How Black Holes Gobble Dark Matter
Artist's schematic impression of the distortion of space-time by a supermassive black hole at the center of a galaxy. The black hole will swallow dark matter at a rate which depends on its mass and on the amount of dark matter around it.
Credit: Felipe Esquivel Reed

To better understand the inexplicable dark matter that seems to dominate the universe, scientists have modeled how supermassive black holes gobble up this strange brand of cosmic stuff.

Dark matter does not reflect light or interact with ordinary matter except through gravity. So when dark matter is swallowed by a black hole, it behaves somewhat differently than normal matter, the new study found.

For example, as normal matter falls toward a black hole, it heats up and radiates light. The radiation can be so intense that it actually halts the accretion of matter onto the black hole at what scientists call the Eddington limit. But because dark matter doesn't emit light, this effect doesn't apply to it.

Without counting dark matter in the mix, the Eddington limit would make it hard to account for how some supermassive black holes got so massive.

"There's probably a consensus that you do have to invoke at least some accretion of dark matter onto the black holes to explain how they got so big," said study leader Xavier Hernandez, an astrophysicist at the National Autonomous University of Mexico.

And because the Eddington limit does not apply to dark matter, the researchers discovered that if dark matter is sufficiently dense in the immediate vicinity of a black hole, it can fall in extremely rapidly in a process they call runaway accretion.

"The bigger it gets the faster it accretes, and the faster it accretes the bigger it gets, so you end up in trouble," Hernandez told SPACE.com.

Runaway accretion would cause black holes to balloon so quickly, engorging so much dark matter that the entire surrounding galaxy would be distorted beyond recognition.

"When you go the extreme range of black hole masses, you find this effect can be dominant and will actually lead to the black hole swallowing up the whole galaxy," Hernandez said.

But this does not seem to have happened to the supermassive black holes thought to occupy the centers of most galaxies, including the heart of our own Milky Way. That means that dark matter probably can't be too dense there.

Since some models of galaxy formation do call for extremely dense dark matter clumps at the hearts of galaxies, these findings may force those models to be reconsidered, Hernandez said.

Another explanation for the lack of runaway accretion in most supermassive black holes could be that the physics of how dark matter interacts with itself is still misunderstood. Just because dark matter doesn't interact much with normal matter doesn't necessarily mean it doesn't interact much with other dark matter, Hernandez said.