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 (AGN), 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 (VLA)
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 (ASIAA), 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."
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