also merge, spiraling slowly towards each other before joining together to become one larger black hole.
We now know, thanks to gravitational wave astronomy, that merging supermassive black holes make gravitational waves lapping through space-time. It is possible that if the gravitational waves were stronger in one direction, the recoil of gravity could kick the merged black hole in the opposite direction.
It would be great to find evidence of this. First, the recoil from a black hole’s fusion has yet to be detected, meaning that it is still hypothetical. But we also don’t know whether supermassive black holes can really merge with each other.
According to numerical simulations of supermassive black hole mergers, they cannot. That’s because as their orbit gets smaller, so does the area in space to which they can transfer energy. By the time the black holes are one parsec apart (about 3.2 light years), this region of space is theoretically no longer large enough to support further orbital decay, so they remain in a stable binary orbit, potentially billions years. This is called the last parsec problem.
There are several indications that such a fusion may have occurred in the heart of A2261-BCG. For starters, there is the size of the core. In 2012, scientists suggested that two merging black holes could have ejected a slew of stars from the core, blowing up 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 contained in the gravitational pull of such a massive object as the merged supermassive black hole as it drove out of 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 to take a closer look at the center of A2261-BCG, and combined it with archival data to look for low levels of supermassive black hole activity. Radio waves had previously shown that the last supermassive black hole activity in the center of the galaxy occurred about 48 million years ago, so the team was also very careful to investigate that area.
They also looked at stellar concentrations around the galactic core.
What the team did find is that the density of the hot gas decreases as the center approaches; so 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 the X-rays associated with black hole activity.
Since black holes do not emit detectable radiation on their own and we can usually only detect them when they feed, it is possible that there is a black hole in the center of A2261-BCG. If so, it is either at rest or it is accelerating matter too slowly to be noticed by our current instruments.
The other explanation is that the black hole has stepped much further than we looked. More sensitive instruments in the future can help answer this fascinating question.
The study has been accepted by AAS Journals and is available on arXiv.