Astronomers have discovered a black hole containing 200,000 times the mass of the Sun buried in gas and dust in the galaxy Mrk 462
Black holes are extremely dense, with gravitational pulls so fierce that not even light escapes. Cygnus X-1 is the largest known stellar mass black hole in the Milky Way and one of the strongest X-ray sources seen from Earth.
There are three categories of black holes. The smallest, like ‘the Unicorn’, are the so-called stellar-mass black holes formed by the gravitational collapse of a single star. There are gigantic ‘supermassive’ black holes like the one at the center of our galaxy, 26,000 light years from Earth, which is four million times the mass of the sun. Some intermediate mass black holes with intermediate masses have also been found.
Mrk 462 contains only several hundred million stars, making it a dwarf galaxy. By contrast, our Milky Way is home to a few hundred billion stars. This is one of the first times that a deeply buried or “obscured” supermassive black hole has been found in a dwarf galaxy.
The discovery of this supermassive black hole in a relatively small galaxy could help astronomers unravel the mystery surrounding how the largest black holes grow. They identified it using NASA’s Chandra X-ray Observatory.
“This black hole in Mrk 462 is among the smallest supermassive or monstrous black holes,” said Jack Parker of Dartmouth College in New Hampshire, who led the study with his colleague Ryan Hickox, also of Dartmouth. “Black holes like this are notoriously hard to find.”
The unusually large intensity of high-energy X-rays compared to low-energy X-rays, along with comparisons with data at other wavelengths, indicates that black hole Mrk 462 is heavily obscured by gas.
“Because buried black holes are even more difficult to detect than exposed ones, finding this example could mean that there are many more dwarf galaxies with similar black holes,” Hickox said.
“This is important because it could help address an important question in astrophysics: How did black holes get so big so early in the universe?”
A large fraction of dwarf galaxies with supermassive black holes support the idea that the seeds of small black holes from the first generation of stars grew astonishingly fast to form the billion solar-mass objects in the early universe.
A smaller fraction would tip the balance in favor of the idea that black holes began life weighing tens of thousands of suns.
These expectations apply because the conditions necessary for the direct collapse of a giant cloud to a medium-sized black hole should be rare, so a large fraction of dwarf galaxies are not expected to contain supermassive black holes.
Stellar-mass black holes, on the other hand, are expected in all galaxies.
“We cannot draw strong conclusions from one example, but this result should encourage much more extensive searches for buried black holes in dwarf galaxies,” Parker said. “We are excited about what we could learn.”