It's the ultimate whodunnit: what kills galaxies? A new study, published today in the journal Nature, names strangulation as the primary cause of galactic death.
Galactic death happens after galaxies are unable to access the raw materials they need to make new stars. Researchers from the University of Cambridge and the Royal Observatory Edinburgh now say that metal levels of dead galaxies provide the clues we need to solve the mystery, functioning like forensic evidence and allowing scientists to determine the cause of galactic death.
Galaxies in our Universe are either alive or dead. Galaxies which are alive produce stars, and dead galaxies do not. Their numbers in the Universe are about equal.
We already know that living galaxies like the Milky Way contain abundant cold gases like hydrogen. These are essential to the production of new stars. Intuitively, dead galaxies have less in the way of cold gases. However, these levels did not explain how galaxies died in the first place.
Astronomers offer two hypotheses for galactic cause of death. One is that the cold gas galaxies need to create new stars is somehow suddenly 'sucked out' of them-by either external or internal forces. The other is that the galactic supply of cold gas is somehow halted; this would strangle a galaxy to death over a lengthy period of time.
The Cambridge team analyzed metal levels in more than 26,000 galaxies of average size relatively near to us in the Universe. The data was from the Sloan Digital Sky Survey, and with it the team hoped to solve this cause of death mystery.
"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."
If the first theory is correct and the process is sudden, the metal content of dead galaxies should resemble that of their living neighbors as the star formation would suddenly cease. However, if galaxy strangulation was the issue, we could expect to see levels of metal in dying galaxies rising until the cold gases were entirely gone.
The immense amount of time involved in the dying process means that individual analysis is impossible. However, statistical analysis of the differing metal content of galaxies living and dead allows the researchers to determine the cause of death for most average-sized galaxies.
"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," coauthor of the study Professor Roberto Maiolino, says. "This isn't what we'd expect to see in the case of sudden gas removal, but it is consistent with the strangulation scenario."
Next the team compared stellar age differences of living and dead galaxies without regard to metal level differences in order to test their results. The scientists found an average age difference of four billion years which supports the idea of slow strangulation galactic death, as the metallicity analysis suggests.
"This is the first conclusive evidence that galaxies are being strangled to death. 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."
