IT’S A WHO-DUNIT on a cosmic scale: What is killing the galaxies? Even they, bright beacons of the skies, meet their eventual demise.
New analysis from a group of astronomers at the University of Cambridge and the Royal Observatory Edinburgh points to the cause. Most galaxies cease producing new stars as their fuel runs low and they suffer a slow suffocation. The study, released today in Nature, challenges the traditional view that star formation stops suddenly.
“It’s a completely new approach,” says Yingjie Peng, an astronomer at the Kavli Institute for Cosmology and one of the authors. “We are the first to trying answering this mystery by looking at the materials, the metals, in the stars.”
As stars consume fuel—hydrogen gas—they fuse materials into heavier elements, like magnesium or iron. The abundance of these metals is a clue to how long the solar fires in that galaxy have burned.
With data from the Sloan Digital Sky Survey, Peng and his collaborators, Roberto Maiolino and Rachel Cochrane, gauged the metal content of more than 26,000 average-sized galaxies. If a galaxy died suddenly, through the gas being sucked into a supermassive black hole at its center for instance, then metal formation would stop quickly. Its current metal levels should match those just prior to death. But if, instead of going out in a burst of glory, most galaxies wasted away, then metal content should keep rising slowly until the fuel was all burned out.
Average-sized galaxies that were extinguished, they found, had significantly higher levels of metal than similar-sized galaxies that were still forming stars. “This isn’t what we’d expect to see in the case of sudden gas removal, but it is consistent with the strangulation scenario,” Maiolino said in a statement.
Not all galaxies live and die in the same way, however. Like people, they come in different flavors. “There are basically two populations,” says Andrea Cattaneo, an astronomer at the Observatoire de Paris and the theorist who predicted the strangulation tested by Peng and his team.
The enormous ones burn bright and run out of steam quickly, like rockstars who live fast and die young. And, like celebrities, these get the most attention and are the best studied. But the majority of mortals, who live without being particularly brilliant or rich, count on making it to retirement age. Their galactic counterparts live on the order of 4 billion years (after strangulation has begun)—which seems a ripe old age considering the universe is just about 13 billion years old.
But, even if the cause of death for most mid-sized galaxies seems solved, Peng says, the agent of death is still a mystery. This, he says, is a puzzle for the next chapter.