You would think of a black hole with the mass of a decent size universe would be easy to find. But then you are not looking for the one in the center of a galaxy in Abell 2261.
Abell 2261 is a ridiculously large galaxy cluster about 2.7 billion light-years away. It has thousands galaxies in it, and astronomers measure its total stellar content equal to the mass of a quadrillion (1015) Sun tanning.
So yes, it is a solid cluster.
Like most large clusters, it has a large galaxy in the center. It has no official name, but astronomers call it Abell 2261 BCG, for Brightest Cluster Galaxy. In general, galaxies in the centers of clusters are the largest and brightest; they are literally at the bottom of the cluster’s gravity source, and everything falls into it. Mergers with smaller galaxies are common, so the central galaxy usually gets huge. In this case, the central galaxy is more than a million light-years across, greatly shading our own Milky Way (which is about 120,000 light-years wide).
We also know that every major galaxy has a supermassive black hole at its core. Those in the centers of central galaxies are also generally very large, for the same reason that their host galaxies are: They are gourmets who feast on gas and stars and everything else that falls into the cluster center.
Looking at various parameters of Abell 2261 BCG, astronomers estimate that it should have a central black hole with a weight that crushes the soul 10 billion times the mass of the sun. That’s a big black hole; compare it to the one in the center of our Milky Way Galaxy, called Sgr A *: it has a mass about 4 million times that of the sun, making Abell 2261 about 2500 times that. Yikes.
Except … there’s no evidence that black is, uh, There.
Shameful. How do you miss such a monster? Usually, again because the center of a cluster is like the drain in a particularly large sink, so much stuff should fall that it accumulates in a disk around the black hole. That drive is huge and incredible hot. The material in it glows so brightly that it can surpass entire galaxies and is clearly visible in the Universe.
Hiding something like that is difficult.
But Abell 2261 BCG is a strange galaxy. Its images in visible light show that the galaxy’s core is unusually large and has some other unusual features. Plus, it’s not centered in the galaxy itself! That’s really weird. The eccentric nature can be seen in both the distribution of stars and hot gas in the galaxy.
This sometimes happens after a merger; when a large galaxy eats a smaller galaxy, things can get out of hand. That leads to an interesting idea: Perhaps the black hole ate too large a meal and was thrown from the center of the galaxy.
If the central galaxy merged with another galaxy that also had a supermassive black hole, that second galaxy would fall to the center, to the larger black hole. In a few billion years, the two could get so close that they orbit each other, and then they would eventually merge, eat each other, form a single larger black hole, and release a truly tremendous amount of energy in gravitational waves.
That burst of energy can sometimes not be in the middle. We’re talking dizzying energies here, such as converting thousands of times the mass of the sun into pure energy. If that explosion is even slightly off-center, a little bit asymmetrical, it can give a tremendous kick to the resulting black hole and throw away the galactic core.
Wondering if that happened in Abell 2261 BCG, astronomers looked carefully at the core. Intriguingly, there are four very bright star clusters, as well as a spot that is bright in radio waves. Could one of those five objects be the location of the missing black hole? Maybe it dragged those stars with it, or it hit gas and caused it to emit radio waves.
If that’s true, the black hole should be a pretty strong X-ray source. So astronomers used the Chandra X-ray Observatory to view Abell 2261 BCG for a long time – 100,000 seconds, almost 28 hours – and added that observation to an older 35,000 second (almost 10 hours) observation to get a really deep picture from the core of the galaxy.
And what they found was … Nothing. Not a trace of a black hole in any of those five places, nor in the center of the galaxy itself. They were also able to eliminate several other possible locations.
That is bizarre. Either the black hole doesn’t exist – which is so unlikely it can be rejected – or it just isn’t eating enough material to glow in X-rays. That is also quite unlikely. Now our local supermassive black hole, Sgr A *, doesn’t glow much in X-rays because it’s not currently feeding, so in principle such a quiet black hole is possible.
But this one would be a huge black hole in the center of a huge galaxy in the center of a huge cluster, and it has that massive mass of ten billion suns! Such a black hole being quiet is real really peculiar.
So either way, this is a bona fide Class A astronomical mystery. The Case of the Missing 20 Undecillion-tons Elephant in the Room.
If the black hole is there (and I think it is), it is not at all clear how to find it. Maybe we should look at other wavelengths, or make deeper observations, or both. Maybe it is really quiet and cannot be found. But if that’s true, then why?
Sometimes science is frustrating when you want to answer questions. And even when you get answers, it just leads to more and more puzzling questions. But the universe is a pretty strange place. We have to keep asking those questions if we ever want to find out.