Why black holes are cannibals

Black hole feeding frenzies are fueled by galactic collisions, suggests a new study that confirms astronomers' suspicions.

Astronomers have had their eyes on a certain class of galaxies that appear to contain central black holes that gorge on gas and dust. So far, scientists have been unsure what triggers these giant meals, but new radio observations may help explain how they work.

Seyfert galaxies are a type of galaxy known as Active Galactic Nuclei, thought to host supermassive black holes in their centers. Seyferts are slightly tamer versions of the extremely luminous AGN called quasars and blazars.

Scientists guessed that recent interactions with neighboring galaxies might have stirred up Seyferts' gas and dust and propelled it toward their giant black holes. But when optical telescopes observe Seyferts in light visible to the human eye, the Seyferts show no sign of close encounters with other galaxies.

Now astronomers have used the Very Large Array telescope to photograph these objects in radio light, and found that the majority of Seyferts do indeed seem to have recently collided with a neighbor. For comparison, the researchers observed non-Seyfert galaxies and found that very few showed signs of an interaction.

"This comparison clearly shows a connection between close galactic encounters and the black-hole-powered activity in the cores," said Ya-Wen Tang, who began this work at the Institute of Astronomy & Astrophysics, Academia Sinica, in Taiwan and now is a graduate student at the National Taiwan University. "This is the best evidence yet for the fueling of Seyfert galaxies. Other mechanisms have been proposed, but they have shown little if any difference between Seyferts and inactive galaxies."

The VLA telescope was able to study the galaxies' hydrogen gas by observing the radio waves emitted by hydrogen atoms. The hydrogen gas in many Seyferts left a clear signal of being disturbed by a collision with another galaxy.

"Our results show that images of the hydrogen gas are a powerful tool for revealing otherwise-invisible gravitational interactions among galaxies," said Jeremy Lim, also of ASIAA. "This is a welcome advance in our understanding of these objects, made possible by the best and most extensive survey ever made of hydrogen in Seyferts."

The new study helps scientists better understand these violent systems, where gas and dust swirl around dense black holes that eventually gobble up the incoming material.

"The VLA lifted the veil on what's really happening with these galaxies," said Cheng-Yu Kuo, a graduate student at the University of Virginia. "Looking at the gas in these galaxies clearly showed that they are snacking on their neighbors. This is a dramatic contrast with their appearance in visible starlight."