Black Holes Crowd the Milky Way

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Illustration by Maren Slobody.
Illustration by Maren Slobody.

To detect a supermassive black hole, astrophysicists need a supermassive computer. With the use of supercomputer simulations, two UC Santa Cruz astrophysicists discovered that up to 2,000 black holes may inhabit the halo of dark matter that surrounds the Milky Way.

“This is not paper and pencils,” said Piero Madau, a UCSC astrophysicist who worked on the project. “This is state of the art, fancy stuff.”

Valery Rashkov, an astrophysics graduate student, conducted the black hole research. He submitted a paper about the study on March 15 to Astrophysical Journal — the foremost international astrophysics research journal — where it is currently undergoing the peer review process.

To find the black holes in the galaxy, Rashkov and his mentor Madau added black holes to the supercomputer simulation Via Lactea 2 (VL-2) ­— a simulation originally created in 2007 by a team of UCSC astrophysicists using the supercomputer at Oak Ridge National Laboratory.

“We used a trick to add black holes to the original simulation,” said Madau, who was part of the VL-2 research team. “That trick allowed us to find a solution to the problem without rerunning the simulation from scratch.”

Like a film of a multi-billion year exodus, the simulation uses glowing spots of ochre against a black screen to represent smaller patches of dark matter, a mysterious particle that dictates the formation of cosmic structure. It simulates how these smaller patches came together to form the larger dark matter structure in the Milky Way today.

Halos of dark matter envelop known galaxies, Madau said. By following the movement of these halos of dark matter in VL-2, astronomers were able to follow the movement of the galaxies the dark matter enveloped. Like the supermassive black hole at the center of the Milky Way, these smaller galaxies may host black holes too.

“If you believe that each of these smaller galaxies have a black hole at its center, when they fall in to form the Milky Way, these black holes won’t go away,” Rashkov said.

Adding black holes to these dark matter halos enabled Rashkov to observe how black holes may have merged with one another in the formation of the galaxy. Black holes are extremely dense, which means they have an immense gravitational pull. When they come in close proximity to one another, they gravitationally suck one another in and merge. When two black holes of different masses merge, the asymmetrical factors cause an excess of velocity in one direction, or, what Rashkov calls a “kick.”

“If that kick is very large, it could actually remove [the black hole] from the Milky Way,” Rashkov explained.

Photo courtesy of Valery Rashkov.
Projected distribution of black holes in the Milky Way’s galactic halo. Photo courtesy of Valery Rashkov.

Through studying these kicks, astronomers found destinations in the Milky Way’s outer dark matter halo of where each black hole was “kicked.” Rashkov’s results showed that up to 2,000 black holes may reside in and around the galaxy.

“We used to think about black holes and galaxies as systems that are isolated. There’s a galaxy and there’s a supermassive black hole at its center,” Madau said. “Supermassive black holes in galaxies are not supposed to be isolated. They’re supposed to be surrounded by a family of black holes that are just like them.”

Astronomers will be able to test these models as observation and simulation technologies improve. When this happens, these studies will potentially elucidate the channels of formation of black holes that inhabit the Milky Way — processes that are currently a mystery, Rashkov said.

“That would really be the cool implication,” Rashkov said. “We would finally pin down in what kind of parent, what kind of dwarf dark matter halos [the black holes] formed.”

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