Research News

Black holes feed faster than previously expected

New finding might explain why quasars flare and fade so quickly

A Northwestern University-led study supported by the U.S. National Science Foundation is changing the way astrophysicists understand the feeding habits of supermassive black holes.

While previous researchers have hypothesized that black holes feed slowly, new simulations indicate that black holes consume much faster than conventional understanding suggests.

The study is published in The Astrophysical Journal.

According to high-resolution 3D simulations, spinning black holes twist up the surrounding space-time, ultimately ripping apart the violent whirlpool of gas, or accretion disk, that encircles and feeds them. This results in the disk tearing into inner and outer subdisks. Black holes first devour the inner ring. Then, debris from the outer subdisk spills inward to refill the gap left behind by the wholly consumed inner ring, and the feeding process repeats.

One cycle of the endlessly repeating eat-refill-eat process takes mere months — a shockingly fast timescale compared to the hundreds of years researchers previously proposed.

The new finding could help explain the dramatic behavior of some of the brightest objects in the night sky, including quasars, which abruptly flare up and then vanish without explanation.

"Classical accretion disk theory predicts that the disk evolves slowly," said Northwestern’s Nick Kaaz, who led the study. "But some quasars, which result from black holes eating gas from their accretion disks, appear to drastically change over time scales of months to years. It looks like the inner part of the disk, where most of the light comes from, gets destroyed and then replenished."

Accretion disks surrounding black holes are physically complicated objects, making them incredibly difficult to model, according to paper co-author Alexander Tchekhovskoy. Conventional theory has struggled to explain why these disks shine so brightly and then abruptly dim — sometimes to the point of disappearing completely.

Previous researchers had mistakenly assumed that accretion disks are relatively orderly. In those models, gas and particles swirl around the black hole — in the same plane as the black hole and in the same direction of the black hole’s spin. Then, over a time scale of hundreds to hundreds of thousands of years, gas particles gradually spiral into the black hole to feed it.

The researchers’ simulation, one of the highest-resolution simulations of accretion disks to date, indicates that the regions surrounding the black hole are much messier and more turbulent places than previously thought.