The research team, which included scientists at the California Institute of Technology and Lancaster University, United Kingdom, combed through available data from the COSMOSUltraVISTA 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 formation activity in galaxies.
External mechanisms, the research team notes, include drag generated from an infallinggalaxy within a cluster of galaxies, which pulls gas away; multiple gravitational encounters with other galaxies and the dense surrounding environment, resulting in material being stripped away from the galaxy; and the halting of the supply of cold gas to the galaxy, thus strangling the galaxy of the material needed to produce new stars over a prolonged period of time.
The researchers explain that internal mechanisms include the presence of a black hole (in which jets, winds, or intense radiation heat up hydrogen gas in the galaxy or blow it out completely, thus preventing the gas from cooling and contracting to form stars) and "stellar outflow" (for example, high-velocity winds produced by massive young stars and supernovae that push the gas out of the host galaxy).
"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 Darvish, a former graduate student in the UC Riverside Department of Physics and Astronomy and the first author of the research paper that appears today in The Astrophysical Journal. "On the other hand, internal processes are the dominantmechanism for shutting off star-formation before this time, and closer to the beginning of the universe."
