A group of researchers has partially solved a galactic murder mystery, which has baffled astronomers for years -- how do galaxies die and what kills them? According to a new study, published in the journal Nature on Wednesday, galaxies primarily die due to strangulation as they are cut off from the raw materials needed to produce new stars.

Scientists know that “alive” galaxies, such as the Milky Way, are rich in cold gas necessary to produce new stars while “dead” galaxies that do not produce stars have very low supplies of such gas. To find exactly what kills the galaxies, astronomers have come up with two hypotheses for galactic death -- either the cold gas is “sucked” out of the galaxies by internal or external forces, or the supply of that gas somehow gets blocked, eventually strangling the galaxies to death.

“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,” Yingjie Peng, a research associate at the University of Cambridge and the study’s lead author, said in a statement. “So looking at levels of metals in dead galaxies should be able to tell us how they died.”

As part of the study, the researchers used data from the Sloan Digital Sky Survey at Apache Point Observatory in New Mexico to analyze metal levels in more than 26,000 galaxies of average size.

If galaxies die after the cold gas is pulled out of them, the metal content of a dead galaxy should remain the same as before its death because star formation would also suddenly stop. But, if the galactic death is caused by strangulation, the metal content would keep rising and eventually stop, as star formation would continue until the available cold gas is completely exhausted.

“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,” Roberto Maiolino, a professor in Experimental Astrophysics at the University of Cambridge and the study’s co-author, said in the statement. “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 also examined the stellar age difference between star-forming and dead galaxies, and found an average age difference of four billion years -- the time it would take for a star-forming galaxy to be strangled to death.

“What’s next though, is figuring out what’s causing it,” Peng said. “In essence, we know the cause of death, but we don’t yet know who the murderer is, although there are a few suspects.”