How Galaxies Strangle to Death

It has long been a mystery to astronomers as to how galaxies die and what kills them exactly. Now, a new study has found that most galaxies quite literally strangle to death.

Described in the journal Nature, when galaxies are cut off from the raw materials needed to make new stars, that's when they meet their maker, so to speak. Researchers from the University of Cambridge and the Royal Observatory Edinburgh came to this realization after finding levels of metals contained in dead galaxies, which provided key "fingerprints" to determining the cause of their demise.

Quite plainly, there are two types of galaxies in the Universe: roughly half are "alive" galaxies that produce stars, and the other half are "dead" ones which don't. Lucky for us, our own Milky Way galaxy is still alive and well, rich in the cold gas - mostly hydrogen - needed to produce new stars. Meanwhile, dead galaxies have a very low supply of star-producing gas.

Until now, the reasoning behind dead galaxies was unknown. Two popular theories for this phenomenon were either that the cold gas needed to form new stars was suddenly being "sucked" out of the galaxies by internal or external forces, or the supply of incoming cold gas was somehow stopped, slowly strangling the galaxy to death over a prolonged period of time.

In order to put the mystery to rest, the research team used data from the Sloan Digital Sky Survey to analyze metal levels in more than 26,000 average-sized galaxies located in our corner of the Universe.

"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," study lead author Dr. Yingjie Peng said in a news release. "So looking at levels of metals in dead galaxies should be able to tell us how they died."

If the first theory is correct, then a dead galaxy would have the same metal content as an alive one, given that star formation would simply come to a halt. However, if strangulation were the culprit, then the galaxy's metal content would keep rising and eventually stop, as star formation could continue until the existing cold gas gets completely used up.

"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, a co-author of the study. "This isn't what we'd expect to see in the case of sudden gas removal, but it is consistent with the strangulation scenario."

By further examining their results, the team concluded that four billion years is about the time it would take for a star-forming galaxy to be strangled to death - a long, slow, painful death.

"This is the first conclusive evidence that galaxies are being strangled to death," Peng concluded. "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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