Researchers say there are two types of galaxies in our Universe. Ones that are ‘alive,’ which produce stars, and ones that are ‘dead,’ which don’t.

The main difference between the two appears to be the amount of cold gas. Alive galaxies have an abundance of this cold gas while dead galaxies don’t have much.

Researchers from the University of Cambridge and the Royal Observatory Edinburgh offer two ideas for what causes the low amount of cold gas in dead galaxies. First, the gas needed to produce new stars gets ‘sucked’ out of the galaxy by some internal or external force. The other theory points to the supply of incoming cold gas being cut off somehow. This leads to the galaxy slowly strangling to death.

Certain levels of metals contained in the dead galaxies provided researchers key ‘fingerprints.’

“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 of Cambridge’s Cavendish Laboratory and Kavli Institute of Cosmology, and the paper’s lead author. “So looking at levels of metals in dead galaxies should be able to tell us how they died.”

Researchers looked at the difference in metal content between alive and dead galaxies to figure out which of the two theories held up.

If the cold gas were quickly sucked out of the galaxy, its metal content would be the same as just before it died. And, star formation would stop.

In the strangulation theory, metal content would keep rising and eventually stop, but star formation would continue until the cold gas was used up.

galaxy strangulation

Investigating individual galaxies is impossible due to the sheer amount of time required. Instead, researchers analyzed the difference of metal content of alive and dead galaxies and found how most galaxies of average size died.

“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 new 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.

Researchers also looked at the difference in age between alive and dead galaxies. Dead galaxies were four billion years older.

Peng says the next task will be figuring out the exact cause of the galactic strangulation.

The study is published in Nature.

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