Why Galaxies Stop Making Stars

One of the most elusive questions in astrophysics is why galaxies stop producing new stars. At a certain point in a galaxy's life it retires as an active, star-producing entity and becomes what astronomers call "quenched," but until recently the physical mechanisms of this transition were not well understood.

New research published Friday in the Astrophysical Journal has shed some light on this longstanding astromystery by using a massive dataset of 70,000 galaxies to study their evolution over the last 11 billion years. The data was culled from the COSMOS UltraVISTA Survey, an astronomical survey designed to probe how time and galactic environments influence a galaxy's formation.

The researchers' goal was to help settle the debate as to whether galaxy quenching is a result of external or internal forces on the galaxy, or some mixture of the two. Examples of external methods that might halt star formation include gravitational encounters with other galaxies that pull the material necessary for stars away or a reduced supply of the cold gas which is necessary for star formation. Internal processes that could hinder stellar production include the presence of black holes, which heat up the galaxy's hydrogen gas and thereby prevent the gas from cooling and contracting to form a star, as well as high velocity winds produced by massive stars that push necessary materials out of the 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 Behnam Darvish, a postdoctoral researcher at Caltech. "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."

The team's research demonstrates that the quenching of galaxies is a highly dynamic process that evolves over time. Whether this process of shutting down star production is fast or slow is a result of a complex interplay of both external and internal factors. Generally speaking however, the team found that external factors act on a relatively short timescale (about 1 billion years) and are more efficient at quenching massive galaxies. Internal processes, by comparison, are more efficient at quenching in dense galaxy clusters.

"The time-scale is very important," said Bahram Mobasher, a professor of physics and astronomy at University of California, Riverside. "A short time-scale suggests that we need to look for external physical processes that are fast in quenching. Another important result of the work is that internal and external processes do not act independently of each other in shutting-off the star formation."

According to the researchers, now that they have a better grip on the processes which account for galaxy quenching, they intend to see how the processes play out on an even larger scale by extending the scope of their research to the cosmic web, which is made of the approximately 100 billion galaxies in the universe.