Black holes are a bit like celebrities — the larger they are, the more activity they have swirling about them. It came as some surprise then, when astronomers spied a supermassive black hole in a relatively quiet neighborhood of the universe. The gravity gobbler has the weight of 18 billion of our suns and is found in a giant elliptical galaxy that should have a much more impressive bulge of stars near its center for a black hole of that size.
"The newly discovered supersized black hole resides in the center of a massive elliptical galaxy, NGC 1600, located in a cosmic backwater, a small grouping of 20 or so galaxies," said astronomer Chung-Pei Ma, head of the MASSIVE Survey, a study of the most massive galaxies and supermassive black holes in the local universe.
Another way to think of the discovery is that it would be common to find a skyscraper (the black hole) in a city like New York (the surrounding galaxies), but less so in the middle of a small town.
Ma says that there are plenty more galaxy clusters that are equally as uncrowded as NGC 1600, so it may be possible that this black hole is just the first of many such discoveries that would have astronomers rethinking the black-hole-to-surrounding-stellar-material ratio. "Maybe there are more monster black holes out there that don't live in a skyscraper in Manhattan, but in a tall building somewhere in the Midwestern plains" Ma says.
Prior to discovering this black hole, astronomers had developed a correlation between the bulge of stars at the center of a galaxy and their accompanying black holes. As the black hole increased in size, went the thinking, so did the disc of stars. In the case of NGC 1600 though, the black hole is 10 times bigger than this correlation would indicate.
One theory about why this black hole seemingly breaks the rules is that it once started out as two black holes. As those two voids collapsed into each other, they became extraordinarily powerful and began devouring all of the gas created from continued galactic collisions. That would leave very little material in the proximity of the black hole, which would mean its central star bulge would be minimal.
"Now, the black hole is a sleeping giant," Ma says. "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. The velocity measurements give us an estimate of the black hole's mass."
Those measurements were obtained using the Gemini telescope in Hawaii along with the Hubble Space Telescope. The findings of the team have been reported in the April 6 issue of the journal Nature.