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By Devin Coldewey

The galaxies we can see tend to fall into one of two categories: "alive," meaning a galaxy is still producing stars, and "dead," if not. But how does a galaxy die, exactly? A new study by astronomers at the University of Cambridge, published in Nature, says that dead galaxies were "strangled" to death slowly, rather than being suddenly rendered sterile by a cosmic event or interfering stellar object.

One of the things necessary to create new stars is cold hydrogen (i.e. not hot and exploding) — in astronomical amounts, of course. By tracking the levels of this and other requisite elements in thousands of galaxies spied in the Sloan Digital Sky Survey, researchers could tell whether that star-forming material slowly ran out or was quickly sucked out or otherwise rendered unusable.

Artist's impression of how galaxies may interact and strangle themselves or one another.Re-Active

"We found that for a given stellar mass, the metal content of a dead galaxy is significantly higher than a star-forming galaxy of similar mass," said study co-author Roberto Maiolino. "This isn't what we'd expect to see in the case of sudden gas removal, but it is consistent with the strangulation scenario."

Other variables backed up the strangulation hypothesis, such as that star-forming galaxies were, on average, about 4 billion years younger than dead ones, which tallies with the researchers' estimates of how fast the galaxies' supplies would be consumed. But what the "strangler" is, or whether there even is one, is still unknown.

"This is the first conclusive evidence that galaxies are being strangled to death," said lead author Yingjie Peng. "What's next though, is figuring out what's causing it. In essence, we know the cause of death, but we don't yet know who the murderer is, although there are a few suspects."

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