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.

This collage of visible and infrared images shows the clouds of interstellar dust and gas falling into the center of the active galaxy Markarian 573. This galaxy has an unusually bright nucleus that was first noted in 1978 by the late Armenian astronomer B.E. Markarian who cataloged this and similar galaxies. The black-and-white image on the upper left was taken with the NASA Hubble Space Telescope's visible-light camera, the Wide Field and Planetary Camera 2 (WFPC2). It shows the main body of this nearly face-on spiral galaxy. The blue box at the center surrounds a narrower view taken by the Hubble telescope's infrared camera, the Near Infrared Camera and Multi-Object Spectrometer (NICMOS). By forming the ratio of the visible and near-infrared images, shown in the picture at the lower right, the central starlight and bright nucleus of the galaxy are suppressed, revealing clouds of dust (dark regions) strung along a pair of broken spiral arms surrounding the active nucleus at the very center. Gas and dust in these arms is thought to be flowing into the center where it will eventually feed an active supermassive black hole. Gas that has already reached the center falls into this black hole and heats up violently, causing it to shine brightly at wavelengths from X-rays to radio waves. Photo: Courtesy OSU

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.

The six images in this montage show dusty spirals of interstellar gas found in the hearts of active galaxies by the Hubble Space Telescope's visible and near-infrared cameras. By combining visible and near-infrared images of the centers of these galaxies, astronomers can supress the light from the billions of stars in the centers, revealing dark clouds of interstellar dust and gas. These dark clouds are seen spiralling inwards towards the very centers of the galaxies where it may ultimately be consumed by a supermassive black hole at the center. As the gas and dust falls into the black hole, it is heated violently, causing it to shine out at wavelengths from X-rays to radio waves, making the nucleus appear nearly as bright as the billions of stars around it. It is this brilliant release of energy the centers of these galaxies that has made them objects of particular interest. Photo: Courtesy OSU

"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.