Using the NASA/ESA Hubble Space Telescope, astronomers have discovered a supermassive black hole with a mass of 17 billion Suns in the center of the relatively isolated elliptical galaxy NGC 1600.
The elliptical galaxy NGC 1600, approximately 200 million light-years away – shown in the center of the Hubble image and highlighted in the box – hosts in its centre one of the biggest supermassive black holes known. Image credit: NASA / ESA / Digital Sky Survey 2.
Until now, the biggest supermassive black holes — those which have a mass of more than 10 billion solar masses — have only been found at the cores of very large galaxies in the centers of massive galaxy clusters.
“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 Dr. Chung-Pei Ma, an astronomer with the University of California-Berkeley and co-author of a study published in the journal Nature.
This small group of galaxies, including NGC 1600, is located in the constellation Eridanus, approximately 200 million light-years away.
“Even though we already had hints that NGC 1600 might host an extreme object in the center, we were surprised that the black is 10 times more massive than predicted by the mass of the galaxy,” said co-author Dr. Jens Thomas, from the Max Planck-Institute for Extraterrestrial Physics in Germany.
Based on previous Hubble surveys of supermassive black holes, scientists had discovered a correlation between a black hole’s mass and the mass of its host galaxy’s central bulge of stars: the larger the galaxy bulge, the more massive the black hole is expected to be.
“It appears from our finding that this relation does not work so well with extremely massive black holes,” Dr. Thomas said.
“These monster black holes account for a much larger fraction of the host galaxy’s mass than the previous correlations would suggest.”
One idea to explain the enormous size of the NGC 1600’s black hole is that it merged with another black hole long ago when galaxy interactions were more frequent.
When two galaxies merge, their central black holes settle into the core of the new galaxy and orbit each other. Stars falling near the binary black hole, depending on their speed and trajectory, can actually rob momentum from the whirling pair and pick up enough velocity to escape from the galaxy’s core. This gravitational interaction causes the black holes to slowly move closer together, eventually merging to form an even larger black hole. The supermassive black hole then continues to grow by gobbling up gas funneled to the core by galaxy collisions.
The frequent meals consumed by NGC 1600 may also be the reason why the galaxy resides in a small group. According to the team, most of the galaxy’s gas was consumed long ago when the black hole blazed as a brilliant quasar from material streaming into it that was heated into glowing plasma.
Finding the extremely massive black hole in NGC 1600 leads astronomers to ask whether these objects are more common than previously thought.
“There are quite a few galaxies the size of NGC 1600 that reside in average-size galaxy groups,” Dr. Ma said.
“We estimate that these smaller groups are about 50 times more abundant than large, dense galaxy clusters.”
“So the question now is: is this the tip of an iceberg? Maybe there are a lot more monster black holes out there.”
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Jens Thomas et al. A 17-billion-solar-mass black hole in a group galaxy with a diffuse core. Nature, published online April 6, 2016; doi: 10.1038/nature17197
