​New York, July 9 (IANS) Scientists may finally have an explanation for why some galaxies stop creating stars after a team of astrophysicists painstakingly analysed around 70,000 galaxies to understand the forces influencing star formation activity in them.

The international research team, led by researchers from the University of California, Riverside, combed through available data from the COSMOS UltraVISTA survey that give accurate distance estimates for galaxies over the past 11 billion years, and focused on the effects of external and internal processes that influence star creation in galaxies.

The processes that cause galaxies to "quench", that is, cease star formation, are not well understood and constitute an outstanding problem in the study of the evolution of galaxies. 

"By using the observable properties of the galaxies and sophisticated statistical methods, we show that, on average, external processes are only relevant to quenching galaxies during the last eight billion years," said study lead author Behnam Darvish from the University of California.

"On the other hand, internal processes are the dominant mechanism for shutting off star-formation before this time, and closer to the beginning of the universe," he added.

External processes include drag generated from an infalling galaxy within a cluster of galaxies, multiple gravitational encounters with other galaxies and the dense surrounding environment and the halting of the supply of cold gas to the galaxy.

Internal mechanisms include the presence of a black hole and "stellar outflow" (for example, high-velocity winds produced by massive young stars and supernovae that push the gas out of the host galaxy).

The finding, published recently in the Astrophysical Journal, gives astronomers an important clue towards understanding which process dominates quenching at various cosmic times. 

As astronomers detect quenched non-star-forming galaxies at different distances (and therefore times after the Big Bang), they now can more easily pinpoint what quenching mechanism was at work.​