British astronomers have found out the answer to an astronomical mystery -- how do galaxies die? Evidence shows that they are "strangled to death," which occurs after galaxies are cut off from the raw materials needed to make new stars.

There are two types of galaxies in the Universe: roughly half are "alive" galaxies which produce stars, while the other half are "dead" ones which don't. Alive galaxies such as the Milky Way are rich in the cold gas -- mostly hydrogen -- needed to produce new stars, while dead galaxies have very low supplies.

Previous studies have come up with two main hypotheses for galactic death: either the cold gas needed to produce new stars is suddenly "sucked" out of the galaxies by internal or external forces, or the supply of incoming cold gas is somehow stopped, slowly strangling the galaxy to death over a prolonged period of time, according to the report recently published by the Journal Nature.

To answer the question, researchers from the University of Cambridge and the Royal Observatory Edinburgh 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, according to the report.

Researchers have found that levels of metals contained in dead galaxies provide key "fingerprints," making it possible to determine the cause of death.

If galaxies are killed by outflows suddenly pulling the cold gas out of the galaxies, then the metal content of a dead galaxy should be the same as just before it died, as star formation would abruptly stop.

In the case of death by strangulation however, the metal content of the galaxy would keep rising and eventually stop, as star formation could continue until the existing cold gas gets completely used up.

Professor Roberto Maiolino, co-author of the new study, said they 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, and "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 researchers were then able to independently test their results by looking at the stellar age difference between star-forming and dead galaxies. They said the conclusion is in agreement with the time it would take for a star-forming galaxy to be strangled to death as well.

Dr. Yingjie Peng of Cambridge University, the paper's lead author, said the next step is to figure out what's causing such strangulation. "We know the cause of death, but we don't yet know who the murderer is, although there are a few suspects."
Source: XINHUA