Image: Markarian 573
The orangish picture focuses on the very center of the active galaxy Markarian 573. The image, which combines readings taken in visible and near-infrared light, traces a spiral of galactic dust with what scientists believe is a supermassive black hole at its center.
By Alan Boyle Science editor

Scientists have used the Hubble Space Telescope to trace the spiraling path of galactic material, swirling toward what they believe are voracious black holes at their centers. Unlike the quiet black hole thought to exist at the heart of our own galaxy, these monsters suck in an avalanche of dust — sparking a cosmic inferno. That’s just one of the latest revelations about the universe’s most violent phenomena.

Astronomers believe that black holes are created when a massive star reaches the end of its life and collapses under its own weight into something so dense that not even light can escape its gravitational pull. In addition, the crushing gravitational centers of galaxies — including our own Milky Way — are thought to be anchored by supermassive black holes.

Since black holes can’t be seen directly, scientists infer their existence from indirect effects: the motions of surrounding stars, the “gravitational lensing” that bends light from even more distant objects, and especially the blasts that occur when material swirls down the black hole. Theories predict that the swirling cosmic gas and dust should be whipped up like atoms in an accelerator, throwing off X-rays and other high-energy particles — and indeed, scientists have found patterns of emissions that match up with what they would expect from regions around black holes.

A step further
Astronomers from Ohio State University took the theories a step further, using Hubble images taken in visible light and near-infrared wavelengths. Associate Professor Richard Pogge and graduate student Paul Martini looked at 24 spiral galaxies with extremely bright centers, zeroing in on the most central 1,000 light-years. That distance is huge — about 6 quadrillion miles or 10 quadrillion kilometers — but it accounts for just 1 percent of the diameter of such galaxies.

“We looked at a region people were unable to study before,” Martini said in a written statement.

Pogge explained that starlight passes more easily through galactic dust in near-infrared wavelengths than in the visible wavelengths. Thus, by tinkering with the two views, the researchers could develop images that highlighted the dust patterns rather than the starlight.

“We found something unexpected in our initial work,” Pogge told The researchers saw swirling patterns in 20 of the 24 galaxies, indicating to them that a huge cyclone of dust was whirling into supermassive black holes. The other four galaxies just showed a “big pile of dust” with no obvious pattern, Pogge said.

Pogge and Martini theorize that the nuclear spirals represent the mechanism for “feeding” black holes so that they start throwing off radiation. In contrast, the black hole at the Milky Way’s center — estimated to be 3 million times as massive as our sun — may be inactive because it isn’t receiving “nourishment” from its surroundings.

“Before black holes become active, you have to feed them,” Pogge said.

The researchers believe that inactive galaxies might be in equilibrium, with material cleared out at a safe distance from the central black hole. Pogge said it would take some kind of disturbance to set off a feeding spiral, perhaps a tidal encounter with another galaxy, or shock waves from a cosmic eruption.

The researchers published their findings on active galaxies last December in the Astronomical Journal, and now they’re planning the next step: observing inactive galaxies to see how their central dust patterns differ.

“That’s where we’re pushing next,” Pogge said. “In fact, that’s a project which just got approved.”

But wait ... there's more
Here are some other black hole projects that have recently borne fruit, based on research reports:

Astronomers at the University of Nottingham and the University of Birmingham in England have developed evidence that supermassive black holes at galactic centers gradually put on weight by consuming a steady diet of gas and stars.

The astronomers analyzed starlight from 23 galaxies thought to have such black holes, including the neighboring Andromeda Galaxy, to estimate how old each galaxy was. Then they calculated how massive the central black holes appeared to be. They found that younger galaxies tended to have relatively less massive black holes, while older galaxies had progressively bigger black holes.

“One of the basic properties of a black hole is that material can fall into it, but can’t get out again,” the University of Nottingham’s Michael Merrifield said in a written statement. “What we seem to be seeing is the consequence of this one-way traffic, with gas and stars from the surrounding galaxy dragged in by gravity, making each black hole more and more obese as it gets older.”

The research is to be presented at the OXCAM2 conference in Oxford on March 27, and a paper on the subject will be published in the April 1 issue of the Monthly Notices of the Royal Astronomical Society.

British and French astronomers have reported strong evidence for the existence of a rare type of black hole, called a Type 2 quasar. Using NASA’s Chandra X-Ray Observatory, they discovered a powerful source of X-rays that appears to be a giant black hole, hidden from optical telescopes by a veil of obscuring material.

Image: Black hole emission
An image made by the Chandra X-ray Observatory shows a powerful source of X-rays, thought to represent emissions from a rare type of black hole. The spread-out appearance of the X-ray source is an instrumental artifact. The distribution of X-rays is consistent with this source being pointlike.
The findings come from a team led by British astronomers Andrew Fabian of the Institute of Astronomy in Cambridge and Ian Smail of the University of Durham. They used Chandra, Hubble and the James Clerk Maxwell submillimeter telescope in Hawaii. By concentrating their search near two galaxy clusters, the astronomers exploited a gravitational lensing effect that can lead to a significant brightening of distant sources.

Four new X-ray sources and seven submillimeter sources were discovered. The brightest X-ray source is concentrated in the center of a distant galaxy. The pointlike source has a deficit of low-energy X rays, consistent with absorption by a thick cloud of gas. The combination of powerful X-ray emission, absorption of low-energy X rays, and the relatively normal optical appearance of the galaxy led the scientists to conclude that the source is a strong contender to be a genuine Type 2 quasar.

Type 2 quasars have been predicted to exist by a popular model for quasars. Their discovery would confirm the so-called unified model for quasars.

A paper describing these results is to be published in the Monthly Notices of the Royal Astronomical Society.

Two jets of high-speed particles from what appears to be either a black hole or neutron star are shooting off in opposite directions and punching their way into the supernova remnant that surrounds them, stretching what was once a spherical shell of shimmering gas into an egg-shaped remnant.

Samar Safi-Harb at NASA’s Goddard Space Flight Center in Maryland probed the interaction between the jets and the surrounding remnant, known as SNR W50 with the Rossi X-ray Timing Explorer satellite. She presented her findings Thursday at the Rossi 2000 meeting at Goddard.

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