There are two types of galaxies: ‘alive’ and ‘dead’ ones. Those galaxies that are still alive are called so because they still produce stars inside, while the dead ones are stripped and devoid of their stellar nurseries. In a case of forensic astronomy of the utmost importance, a team at the University of Cambridge and the Royal Observatory Edinburgh has revealed the leading cause of death for most deceased galaxies of average size: death by strangulation. What remains now is to identify the killer, the researchers say.

Artist’s impression of one of the possible galaxy strangulation mechanisms: star-forming galaxies (fed by gas inflows) are accreted into a massive hot halo, which ‘strangles’ them and leads to their death. Image: Cambridge University

Alive galaxies, such as our own Milky Way, are rich in cold gas like hydrogen which are essential to birthing new stars. Stars begin as vast clouds of cold molecular hydrogen and helium left over from the Big Bang. These vast clouds can be hundreds of light years across and contain the raw material for thousands or even millions of times the mass of our Sun. Sometimes these clouds are seeded with heavier elements which were fused fore in stars now long dead. When there’s little cold gas, scientists know that the galaxy in question is dead, for stars are its vital force and if its not replenished it will surely succumb.

There are two scenarios under which the cold gas might become depleted: 1) the cold gas is suddenly sucked out of the galaxy by some internal or external force; 2) the local supply of cold gas is stopped, which is like cutting a person’s air supply – death by strangulation. There’s a way out of this dilemma: by analyzing metal levels in the galaxies. With this in mind, using the Sloan Digital Sky Survey the team analyzed metal levels in more than 26,000 average-sized galaxies.

“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 there’s an outflow which suddenly pulls out the cold gas, then we’d expect star formation to cease immediately (within a couple millions years in a cosmic time frame). This in turn implies detecting metal levels for a dead galaxies that are the same as those right before it died. In the case of strangulation, on the other hand, the metal concentration should keep rising albeit at a slower rate, then stop.

Using a statistical method, the team was able to infer the  the difference of metal content of alive and dead galaxies. Their analysis shows that for most galaxies of average size, strangulation is indeed the cause of death. To verify their model, the researchers then compared the stellar age between star-forming and dead galaxies, regardless of metal levels, and found there was a four billion years difference. This is about the time it would take for a star-forming galaxy to become slowly strangled. The study, published in Nature, is the first to provide conclusive evidence that galaxies are strangled to death.

“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.”