According to a new study published in the journal Nature, the primary mechanism for stopping star formation in galaxies is the so-called ‘strangulation,’ in which the supply of cold gas to the galaxy is halted.
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 credit: University of Cambridge.
Galaxies are broadly divided into two main types – star-forming and quiescent. The primary mechanism responsible for terminating star formation in galaxies and transforming them into quiescent systems is still unclear.
In order to finally solve this mystery, a team of astronomers led by Dr Yingjie Peng of Cambridge University’s Cavendish Laboratory and Kavli Institute of Cosmology used data from the Sloan Digital Sky Survey to analyze metal levels in more than 26,000 local 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. So looking at levels of metals in dead galaxies should be able to tell us how they died,” explained Dr Peng, who is the lead author on the study.
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.
While it is not possible to analyze individual galaxies due to the massive timescales involved, by statistically investigating the difference of metal content of alive and dead galaxies, Dr Peng and his colleagues, Prof Roberto Maiolino and Rachel Cochrane of Cambridge University’s Cavendish Laboratory, were 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,” Prof Maiolino explained.
“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 astronomers 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 4 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.
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Y. Peng et al. 2015. Strangulation as the primary mechanism for shutting down star formation in galaxies. Nature 521, 192–195; doi: 10.1038/nature14439
