Why do galaxies stop making stars?

A team of astronomers from the University of California, Riverside may have finally solved a longstanding mystery by explaining exactly why some galaxies stop forming stars, according to new research published in the latest edition of The Astrophysical Journal.

Study authors Behnam Darvish and Bahram Mobasher, along with a group of colleagues at the UK’s Lancaster University and the California Institute of Technology , studied a large sample of more than 70,000 galaxies to discover both the internal and external processes that influence the formation of stars and to discover why this process eventually comes to an end.

While galaxies come in different shapes (elliptical, spiral and irregular) and sizes, they also can be either blue or red depending upon whether or not they are still actively forming stars (blue do, but red ones do not). However, the reasons that galaxies cease star formation remained a bit of a mystery, but now, Darvish and Mobasher have identified some of the factors responsible.

They identified several external mechanisms, including drag generated from an infalling galaxy in a cluster that pulls away gas and multiple gravitational encounters with other galaxies and the dense surrounding environment, which causes material to be stripped away, and internal factors like the presence of a black hole or stellar outflow, that play a role in ending star formation.

Internal, external factors both play key roles at different times

The study authors, who reviewed data from the COSMOS UltraVISTA survey, also found that the halting of the supply of cold gas to a galaxy, causing it to have the supply of materials which it needs to produce new stars, to become cut off over a prolonged period of time.

“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,” Darvish, a former graduate student in UC Riverside’s Department of Physics and Astronomy and first author of the study, explained in a statement.

“On the other hand,” he added, “internal processes are the dominant mechanism for shutting off star-formation before this time, and closer to the beginning of the universe.” This discovery will help improve their understanding of which processes are responsible for halting star formation at different points in the evolution of a galaxy, as they detected these so-called “quenched” galaxies at different distances (and different points in time) during future studies.

Despite the breakthrough, it remains unclear if internal factors, external ones or a combination of the two are primarily responsible for quenching, and astronomers are not sure when the different types of processes begin to play important roles in galaxy evolution. However, the UCR-led team now plans to expand their research to analyze the environment of galaxies on a larger scale.

“We found that on average the external processes act in a relatively short time-scale, around one billion years, and can more efficiently quench galaxies that are more massive. Internal effects are more efficient in dense clusters of galaxies,” said Mobasher, an astronomy and physics professor. “The time-scale is very important. 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.”

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Image credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA).

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