A new study, published in the science journal Nature, has found that galaxies filling half of the universe died due to strangulation after losing access to the raw materials essential for making new stars.
Researchers from the University of Cambridge and the Royal Observatory, Edinburgh, determined the cause of death by looking at the levels of metals contained in the carcasses of the galaxies.
Dead galaxies make up roughly half of the observable universe. Alive ones, such as our Milky Way, are still producing stars and are rich in cold gases like Hydrogen, a key element needed for building stars.
According to the astronomers either the galaxies suddenly lost their star building cold gases for internal or external reasons, or they were cut off from the supply of the gas somehow.
To solve the mystery scientists analysed 26 thousand mid-size galaxies in the universe and examined the metal levels of each.
“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, the paper’s lead author, in the press release of Cambridge University.
“So looking at levels of metals in dead galaxies should be able to tell us how they died.”
The metal contents of dead galaxies should be unchanged if they died suddenly. On the other hand, if the galaxies died due to cold gas running out, metal levels should have continued to rise until all the cold gas was consumed.
“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 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.”