One would think a black hole with dough of a decent size galaxy it would be easy to find. But then you’re not looking for the one in the center of a galaxy in Abell 2261.
Abell 2261 is a ridiculously huge cluster of galaxies about 2.7 billion light-years away from us. Has thousands of galaxies and astronomers measure their total stellar content to match the mass of a gang (1015) Only.
So yes, it’s a strong cluster.
Like most large clusters, it has a large galaxy sitting in the center. It does not have an official name, but astronomers call it Abell 2261 BCG, for the brightest galaxy. In general, the galaxies in the center of the clusters are the largest and brightest; they are literally at the bottom of the gravity well of the cluster, and everything falls into it. Fusions with smaller galaxies are common, so the central galaxy usually grows huge. In this case, the central galaxy is more than a million light-years in diameter, pushing our own Milky Way enormously (which is about 120,000 light-years wide).
We also know that every large galaxy has a supermassive black hole in its core. The ones in the center of the central galaxies are also usually huge, for the same reason that their host galaxies are: they are gourmands, they feast on gas and stars and everything that falls in the center of the cluster.
Observing various parameters of Abell 2261 BCG, astronomers estimate that it should have a central black hole that weighed in a soul crusher 10. billion times the mass of the Sun. This is a big black hole; compare it with the one in the center of our Milky Way galaxy, called Sgr A *: it has a mass of about 4 million times the Sun, which makes that of Abell 2261 is about 2,500 times larger. Yikes.
Except … there’s no evidence that black is, there.
Shameful. How do you miss a monster like that? Usually again, because the center of a cluster is like the drain of a particularly large sink, so many things would have to fall because they pile up on a disk around the black hole. This record is huge and incredibly hot. The material shines so fiercely that it can shine entire galaxies and be visible in the universe.
Hiding something like that is hard.
But Abell 2261 BCG is a strange galaxy. His images in visible light show that the core of the galaxy is unusually large and has some other unusual features. Also, it is not focused on the galaxy itself. This is really weird. Off-center nature is 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 one, things can unload. This leads to an interesting idea: perhaps the black hole ate a meal too big and was ejected from the center of the galaxy.
If the central galaxy merged with another galaxy that also had a supermassive black hole, the second would fall toward the center, toward the larger black hole. After a few billion years, the two could get so close around each other and eventually merge, eating each other, forming a single larger black hole. and releasing a large amount of energy into gravitational waves.
This burst of energy can sometimes be off-center. We talk about amazing energies, like turning the mass of the Sun into pure energy thousands of times. If this explosion is even a little off-center, a little asymmetrical, it can deal a major blow to the resulting black hole, moving it away from the galactic core.
Wondering if this happened at Abell 2261 BCG, astronomers looked at the core carefully. Interestingly, there are four very bright star clusters, as well as a bright spot on the radio waves. Could one of these five objects be the site of the missing black hole? Maybe he dragged those stars with him or hit some gas and caused it to emit radio waves.
If true, the black hole should be a fairly strong source of X-rays. So astronomers used Chandra’s X-ray Observatory to look at Abell 2261 BCG for a long time (100,000 seconds, almost 28 hours). and they added this observation to a previous one of 35,000 seconds (almost 10 hours) to get a very deep picture of the galaxy’s core.
And what they found was … Nothing. There is no hint of a black hole in any of these five points, nor in the center of the galaxy itself. They were also able to remove other possible locations.
This is strange. Either the black hole doesn’t exist, which is so unlikely to be ruled out, or it just doesn’t eat enough material to shine with X-rays. It’s also pretty unlikely. Now, our local supermassive black hole, Sgr A *, doesn’t shine much in X-rays because it doesn’t feed now, so in principle, a still black hole is possible.
But this would be a huge black hole in the center of a huge galaxy in the center of a huge cluster and has a huge mass of ten billion suns. A black hole like this to be quiet is really, really peculiar.
Anyway, this is a good quality astronomical mystery. The case of the missing elephant of 20 billion tons in the room.
If there is the black hole (and I think so), it is not at all clear how to find it. Maybe we need to look at other wavelengths or make deeper observations or both. Maybe he’s really at rest and can’t be found. But if that is true, then Because?
Sometimes, if you want to answer questions, science is frustrating. And even when you get answers, it generates increasingly puzzling questions for you. But the Universe is a rather strange place. We need to keep asking these questions if we ever want to find out.