The general rule of thumb for black hole hunters is that really big, “supermassive” black holes occur in really big clusters of galaxies. The largest black hole ever recorded, for instance, in galaxy NGC 4889, weighs over 21 billion times the mass of our sun and resides in the Coma Cluster, a group of about 10,000 galaxies that stretches 20 light years from end to end.
So it came as a surprise for scientists to find an enormous black hole in the center of galaxy NGC 1600, part of a “cosmic backwater” of about 20 galaxies. The group responsible for the discovery is the MASSIVE survey team, who are dedicated to studying the most massive galaxies and black holes in our local universe. The discovery opens the possibility that there are many more supermassive blackholes in the universe than previously believed.
“Rich groups of galaxies like the Coma Cluster are very, very rare, but there are quite a few galaxies the size of NGC 1600 that reside in average-size galaxy groups,” University of California, Berkeley, astronomer Chung-Pei Ma, who leads the MASSIVE Survey, says in a press release. “So the question now is, ‘Is this the tip of an iceberg?’ Maybe there are a lot 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.”
“What this is saying is that you don’t need these galaxy clusters to grow very massive black holes,” Poshak Gandhi of the University of Southampton tells Nicola Davis at The Guardian. “That throws a wrench in the works of our understanding of how these monster black holes form – it throws the field wide open.”
By studying the movement of stars within NGC 1600, Ma and her team were able to estimate that a black hole with the weight of 17 billion solar masses resides at its center. The lack of stars in the immediate vicinity and the black hole’s size suggest that NGC 1600 was once a binary system of two galaxies each with a large black hole at its core.
As gravity pulled the galaxies closer and as their black holes merged, it destabilized stars and spit them away from the center of the galaxy, as many stars as there are in the Milky Way. This has left a distinctive “scoured” core in the galaxy, Ma and her team report this week in the journal Nature
“Each time they eject a star [the black holes] lose a bit of energy and the binary becomes smaller,” Jens Thomas from the Max Planck Institute for Extraterrestrial Physics and an author on the paper tells Davis. “At some point the two black holes are so close to each other that they merge.”
NGC 1600 now gives Ma and the MASSIVE Survey a new template to look for in their search for black holes, which may take a second look at the galactic equivalents of Omaha.