Contact: Richard Pogge
Pogge@astronomy.ohio-state.edu
614-292-0274
COLUMBUS, Ohio — Astronomers at Ohio State University used
an innovative imaging technique to discover swirling masses of
interstellar dust spiraling into the center of nearby galaxies.
The researchers believe this interstellar dust is feeding
supermassive black holes.
Despite mounting circumstantial
evidence that black holes
occupy the heart of most
galaxies, astronomers haven’t
seen compelling evidence of
the material that might “feed”
those black holes until now.
Their study appeared in a
recent issue of the
Astronomical Journal.
Using NASA’s Hubble Space Telescope, Richard Pogge,
associate professor of astronomy, and Paul Martini, astronomy
graduate student, devised a plan to point two cameras — one that
records visible light and one that records infrared — at nearby
galaxies that may contain supermassive black holes. They
combined infrared and visible-light images to create single images
of the interstellar dust clouds in the centers of these galaxies.
“Imagine if we could take a picture that showed only the dust in a
galaxy,” said Pogge. “We can’t exactly do that, but we can get
pretty close.”
The Hubble telescope enabled
the Ohio State astronomers to
zero in on the very center of 24
nearby spiral-shaped galaxies
with extremely bright centers.
Astronomers believe the
centers of these galaxies are
bright because they contain
active supermassive black
holes consuming matter from
their surrounding galaxies.
Astronomers call a black hole “active” when its powerful gravity
tears material apart, releasing radiation and brightening the galaxy’s
center. Only 1 percent of galaxies that should contain supermassive
black holes appear to be in an active state.
Most pictures of these active galaxies show the giant arms of gas
and dust that give spiral galaxies their shape. Pogge and Martini
focussed instead on only the central 1,000 light years —
approximately 1 percent of the total diameter of these galaxies.
“We looked at a region people were unable to study before,” said
Martini.
Within 20 of the 24 galaxies they photographed, they saw a
secondary, mini-spiral of dust that appeared to go directly into the
center where the supermassive black hole resides.
These “nuclear spirals” may be the feeding mechanism that
activates black holes, Pogge and Martini said.
Black holes like the one in our own Milky Way may be inactive,
Pogge said, because they aren’t receiving any nourishment from
their host galaxy.
“Before black holes become active, you have to feed them,” Pogge
said.
And supermassive black holes have voracious appetites.
Astronomers calculate that black holes must consume stars, gas, or
dust in amounts up to the mass of our sun every year to remain
active.
Martini explained that over time the material in an inactive galaxy
may reach an equilibrium in which it orbits the central black hole at
a distance just out of the hole’s reach. The black hole wouldn’t
receive any fuel, he said, until some kind of disturbance triggered
an avalanche of material into the center.
The disturbance could come in the form of a collision with another
galaxy, or shock waves — the equivalent of sound waves flowing
from some cosmic event.
“It wouldn’t have to be a large disturbance to start with,” Martini
said. “A small nudge could propagate and have a very large effect.”
Pogge and Martini think the nuclear spirals form when material
orbiting near the centers of these galaxies gets caught up in
propagating shock waves, and the material is robbed of its orbital
energy, permitting it to fall inwards.
At a time when astronomers are painting portraits of black holes as
hungry monsters dwelling at the center of most galaxies, Pogge,
Martini, and others wonder why the monster in our own galaxy is
asleep.
“All the present data suggests we have a three-million solar mass
black hole in our own Milky Way, but it’s about as quiescent as
they get,” said Pogge.
That’s why the researchers are joining with colleagues to use their
imaging technique to examine seemingly dormant galactic nuclei like
our own to see whether these galaxies lack the mini-spiral
structures seen in their brighter cousins.
Contact: Richard Pogge, (614) 292-0274;
Pogge@astronomy.ohio-state.edu
Paul Martini, (614) 292-9242;
Martini@astronomy.ohio-state.edu
Written by Pam Frost, (614) 292-9475;
Frost.18@osu.edu
