"Dead" galaxies — so called because they're no longer producing stars — probably die from strangulation when they've run out of the raw materials for star formation, researchers have discovered.

"Living" galaxies, such as our own Milky Way, have abundant gases, predominantly hydrogen, from which new stars are created, whereas dead ones have run out of it, they say.

The mystery has been where that gas has gone.

There have been two main schools of thought on that; one holds the gas has been rapidly "sucked" away from the galaxy through internal or external forces such as the gravitational pull of a passing galaxy, leading to a relatively quick galactic death.

The other theory holds that fresh incoming supplies of gas are somehow blocked, causing the galaxy to slowly strangle to death.

To try and pin down an answer, researchers analyzed data on around 26,000 galaxies in our neighborhood of the universe, looking for metal levels within them.

"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 Yingjie Peng of the University of Cambridge in England, the study's lead author. "So looking at levels of metals in dead galaxies should be able to tell us how they died."

If a galaxy died quickly through sudden loss of star-forming gas, metal formation would stop quickly, the researchers point out.

If, on the other hand, the galaxy slowly withered and died through being "starved" of incoming gas, then metal levels should continue rising until the end as the galaxy consumed every bit of gas it still possessed, they said.

Dead galaxies in their survey had much higher amounts of metals than the live galaxies still forming stars displayed, a finding consistent with how strangulation would lead galaxies to evolve over time until their death, the researchers say.

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

The strangulation theory seems to hold for galaxies of masses up to around 100 billion times that of our sun, the researchers note; for galaxies larger than that, there was insufficient evidence to strongly point to either the strangulation or the sudden-removal hypothesis.

Although they are confident they've identified strangulation as the most common cause of galactic death, they say they need to identify 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," Peng says.