For well over two decades, astronomers have wondered what kills most galaxies in the universe. Now a group of researchers led by Yingjie Peng of the University of Cambridge has solved the so-called “cosmic murder mystery.” There are two classes of galaxies: alive, gas-rich ones that still produce stars, and second, the ‘dead’ ones that can no longer create new stars.
Researchers studied over 26,000 galaxies
Findings of the study were published in the journal Nature. Astronomers proposed two theories for what kills galaxies. One is ‘galactic strangulation,’ in which the supply of cold hydrogen necessary for star formation is slowly chocked off. The second theory involves the quick removal of gas from a galaxy, likely caused by the gravitational pull of another galaxy, reports Space.com.
Peng and his colleagues analyzed over 26,000 nearby galaxies. Analysis revealed that most galaxies are strangled to death slowly, instead of being suddenly rendered dead by cosmic events. Stars are largely made up of hydrogen and helium. Peng focused on the concentration of ‘metals,’ that are heavier than hydrogen and helium both, in galaxies. These metals come from the fusion of hydrogen and helium from heavier metals.
Evidence consistent with strangulation of galaxies
Astronomers found that the metal content of a dead galaxy was significantly higher than that of an active, star-forming galaxy of similar mass. It is consistent with the strangulation of galaxies, said Roberto Maiolino, the co-author of the study. Other variables also supported the idea of death by strangulation.
Computer simulations showed that strangulation usually takes four billion years to kill a galaxy. That’s in line with the age difference between star-forming and dead galaxies. The star-forming galaxies were at least four billion years younger than dead ones.
Peng said the strangulation theory applies to galaxies that are up to 100 billion times bigger than sun. That covers almost 95% of all galaxies. For even larger galaxies, their evidence was not conclusive.