Astroboffins think they have one of the oldest murder mysteries in the universe: what kills galaxies, and how does it happen? A study just published states that the primary cause of galactic death is strangulation – a process in which the galaxies' supply of materials needed to form new stars is choked off.
There are at least one hundred billion galaxies in the universe, but they may all be sorted into two categories: the living and the dead. Living galaxies, such as our own Milky Way, are rich in cold gas – mostly hydrogen – which is needed to produce new stars. Dead galaxies, however, have too low a supply of star-forming material to perpetuate this process. What had been unknown is how living galaxies become dead ones.
Scientists have concluded this whodunit in a study with the pleasingly explicit title Strangulation as the primary mechanism for shutting down star formation in galaxies, published today in the journal Nature.
There were two prevailing hypotheses about how galactic death occurred. The first was that the cold gas needed to produce new stars was "suddenly" removed from the galaxies, by being sucked up by an external force such a passing galaxy, or an internal force such as a black hole.
The alternative hypothesis suggested that the supply of incoming gas was somehow stopped, slowly choking the galaxy to death.
Astro-boffins from the University of Cambridge and the Royal Observatory Edinburgh used data from the Sloan Digital Sky Survey, to analyse metal levels in more than 26,000 average-sized galaxies in what can loosely be described as our universal neighbourhood.
"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, the paper’s lead author. "So looking at levels of metals in dead galaxies should be able to tell us how they died."
If galaxies are killed by outflows which pull the cold gas out of the galaxies "suddenly" – over a mere four billion years – then the metal content of the deceased galaxy should be the same as just before it passed away, as the star formation would "abruptly" stop.
If the galaxies were strangled, however, the metal content would keep rising, as star formation could continue until the existing gas gets completely used up.
A University of Cambridge press release notes that "it is not possible to analyse individual galaxies due to the massive timescales involved," but "by statistically investigating the difference of metal content of alive and dead galaxies [... researchers are] able to determine the cause of death for most galaxies of average size."
"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 managed to independently test their results by measuring the stellar age difference between star-forming and dead galaxies, independent of metal levels, and found an average age difference of four billion years – which exactly correlates with the time it would take for a star-forming galaxy to be strangled to death.
"This is the first conclusive evidence that galaxies are being strangled to death,” said Peng. "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."
Talking to The Register, Dr Peng fessed up to the identites of those whom he was supicious of.
"First, I would consider the galaxy environment effect. Due to a crowded intergalactic environment, a star-forming galaxy could fall into the hot halos which surround galaxies. This would halt the inflow of cold gas to the galaxy, suffocating it to death."
The external force could also be tidal stripping – which occurs when a galaxy interacts with a more massive one – or ram pressure stripping, which happens as a galaxy moves within a galaxy cluster. This can cause a strong drag force on the galaxy which removes the cold gas.
"Another suspect would be the supermassive black holes which lie at the centre of all galaxies," Peng continued.
Feedback from a supermassive black hole can also launch collimated jets that shock and heat the circum-galactic medium, preventing gas from flowing towards the galaxy. Supermassive black hole feedback occurs when radiation from the accretion disk of a central supermassive black hole drives a wind of gas out of its host galaxy.
The timescale for this scenario should be shorter than one billion years, depending on how strong the outflow is. In contrast, the timescale for strangulation is roughly four billion years.
Dr Peng says he is hopeful that the study's methods can be applied to galaxies larger than 1011 solar masses, such as those the study focused upon, as the metal content of the most massive galaxies provokes further interesting questions. ®