Astronomy Study Reveals Possible Cause of Galaxy ‘Quenching’

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Hubble Space Telescope image of one of the SpARCS clusters used in the study, seen as it appeared when the Universe was only 4.8 billion years old.PC: Jeffrey Chan, UC Riverside.

A new international study supported by W.M. Keck Observatory on Maunakea points to the possible cause behind stars ceasing to form within galaxy clusters in the early universe.

The study, led by Ryan Foltz, an astronomer and former graduate student at the University of California, Riverside, captured the best timescale measurement to date of a process known as quenching. Quenching is the cessation of star formation when a galaxy slips into a galaxy cluster containing trillions of stars. The cause of quenching remains a mystery, though astronomers now believe they understand the causes better.

When a galaxy merges with a cluster, it brings cold gas that has not yet formed new stars. One explanation suggests that before the cold gas can turn into stars, it is “stripped” from the merging galaxy by the hot, dense gas already present in the cluster, thus causing star formation to cease.


Another possibility suggests merging galaxies are “strangled” and stop forming stars because they cease accumulating cold gas once they fall inside a cluster.

A third possibility is that energy from the star formation itself drives much of the cold gas away from the galaxy and prevents it from forming new stars. This “outflow” scenario is predicted to occur on a faster timescale than stripping because the gas is forever lost and unavailable to form new stars.

Ryan Foltz. PC: Kati Hatch.

Because these three hypothetical situations predict quenching on varying timescales over the history of the universe, astronomers wondered if they could compare the number of quenched galaxies against a baseline time period to figure out the dominant cause.


However, until recently, finding distant galaxy clusters was extremely difficult, and it was even harder to measure their properties. Thankfully for them, hundreds of new clusters have been discovered recently with the help of the international Spitzer Adaptation of the Red-sequence Cluster Survey (SpARCS) survey—a study that has now measured more than 70% of the history of the universe.

The UCR-led study used these newly discovered SpARCS clusters to determine that it takes a galaxy longer to stop forming stars as the universe gets older: only 1.1 billion years when the universe was young (four billion years old), 1.3 billion years when the universe is middle-aged (six billion years old), and five billion years in the present-day universe.

“Comparing observations of the quenching timescale in galaxies in clusters in the distant universe to those in the nearby universe revealed that a dynamical process such as gas stripping is a better fit to the predictions than strangulation or outflows,” Foltz said.


The SpARCS team calculated these measurements with 10 nights of observations using
W.M. Keck Observatory’s Multi-Object Spectrograph for Infrared Exploration (MOSFIRE).

“MOSFIRE was key to characterizing the most distant, ultra-faint galaxy clusters in the survey,” said Gillian Wilson, professor of physics and astronomy at UCR and leader of the SpARCS survey. “The superb sensitivity of MOSFIRE, combined with the excellent seeing on Maunakea, also allowed us to analyze multiple galaxies in each of those clusters simultaneously.”

The team also conducted 25 nights of observations with the twin Gemini telescopes in Hawai‘i and Chile.

“Thanks to the phenomenal investment in our work by these observatories, we now believe we have a good idea of how star formation stops in the most massive galaxies in clusters,” said Wilson. “There are good reasons, however, to believe that lower-mass galaxies may quench by a different process. That is one of the questions our team is working on answering next.”

The results of the study were published in the latest issue of The Astrophysical Journal.

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