NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI). Looking at these galaxies, it is hard to believe some are bursting with life and some are 'dead.'

If you've ever felt suffocated because no one will let you grow, it may be a comfort to know you have some galactic-sized friends. Galaxies that can't grow start to die and Cambridge University astronomers think they have established that “dead” galaxies die from suffocation, their supply of the life-giving gasses cut off.

Galaxies that are no longer forming stars are referred to by astronomers as dead. The dead galaxies still emit light, and could be teaming with life around existing suns. However, without a collision with some smaller galaxy, the trajectory for these dead galaxies is set.

If galactic death occurs through a lack of gas that would otherwise combine to form stars, why are some galaxies devoid of gas? Competing theories are that some process sucks the gas out of unfortunate galaxies, or that the supply of new, in-falling gas is cut off, leading galaxies to simply use up what they had before dying of suffocation. Lead author Dr. Yingjie Peng proposed that determining which of these is true could be determined by looking at the metal content of stars in dead galaxies.

The first stars were formed almost entirely of hydrogen and helium. In their lifetime, they fused these to form elements such as beryllium and carbon. In their death throes, these stars forged the heavier elements that enabled the formation of planets such as our own around subsequent generations of stars.

Some of these heavier elements, known to astronomers as metals, became incorporated into subsequent stars, and can be detected in their light.

“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,” says Peng. “So looking at levels of metals in dead galaxies should be able to tell us how they died.”

A galaxy killed by the vampiric sucking of its life-giving gas would be expected to have the same metal content as just before it died, Peng reasoned. However, a strangled galaxy would see its metal content slowly rise, as the gas still present when the suffocation occurred would take a while to get used up.

The two paths for galaxy death, depending on the cause. Credit: Peng et al, Nature.

In Nature, Peng reports on the metal content of 26,000 average-sized galaxies located within 1.5 billion light-years. “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 Professor Roberto Maiolino, co-author of the paper. “This isn’t what we’d expect to see in the case of sudden gas removal, but it is consistent with the strangulation scenario.” The authors acknowledge their work excluded large galaxies, which may die by other means.

Moreover, the age difference of stars in live and dead galaxies averages four billion years, a result consistent with suffocation models. Peng admits, “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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