New research explains why galaxies halt star production

RIVERSIDE, Calif., July 8 (UPI) -- A new study offers a comprehensive survey of the reasons galaxies "quench," or cease star production.

Quenched galaxies are known as passive galaxies. Understanding why these galaxies stop making stars remains a problem for cosmologists keen on understanding the evolution of galaxies.

The new research -- carried out by researchers at the University of California, Riverside and detailed in the Astrophysical Journal -- paints a clearer picture of galactic quenching.

Quenching triggers can be divided into external and internal mechanisms. Which mechanisms are to blame for individual passive galaxies depends on how old the galaxy is.

"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," Behnam Darvish, a former UC Riverside graduate student, explained in a news release. "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."

Both external and internal quenching mechanisms thwart star formation by robbing the galaxy of gas or rendering its gas unsuitable for stellar creation.

Externally speaking, gas can be robbed by drag on an infalling galaxy inside a galactic cluster. Multiple intense gravitational encounters with another galaxy can also rid a galaxy of its gas. A galaxy's external supply of cold gas can also be shut off, starving it of new star-forming material.

Internally, black holes can heat gas so much it can never cool and condense, as is necessary for star formation. Black holes can also generate jets, winds and intense stellar radiation that blow a galaxy's gas out and away. Massive young stars and supernovae can also have similar "outflow" effects.

These mechanisms aren't new, but researchers have struggled to determine which ones most often trigger galactic quenching. It's also unclear whether certain triggers are more common among certain types of galaxies -- big, small, spiral, elliptical, irregular. Astronomers have also struggled to understand how long each mechanism takes, as well as what role quenching plays in a galaxy's evolution.

Researchers haven't answered all these questions, but the new findings begin to offer some clarity.

"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," astronomer Bahram Mobasher said. "Internal effects are more efficient in dense clusters of galaxies."

"The time-scale is very important," Mobasher continued. "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."

Darvish and Mobasher are now looking for even broader star quenching patterns across galactic regions and the entire cosmic web.