Image copyright SPL Image caption Our own Milky Way and the nearby Andromeda (pictured) are both blue, star-forming galaxies

When galaxies stop making stars, their death is usually a slow process that chokes them of the necessary cool gases over about four billion years.

That is the conclusion of astronomers who surveyed thousands of galaxies, living and dead, to assess whether the transition is rapid or slow.

In the dead galaxies they detected high levels of metals, which build up during star formation and point to a slow strangulation process.

The work appears in the journal Nature.

"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," said Dr Yingjie Peng from the University of Cambridge, the paper's lead author.

"So looking at levels of metals in dead galaxies should be able to tell us how they died."

If a galaxy's death was quick and violent, with the cool gas that feeds star formation stripped away by internal or external forces, it would immediately stop forming stars and its metal content would remain the same.

In essence, we know the cause of death, but we don't yet know who the murderer is Dr Yingjie Peng, University of Cambridge

On the other hand, if the galaxy is cut off from its supply of gas but it continues to use up what remains, metal would continue to build up until the galaxy eventually "suffocates".

Slow way to go

In a commentary for Nature, fellow astronomer Andrea Cattaneo from the Observatoire de Paris compared this tell-tale evidence to the high levels of carbon dioxide seen in a strangled human body.

"During [strangulation], the victim uses up oxygen in the lungs but keeps producing carbon dioxide, which remains trapped in the body," wrote Dr Cattaneo.

"Instead of building up CO2, the strangled galaxies accumulate metals - elements heavier than helium - produced by massive stars."

The team led by Dr Peng spotted that accumulation of metal when they compared the spectrum of light emitted by 23,000 red, passive galaxies and 4,000 blue, star-forming ones.

They used data from the Sloan Digital Sky Survey - a vast collection of detailed, multi-coloured images spanning a third of the sky, which has been used to compile a precise 3D map of the universe.

Image copyright SPL Image caption Red galaxies like NGC 2787 have seen the last of their star-forming days

On average, the living, star-forming galaxies were four billion years younger than the dead ones. This matches the amount of time that the astronomers calculate would be needed for the galaxies to burn up their remaining gas supply during the strangulation.

It is also considerably longer than the four minutes it takes to strangle a human - but the analogy stands.

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

One of the likely culprits is overcrowding; if a galaxy is in a busy group or cluster, its collection of gas from from the surrounding environment might face disruption, commencing the strangulation process.

Within galactic clusters, the astronomers saw even more pronounced differences in metal content, which supports this idea.

For very big galaxies, which are relatively rare, the differences dwindle, suggesting that violent galactic deaths are more common at those extremes.