New Insight Into Why Galaxies Stop Forming Stars

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. CREDIT: J. CHAN, UC RIVERSIDE

A team of scientists get best measure yet of why star formation stopped in galaxy clusters in the early universe

Maunakea, Hawaii – Galaxy clusters are rare regions of the universe consisting of hundreds of galaxies containing trillions of stars, as well as hot gas and dark matter.

It has long been known that when a galaxy falls into a cluster, star formation is fairly rapidly shut off in a process known as “quenching.” What actually causes the stars to quench, however, is a mystery, despite several plausible explanations having been proposed by astronomers.

A new international study led by astronomer Ryan Foltz, a former graduate student at the University of California, Riverside, has made the best measurement yet of the quenching timescale, measuring how it varies across 70 percent of the history of the universe.

With the help of W. M. Keck Observatory on Maunakea, Hawaii, the study has also revealed the process likely responsible for shutting down star formation in clusters.

The results are published in today’s issue of The Astrophysical Journal.

Each galaxy entering a cluster is known to bring some cold gas with it that has not yet formed stars. One possible explanation suggests that before the cold gas can turn into stars, it is “stripped” away from the galaxy by the hot, dense gas already in the cluster, causing star formation to cease.

Another possibility is that galaxies are instead “strangled,” meaning they stop forming stars because their reservoirs cease getting replenished with additional cold gas once they fall inside the cluster. This is predicted to be a slower process than stripping.

Ryan Foltz. Credit: Kati Hatch.

A third possibility is that energy from the star formation itself drives much of the cold gas fuel 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 lost forever to the galaxy and is unavailable to form new stars.

Because these three different physical processes predict galaxies to quench on different relative timescales over the history of the universe, astronomers have postulated that if they could compare the number of quenched galaxies observed over a long time-baseline, the dominant process causing stars to quench would more readily become apparent.

However, until recently, it was very difficult to find distant clusters, and even harder to measure the properties of their galaxies. The international Spitzer Adaptation of the Red-sequence Cluster Survey, or SpARCS, survey has now made a measurement of more than 70 percent of the history of the universe, accomplished by pioneering new cluster-detection techniques, which enabled the discovery of hundreds of new clusters in the distant universe.

Using some of their own newly discovered SpARCS clusters, the new UCR-led study discovered 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 (4 billion years old), 1.3 billion years when the universe is middle-aged (6 billion years old), and 5 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.

To make this state-of-the art measurement, the SpARCS team required 10 nights of observations using Keck Observatory’s powerful instrument, the Multi-Object Spectrograph for Infrared Exploration (MOSFIRE).

Gillian Wilson. Credit: I. Pittalwala, UC Riverside.

“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, in whose lab Foltz worked when the study was done. “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 Hawaii 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 W. M. Keck Observatory findings were obtained as the result of a collaboration amongst UC faculty members Gillian Wilson (UCR), Michael Cooper (UC Irvine) and Saul Perlmutter (UC Berkeley). Other authors involved in the study were Adam Muzzin (University of York, Canada), Julie Nantais (Andres Bello University, Chile), Remco van der Burg (European Southern Observatory, Germany), Pierluigi Cerulo (Universidad de Concepción, Chile), Jeffrey Chan (UCR), Sean Fillingham (UCI), Jason Surace (California Institute of Technology), Tracy Webb (McGill University, Canada), Allison Noble (MIT), Mark Lacy (National Radio Astronomy Observatory), Michael McDonald (MIT), Gregory Rudnick (University of Kansas), Ricardo Demarco (Universidad de Concepción, Chile), Christopher Lidman (Australian Astronomical Observatory), Julie Hlavacek-Larrondo (University of Montreal), Howard Yee (University of Toronto, Canada), and Brian Hayden (UCB).

The study was supported by grants from NSF and NASA.


The Multi-Object Spectrograph for Infrared Exploration (MOSFIRE), gathers thousands of spectra from objects spanning a variety of distances, environments and physical conditions. What makes this huge, vacuum-cryogenic instrument unique is its ability to select up to 46 individual objects in the field of view and then record the infrared spectrum of all 46 objects simultaneously. When a new field is selected, a robotic mechanism inside the vacuum chamber reconfigures the distribution of tiny slits in the focal plane in under six minutes. Eight years in the making with First Light in 2012, MOSFIRE’s early performance results range from the discovery of ultra-cool, nearby sub stellar mass objects, to the detection of oxygen in young galaxies only two billion years after the Big Bang. MOSFIRE was made possible by funding provided by the National Science Foundation and astronomy benefactors Gordon and Betty Moore. It is currently the most in-demand instrument at Keck Observatory.


The W. M. Keck Observatory telescopes are among the most scientifically productive on Earth. The two, 10-meter optical/infrared telescopes on the summit of Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community.  We are most fortunate to have the opportunity to conduct observations from this mountain.