Quiet, Old Black Holes Actually Rumble with Roar of Particle Jets

COLLEGE PARK, Md. -- Older supermassive black holes that were once thought to be quiet, actually generate high energy jets of particles powerful enough to put a brake on the formation of new stars, say a team of researchers from the University of Maryland, Stanford University, the University of Cambridge and New Mexico State University.

A new study using data from NASA's Chandra X-ray Observatory shows that most of the energy released by matter falling toward a supermassive black hole is in the form of high-energy jets traveling at near the speed of light away from the black hole. The jets heat up clouds of gas in the galaxy that otherwise would have gradually cooled and coalesced into new stars.

"We see enough energy coming out of these black holes to completely stifle star formation," said study co-author Christopher Reynolds of the University of Maryland. "This is an exciting finding because it demonstrates a direct interaction between black holes and galaxy formation, which is a hot topic in astrophysics right now."

The supermassive black holes studied by the team are relatively old and had been considered "boring" because they generate much less radiation and much less scientific attention than quasars -- rapidly growing supermassive black holes seen in the early Universe.

Reynolds, and first author Steve Allen of Stanford University, independently became interested in studying these seemingly inactive giants, precisely because they were so subdued.

"They attracted our attention because they were too boring," said Reynolds. He explained that, when material is pulled into a black hole, that material is converted to energy that comes out as radiation. However, these black holes were not generating as much radiation as would be expected from the amount of material, or fuel falling toward them.

"There had to be something else happening that would explain this discrepancy," he said. Reynolds said he decided to study this type of black hole using data from Chandra that already existed in the public domain. In discussing the idea with colleagues, he discovered that Allen had just begun such a study. "Rather than trying to compete, I decided join up," he said.

Working together with three other scientists (Andy Fabian of the University of Cambridge, Greg Taylor of New Mexico State University and Robert Dunn, a graduate student at Cambridge) Allen and Reynolds selected nine quiet, older supermassive black holes that could be studied in detail because they were in "nearby" galaxies.

The team discovered that these black holes were actually active and very efficient "jet" engines. The "missing" energy was not coming out as radiation (x-rays, light, etc.), but as jets of high-energy particles that create huge bubbles, or cavities, in the interstellar gas of the galaxies.

The high efficiency of the black hole production of energy via particle jets was calculated in two steps. First, Chandra images of the inner regions of the galaxies were used to estimate the amount of fuel available for the black hole; then, Chandra images were used to estimate the power required to produce the cavities.

"Just as with cars, it's critical to know the fuel efficiency of black holes," said lead study author Allen of Stanford University. "Without this information, we cannot figure out what is going on under the hood, so to speak, or what the engine can do."

The study results, which will appear in an upcoming issue of the Monthly Notices of the Royal Astronomical Society, also shed new light on how black holes so efficiently produce such particle jets. A significant portion of the gas pulled into black holes must approach the event horizon where it is used with high efficiency to produce the jets.

"Though we don't definitely know the mechanism by which these jets are produced, our findings are supportive of the idea that magnetic field lines interact in a way that causes them to work like the elastic bands of a giant sling shot throwing incoming material back out from the black hole," said Reynolds.

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