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Supermassive Black Holes Can Kill Whole Galaxies

Wolfing down your food can be bad for your health, but who knew the same principle could apply to an entire galaxy? Astrophysicists have found that when a supermassive black hole quickly devours gas and dust, it can generate enough radiation to abort all the embryonic stars in the surrounding galaxy. It's not clear what this means for life's ability to take hold in such a bleak environment, but the research shows that the process might have determined the fates of many of the large galaxies in the universe.

Since the mid 1990s, astronomers have known that every galaxy in the universe harbors a supermassive black hole at its center. These monsters, some containing the mass of billions of suns, can swallow gigantic volumes of gas and dust. When so much matter nears the maw and crunches together, enormous amounts of radiation blast far out into space.

Astronomers have also known for years that some large galaxies don't seem to be forming any new stars; they are full of aging suns. Is the extreme activity of supermassive black holes connected to the dearth of star making?

A team led by astrophysicist Asa Bluck the University of Nottingham in the United Kingdom decided to investigate. They selected about 100 galaxies containing active supermassive black holes. Over the past 5 years, they have studied those galaxies in optical, near-infrared, and x-ray wavelengths using orbiting and ground-based telescopes. In particular, the researchers estimated the rate of growth of the black holes from the amounts of radiation they were emitting. The team also compared the masses of the black holes with the total masses of their host galaxies to determine how much matter they had swallowed.

On Friday at a meeting of the Royal Astronomical Society in Glasgow, U. K., Bluck will report that the most active supermassive black holes release staggering amounts of radiation during their most energetic periods, which can last hundreds of millions of years—enough, he says, "to strip apart every massive galaxy in the universe at least 25 times over." The x-ray emissions from these monsters, he adds, dwarf the combined x-radiation from every other source in the universe. Such emissions can sweep away virtually all of the cold, compact dust in a galaxy, thereby preventing that dust from ever congealing into new stars. He adds that of the 100 galaxies the team surveyed, at least one-third have lost their star-making capacity, presumably due to the radiation from their central black holes. In the other galaxies, the supermassive black holes are less active, and the star-making process is evident.

The eventual impact on the health of the galaxy is profound. "Without new stars to replace them," old stars will age, redden, and eventually fade out of existence," Bluck says. "The galaxies will grow dark and die as well."

If life evolves during the central black hole's active stage, then the powerful radiation would almost certainly obliterate it. But after the black hole settles down, Bluck says, conditions would be far more hospitable; no new stars would form and disrupt the galactic neighborhood by churning up gas and dust and emitting their own radiation.

The research represents a "big step forward in the quest to determine why some galaxies aren't making new stars," says astronomer Steven Willner of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. Although the findings confirm that active supermassive black holes emit plenty of x-rays, he explains, whether the radiation actually empties galaxies of their gas "is a good hypothesis—though still far from a certainty."