You’d think a black hole with a decent size table galaxy it would be easy to find. But then don’t look for the one in the center of a galaxy in Abell 2261.
Abell 2261 is a ridiculous group of giant galaxies about 2.7 billion light-years away. It has THOUSAND of the galaxies in it, and astronomers measure its total stellar content to equal the mass of one quadrillion (1015) Sori.
So, yes, it’s a meat group.
Like most large groups, it has a large galaxy at its center. It doesn’t have an official name, but astronomers call it the Abell 2261 BCG for the Galaxy Brightest Cluster. In general, the galaxies in the center of the clusters are the largest and brightest; they are literally at the base of the cluster’s gravitational well and everything falls into them. Mergers with smaller galaxies are common, so the central galaxy usually grows huge. In this case, the central galaxy has a radius of over one million light-years, greatly saving our own Milky Way (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 tend to be really huge, also for the same reason, their host galaxies are: They are gourmets, they celebrate with gas and stars and anything else that falls in the center of the cluster.
Looking at various parameters of the Abell 2261 BCG, astronomers estimate that it should have a central black hole that weighs in a crushing of the soul 10 billion or the table of the Sun. This is 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 the Sun, which makes the one in Abell 2261 about 2,500 times larger. Yikes.
Except … there’s no evidence that black is, um, There.
Embarrassing. How do you miss a monster like that? Usually again, because the center of a group is like a drain in a particularly large sink, so many things should fall so that they pile up in a disk around the black hole. That record is huge too incredible Hot. The material in it shines so brightly that it can transcend entire galaxies and be clearly visible in the universe.
Hiding something like that is hard.
But Abell 2261 BCG is a strange galaxy. Its images in visible light show that the core of the galaxy is unusually large and has other unusual features. Plus, it’s not centered on the galaxy itself! It is very 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 break down. This leads to an interesting idea: Maybe the black hole ate too much mass and was thrown from the center of the galaxy.
If the central galaxy merged with another galaxy that had a supermassive black hole, the second would fall to the center, to the larger black hole. Over a few billion years, the two could get so close that they orbited each other and then eventually merged, eating each other, forming a single larger black hole, and releasing a really vast amount of energy in gravitational waves.
This burst of energy can sometimes be off-center. We are talking here about amazing energies, such as the conversion of thousands of times the mass of the Sun into pure energy. If this explosion is even slightly off-center, a little asymmetrical, it can deal a huge blow to the resulting black hole, throwing it away from the galactic core.
Wondering if this happened in the Abell 2261 BCG, astronomers looked closely at the nucleus. Fascinatingly, there are four groups of very bright stars there, as well as a spot that is bright in the radio waves. Can one of these five objects be the place of the missing black hole? Maybe he shot those stars with them or hit some gas and made radio waves.
If true, the black hole should be a fairly strong source of X-rays. So astronomers used the Chandra X-ray Observatory to look at the Abell 2261 BCG for a long time – 100,000 seconds, almost 28 hours. – and added that observation to one older than 35,000 seconds (almost 10 hours) to get a very deep picture of the galaxy’s core.
And what they found was … Nothing. No sign of a black hole in any of these five spots, nor in the center of the galaxy itself. They managed to eliminate a few other possible locations.
It’s weird. Either the black hole doesn’t exist – which is so unlikely to be repulsed – or it simply doesn’t eat enough material to shine in X-rays. It’s quite unlikely, too. Now, our local supermassive black hole, Sgr A *, does not shine too much in X-rays, because it does not feed right now, so, in principle, a black hole at rest is possible.
But this would be a huge black hole in the center of a huge galaxy in the center of a huge group and it has that huge mass of ten billion suns! A black hole like that to be quiet is really, really strange.
In any case, this is an astronomical mystery of good faith. The case of the elephant lacking 20 billion tons 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 need to look at other wavelengths or make deeper observations or both. Maybe it’s really quiet and can’t be found. But if it’s true, then Why?
Sometimes, if you want to be answered questions, science is frustrating. And even when you get answers, it just leads to more and more puzzling questions. But the Universe is a rather strange place. We must continue to ask these questions if we ever want to realize.