In a new study published in the journal Nature, researchers from the University of Cambridge and the Royal Observatory Edinburgh have found that when galaxies do die they die of strangulation. Strangulation of galaxies occur when the galaxy's materials such as hydrogen and helium are cut off preventing new stars from forming.
Currently scientists estimate that the Universe has more than a hundred billion galaxies that can be divided into living galaxies and dead galaxies. Galaxies such as the Milky Way and other similar living galaxies have a huge reserve of cold case which is used to form new stars. Dead galaxies, on the other hand, have very little stock of star forming materials and are unable to continue creating new stars.
There were two theories about galactic death. The first theory suggested that the sudden removal of cold gas that was needed to form new stars caused it. This occurred probably due to an external force like a passing galaxy or even an internal force like a black hole swallowed up the gas. The second theory stated that the galaxy was simply choked to death as the supply of incoming gas stopped for some reason.
Researchers observed data from the Sloan Digital Sky Survey about more than 26,000 average sized neighboring galaxies and analyzed the metal levels found in them. If the cold gas would suddenly be pulled out of the galaxy, its metal content would remain the same as it was before the galaxy died, because of the sudden stopping of star formation. However, if it was strangled, its metal content would keep rising, because star formation would go on until the supply of gas got exhausted.
"Here we report an analysis of the stellar metallicity (the fraction of elements heavier than helium in stellar atmospheres) in local galaxies, from 26,000 spectra, that clearly reveals that strangulation is the primary mechanism responsible for quenching star formation, with a typical timescale of four billion years, at least for local galaxies with a stellar mass less than 1011 solar masses," the researchers wrote in their study.
Researchers found a somewhat even ratio of gases to metals in the living gases, and higher concentrations of metal in dead galaxies. This indicates that they galaxies are slowly suffocated after their supply of gas was cut off, and they formed the few last stars with the remaining helium and hydrogen. Researchers revealed that the strangulation takes about 4 billion years to finally stop a galaxy's ability to create new stars, which is consistent with the age difference between live and dead galaxies.
