200 million light years from Earth, scientists have discovered a supermassive black hole that is hard to explain. Your average supermassive black hole, weighing 10 billion suns or more, calls very large galaxies home. And its home galaxy is always surrounded by many more large galaxies. Well, until now.
The newest supermassive black hole was discovered in NGC 1600, a pretty big elliptical galaxy. But what makes this black hole different is NGC 1600 sits “in a cosmic backwater, a small grouping of 20 or so galaxies,” said Chung-Pei Ma, a University of California-Berkeley astronomer and head of the MASSIVE Survey. MASSIVE studies the biggest galaxies with the biggest black holes in the local universe.
Did you know: The largest black hole discovered so far weighs an astonishing 21 billion suns. And it sits inside the cramped Coma galaxy cluster with another 1,000 galaxies.
The Hubble Space Telescope helped spot the massive black hole in NGC 1600.
Another fact that surprised researchers was its size compared to its home galaxy. This supermassive black hole is about 10 times more massive than they thought it would be in a galaxy this size. Our previous understanding of black holes showed a correlation between a black hole’s mass and the galaxy’s bulge of stars at its center. The larger the bulge, the larger the black hole. But the correlation isn’t seen in NGC 1600.
“It appears that relation does not work very well with extremely massive black holes; they are a larger fraction of the host galaxy’s mass,” says Ma.
Is NGC 1600 the outlier? Or, do scientists need to tweak their understanding of supermassive black holes? Ma says there are plenty of galaxies the size of NGC 1600 in clusters with a few dozen galaxies. “We estimate that these smaller groups are about 50 times more abundant than spectacular galaxy clusters like the Coma cluster,” Ma added.
The researchers do have a theory to explain what’s going on at NGC 1600’s heart. We may be seeing the result of two big black holes merging. The merging of two black holes into one gigantic one could also explain why it sits where it does. “To become this massive, the black hole would have had a very voracious phase during which it devoured lots of gas,” Ma said.
NGC 1600 is at least three times as bright as its neighbors. Researchers don’t see this kind of brightness gap often. According to NASA, most of the galaxy’s gas was gobbled up by the black hole long ago. Back then, the black hole would have appeared as a brilliant quasar. Today, it sleeps.
“The only way we found it was by measuring the velocities of stars near it, which are strongly influenced by the gravity of the black hole,” Ma said. “The velocity measurements give us an estimate of the black hole’s mass.”