What's 10-billion times as massive as our Sun, several times larger than our solar system, and has a gravitational influence on pretty much anything that comes within 2000 light years of its gaping, cosmic maw? Why, that would be a recently discovered black hole at the center of elliptical galaxy NGC 3842, the smaller — yes, smaller — of two supermassive black holes that astronomers have announced are the most massive they've ever discovered.
The second black hole, which astronomers estimate could be as much as 37-billion times the mass of our Sun, is found at the center of a galaxy named NGC 4889. Both galaxies are a little more than 300 million light-years from Earth.
"For comparison," says the study's lead author, Nicholas McConnell, "these black holes are 2,500 times as massive as the black hole at the center of the Milky Way Galaxy, whose event horizon is one fifth the orbit of Mercury"
"They are monstrous," explains Berkeley astrophysicist Chung-Pei Ma, who co-led the team of astronomers that made the discovery. "We did not expect to find such massive black holes because they are more massive than indicated by their galaxy properties. They're kind of extraordinary."
Astronomers have long assumed that every large galaxy harbors a supermassive black hole at its center. One way astronomers identify and "weigh" these supermassives is by clocking the speed of stars ensnared by their gravitational influence; faster stars require black holes with stronger gravitational pulls — and, by extension, more mass — to keep them from zooming off. (The image up top shows an artist's conception of stars moving in an enormous elliptical galaxy with a supermassive black hole at its center).
Ma and McConnell made a conscious effort to search for supermassive black holes centered in galaxies that themselves belong to what are known as galaxy clusters. The two hypothesized that the growth of a black hole in such an environment would be fed not only by the gas and stars of its own galaxy, but by those of other galaxies in the same cosmic cluster. The researchers fond what they were looking for with the supermassives contained in NGC 3842 and 4889.
But as Ma alludes to above, these two black holes are a lot more massive than any well-established models relating the mass of a galaxy's stars to that of its central black hole would have suggested.
In the past, astronomers have speculated — based on the incredible brightness of celestial objects known as quasars — that black holes as large as these could exist in theory (the light from quasars is believed to be generated by superheated matter as it is consumed by a black hole); but until now, no evidence in the form of a supermassive so large had been discovered. According to Study co-author Karl Gebhardt of the University of Texas at Austin, the unprecedented size of these supermassives could offer new insights on the formation of galaxies and black holes alike.
"If there is any bigger black hole," Ma said, "we should be able to find them in the next year or two. Personally, I think we are probably reaching the high end now. Maybe another factor of two to go at best."
The researchers' findings are published in the latest issue of Nature.
Top image by Lynette Cook via Nature