The massive Abell 2261 galaxy cluster should have a supermassive black hole in its center. But that is not the case. Astronomers have searched everywhere – even between the sofa cushions. What's happening?
Giant black holes, also known as supermassive black holes, are basically everywhere in the heart of almost every known galaxy. Even our own Milky Way has one, a beast over 4 million times the size of the Sun, known as Sagittarius A * (and the subject of the recent Nobel Prize in Physics).
Unfortunately, supermassive black holes are hard to spot in and of themselves – they're black, after all. Instead, astronomers find them in the material that surrounds them. As all these tons of gas and dust push into the event horizon, they heat up and glow to tens of millions of degrees Kelvin. At this temperature the material emits copious amounts of X-rays – an unmistakable sign of a huge black hole.
The huge elliptical galaxy that sits in the center of the Abell 2261 galaxy cluster should have such a black hole with the corresponding intense X-ray emission. However, repeated scans with NASA's Chandra X-ray Observatory have revealed nothing. Nada. Zilch. It is empty.
Previous scans with the NSF's Karl G. Jansky Very Large Array, looking for ghosts of radio emission, showed that there was significant activity in the cluster 50 million years ago – but not today.
What's happening? One possibility is that two (slightly smaller, but still supermassive) black holes have merged in the recent past. When they merge, they emit enormous amounts of gravitational waves, and this emission can "knock out" the resulting black hole from the host galaxy. An indication that this could have happened is the fact that the densest star cluster in the central galaxy is more than 2,000 light-years from the galaxy's true center. That is much further than normal.
But there is also no X-ray emission from this lump. The only possibility is that the black hole is sleeping. When it isn't eating, it won't produce any tell-tale X-rays.
If we're lucky, the giant can soon wake up from his sleep and reveal his true location to the universe – and let us sleep at night.