Death spiral: why the universe is producing fewer stars
Two new studies attempt to explain how gases crucial to star formation are being expelled from galaxies, turning fertile spiral galaxies into fallow elliptical ones.
Why isn't the universe producing as many stars as it once did, even though it still holds a respectable amount of hydrogen gas, the raw material for stars?Skip to next paragraph
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Researchers have pointed to enormous black holes in the centers of galaxies and dense collections of massive, hot young stars as accomplices in the relentless decline, which has been underway during the last 10 billion years. The processes are thought to propel massive galaxies like the Milky Way on an inevitable evolutionary journey from hotbeds of star formation early in the universe's history to galactic geezers – giant elliptical galaxies whose stars are ancient with no new ones in sight.
Now, two teams of astronomers have taken cosmic snapshots of these processes at work in unprecedented detail, opening important windows on the mechanisms contributing to the decline of star formation rates in galaxies.
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One study, set to appear in Friday's issue of the journal Science, focused on a galaxy more than 1.5 billion light-years away as a supermassive black hole at its center was waking up following a dormant period. Jets of plasma erupting near the poles of the black hole and hurtling into space at nearly the speed of light were bulldozing massive amounts of hydrogen gas – 16 to 20 suns' worth – out of the galaxy each year.
Another team, whose results were published in July, has seen large amounts of hydrogen gas streaming from another galaxy 11.5 million light-years away. There, stellar winds and the explosions of hot young stars gathered in dense clusters near the center of the galaxy are causing nine suns' worth of hydrogen to flow out of the galaxy each year.
The findings could help answer several lingering questions.
For years, astronomers have detected elements heavier than hydrogen and helium between galaxies. Those elements require stellar fusion to form, so their existence in a place with no stars was a mystery.
"We always wondered where that comes from," says Juergen Ott, a researcher at the National Radio Astronomy Observatory's Very Large Array in Socorro, N.M.
Evidence had been building to support the idea that supermassive black holes or stellar winds might be responsible. Now researchers have watched as it happens.
At the same time, he says, computer simulations of galaxy evolution during early periods in the universe's history point to a need for some way to shut off star formation. Otherwise the galaxies astronomers see today would look much different. During their periods of activity, the black holes' bulldozing jets could be just such an "off" switch.
And some mechanism has allowed galaxies like the Milky Way to continue forming stars long after theories suggest the galaxy should have run out of gas and star formation should have ended. Researchers suspect that some gas pushed outward by the jets moves too slowly to escape its galaxy's gravity, and eventually falls back in to provide the raw material for new stars.