CAMBRIDGE, England, May 14 (UPI) -- Just as animals need oxygen to survive, galaxies need hydrogen and other cold gases. Those that are cut off from the materials they need to make new stars are considered "dead." Roughly half of all galaxies are dead, no longer making new stars.
But until recently, scientists weren't entirely sure why or how galaxies are killed off, robbed of their star-making abilities. Previously, astronomers offered two basic theories. Cold, life-giving gases are either sucked out by external forces, or internal mechanisms halt the influx of replenishing cold gases.
To solve the mystery, researchers came up with a novel idea -- trace the levels of metals in dead galaxies.
"Metals are a powerful tracer of the history of star formation: the more stars that are formed by a galaxy, the more metal content you'll see," lead study author Yingjie Peng, an astronomer at the University of Cambridge's Cavendish Laboratory and Kavli Institute of Cosmology, explained in a press release. "So looking at levels of metals in dead galaxies should be able to tell us how they died."
Astronomers theorized that if an external force quickly sucked out a galaxy's star-fertilizing gases, then metal levels would be equal before and after death. If, on the other hand, galaxies die by slow strangulation -- their gas supply slowly cut off by internal circumstances -- then metal levels would continue to rise and then abruptly stop when no more gas remained.
"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," co-author Roberto Maiolino said. "This isn't what we'd expect to see in the case of sudden gas removal, but it is consistent with the strangulation scenario."
Their metal analysis suggested it takes roughly four billion years to strangle a galaxy to death. That's the age difference between dead and living galaxies.
"This is the first conclusive evidence that galaxies are being strangled to death," Peng said. "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."
The study was published this week in the journal Nature Letters.