British astronomers seem to have solved one of the oldest astronomical mysteries; they have found out how galaxies die. Evidence collected by them suggests that galaxies die due to strangulation. This happens after galaxies get cut off from raw materials required for making new stars.
For those who don’t know: the Universe is home to two kinds of galaxies, alive and dead. Around 50% of the galaxies are alive and still producing stars; the remaining ones are dead and have stopped producing stars.
Alive galaxies like our home galaxy Milky Way are filled with cold gases (primarily hydrogen), which are essential components for production of new stars. The dead galaxies, on the other hand, have extremely low supplies of these gases.
Studies conducted previously have put forward two main theories of galactic death. According to the first theory, galactic deaths occur due to sudden sucking up of cold gases required for producing new stars by various external or internal forces. The second theory suggests that galaxies die when the supply of the incoming cold gas stops for some reason; according to this theory, galaxies get strangled to death over an extended period of time.
To get rid of the confusion surrounding the causes of galactic death, a group of British Researchers representing the Royal Observatory Edinburgh and the University of Cambridge analyzed the data gathered by the Sloan Digital Sky Survey. The study was conducted for finding out the levels of metal in over 26,000 average-sized galaxies situated in our side of the Universe.
During the study, the researchers came to know that metal levels in the dead galaxies offer key fingerprints that make determining the cause of a galaxy’s death possible.
The metal content of galaxies that died when cold gases were suddenly pulled out of them by outflows will be same as the level of metal they had just before they were killed. This is because star formation in those galaxies stopped abruptly.
In case of galaxies that encountered death by strangulation, initially there will be the continuous increase in their metal levels; then, gradually the metal formation will stop. This happens because star formation continues to take place in these galaxies until the existing cold gases get used up completely.
The study’s co-author Robert Maiolino said that he and his co-researchers have found that for a particular stellar mass, a dead galaxy’s metal content is significantly more than that of a star-producing galaxy of the same mass. According to Maiolino, this difference in metal levels doesn’t indicate that galaxies die due to sudden gas removal. This finding is more consistent with the other theory, the theory of death by strangulation.