As murder mysteries go, it’s a big one: how do galaxies die and what kills them? A new study, published in the journal Nature, has found that the primary cause of galactic death is strangulation, which occurs after galaxies are cut off from the raw materials needed to make new stars.
Astronomers 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.
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.
“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.”
The researchers were then able to independently test their results by looking at the stellar age difference between star-forming and dead galaxies, independent of metal levels, and found an average age difference of four billion years – this is in agreement with the time it would take for a star-forming galaxy to be strangled to death, as inferred from the metallicity analysis.
