Crash diets are never a good idea. But when they happen at a galactic scale, they're much more interesting: Researchers have spotted a black hole between two crashing galaxies that seems to have been stripped of some of the stars that should surround it. In other words, the galactic merger may have led the supermassive black hole to get a little less massive.
The merging galaxies a billion light years away each came with their own baggage: Dust, stars, gas, and a supermassive black hole apiece. It's thought that all spiral galaxies have black holes at their center, which range in size from 1 million to 1 billion times the mass of our sun depending on the size of the galaxy itself.
So the newly merged galaxy has two black holes, which isn't a surprise. One of them looks the way researchers expected it to. But the other had fewer stars around it than one would usually find.
Lead researcher Julie Comerford of the University of Colorado Boulder presented her findings on Tuesday at the annual meeting of the American Astronomical Society. A preprint version of her group's upcoming paper on the work is available online.
“One black hole is starved of stars, and has 500 times fewer stars associated with it than the other black hole,” Comerford said in a statement. “The question is why there’s such a discrepancy.”
It's possible that this black hole is just an odd duck: It could be a rare “intermediate” mass black hole, with a mass just 100 to 1 million times greater than that of the sun. There are small, stellar black holes just a few times more massive than the sun, but scientists suspect there must be medium-sized ones, too. To date, black holes as "medium" as 50,000 times more massive than our sun have been spotted. But it's hard to confirm their existence, or even figure out where scientists should go looking for them.
It could be that one of the two original galaxies that formed the merger – the one that hosted this strange black hole – was actually a dwarf galaxy. Some researchers suspect that dwarf galaxies are more likely to play host to supermassive black holes that are relatively tiny, but this isn't universally accepted.
Alternatively, the merger itself could have stripped away some of the black hole's starry entourage. The gravitational force of two galaxies intermingling would have been massive, and might have caused some of the material surrounding the black hole to disperse.
In any case, it's clear that this merged galaxy has something going on that we haven't seen before.