Black holes may shut down stellar factories

Galaxies stop forming stars despite access to raw materials

DEAD GALAXY  The giant elliptical galaxy NGC 5044, seen in this composite image, is filled with cold gas, yet it is not making stars. New research suggests a black hole may be to blame. The galaxy is filled with threads of warm hydrogen gas (red) that snake through an atmosphere of hot gas (blue). Stars appear white.  

Digitized Sky Survey, Chandra/NASA , Southern Observatory for Astrophysical Research, Very Large Array

Supermassive black holes might slowly suffocate galaxies. The suggestion runs counter to astronomers’ notion that galaxies stop forming stars when they run out of cold gas. But researchers have found a cache of galaxies loaded with cold gas that aren’t making stars. The team’s observations suggest that the galaxies’ central black holes stirred up the gas and shut down the stellar assembly lines.

Over the last decade, astronomers have learned that black holes can drive the fates of entire galaxies. “It’s a bit like an orange affecting the Earth,” says Andrew Fabian, an astronomer at the University of Cambridge. “These black holes are enormously powerful. They’re emerging as an important factor in the way galaxies operate.”

Stanford astrophysicist Norbert Werner and colleagues looked at eight giant elliptical galaxies, all within about 100 million light-years of Earth. Giant ellipticals are the retirement homes of the universe, devoid of vigorous star birth and filled with older stars. So Werner was surprised to see that six of the eight galaxies were filled with cold gas.

NGC 1399, one of two galaxies without cold gas in a new study, shows off its powerful jets in this composite image. The stars (white) in the galaxy sit in a pool of hot gas (blue). The jets (purple) are powered by the supermassive black hole in the galaxy’s center. Digitized Sky Survey, Chandra/NASA, Very Large Array
By combining infrared and X-ray images, the team thinks it’s solved the twin mysteries of the origin of the cold gas and the reason it isn’t collapsing. Filaments of hot gas — over a million degrees Celsius — thread through the pools of cold gas, Werner’s team discovered. The hot gas appears to cool off as it dumps energy into the cold gas, and the extra energy prevents the cold gas from forming stars.

But the hot gas isn’t acting alone. It’s just a tool of the supermassive black holes in the galaxies’ centers.

As the black holes pull in material, they propel jets of protons and electrons at nearly the speed of light thousands of light-years into intergalactic space. The jets carve out gas bubbles in the galaxies’ atmospheres.  Like a boiling pot of water, the bubbles churn up the galaxies’ gas, Werner’s team suggests, dragging around the hot gas filaments and preventing the cold gas from collapsing into new stars.

In the two remaining galaxies, the team saw little or no cold gas. The difference appears to lie with the black holes’ activity. Compared with the galaxies with cold gas, the jets in these galaxies are far more powerful, suggesting that the black holes have consumed all the cold gas and are spitting it back out beyond the reach of the rest of the galaxy, the researchers report February 24 in Monthly Notices of the Royal Astronomical Society.

The emerging picture, Werner suggests, is that the cold gas is part of an energy cycle operating in galaxies. The black holes suck in cold gas; jets erupt from the galactic centers; the jets stir and heat gas throughout each galaxy; and the hot gas cools to make more cold gas, some of which finds its way back to the black hole.

Timothy Heckman, an astrophysicist at Johns Hopkins University, interprets the findings more cautiously. “They have a pretty small sample which is not directly shedding light on why the gas is cooling in some galaxies but not others,” he says. He adds that he is not surprised that the researchers detected cold gas: Radio telescopes had previously shown hints of cold gas in other giant ellipticals. Nevertheless, the energy transfer between the hot and cold gas intrigues him.

Both Heckman and Werner want to investigate what’s happening in the gas right next to the black hole. How the gas feeds the black hole, Werner says, appears to affect the energy balance in parts of the galaxy hundreds of thousands of light-years away. New telescopes such as the Atacama Large Millimeter/submillimeter Array, or ALMA, and ASTRO-H, an orbiting telescope under development by the Japan Aerospace Exploration Agency and scheduled to launch in 2015, can give astronomers a peek at how gas behaves in a black hole’s vicinity. Such facilities, Heckman adds, will go a long way toward helping astronomers figure out if black holes are truly galactic assassins. 

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