Astronomers from the University of California, Berkeley have found evidence of a near-record monster black hole, located in a modestly sized, local galaxy. This discovery suggests supermassive black holes may be more common than previously thought.
Located some 200 million light-years from Earth, the newly found supermassive black hole is believed to have a mass equal to 17 billion suns. Generally, supermassive black holes are found at the heart of very large galaxies located in dense clusters in the universe. But this is the first time astronomers have found such an object lurking at the center of a large galaxy in a relatively empty area of the universe.
Black holes form when matter becomes so dense that not even light can escape its gravitational pull. When these massive galactic objects have masses millions or billions times that of the sun they are referred to as "supermassive."
Discovered in the Coma Cluster in 2011, the largest known black hole tips the scale at 21 billion times more massive than the sun. In comparison, the black hole at the center of the Milky Way galaxy, named Sagittarius A, is about 4 million times the mass of the sun.
The newly found black hole is in a somewhat isolated galaxy known as NGC 1600. This galaxy is said to be in a part of the sky opposite from the Coma Cluster.
"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," explained Chung-Pei Ma, study leader and University of California Berkeley astronomer. "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."
Studying the movement of stars within NGC 1600 suggests the black hole is binary, meaning it was formed by two merging galaxies. Researchers explained that most stars inside the sphere of influence of the black hole - a region about 3,000 light-years in radius - are traveling on circular orbits around the black hole, with very few moving radially inward or outward.
"Somehow the stars have been scared away from the center of very massive galaxies, and either were afraid to come in, or came in and got kicked out," Ma said, adding that the stellar orbits around the center of NGC 1600 indicate the latter, which "may be support for a binary black hole formed by a merger."
While further study is needed, the mass of the supermassive black hole alone gives clues to the galaxy's past. For instance, its size indicates it grew by "feeding" off vast quantities of gas. This, Ma explained, suggests NGC 1600 may harbor an ancient quasar that once blazed brightly but is now asleep.
The new supermassive black hole was studied as part of an ongoing survey of the 100 most massive galaxies within about 300 million light-years of Earth. Aptly named Massive, the survey combines data from the Hubble Space Telescope, the Gemini Telescope in Hawaii and the McDonald Observatory in Texas.
"Probably all of these galaxies harbor black holes in their centers. The question we would like to answer is how massive the black holes are, and whether there are more hefty ones like the one we found in NGC 1600," Ma said. "We don't know right now if NGC 1600 is the tip of an iceberg, or a rare find, perhaps as a result of an unusually voracious phase during its youth."
Their findings were recently published in the journal Nature.
