Yale Graduate School of Arts and Sciences

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Ssssh... The Galaxy is Sleeping

Yale astronomers have looked into the distant universe and discovered that galaxies display one of two distinct behaviors: they are either “awake” or “asleep,” actively forming stars or not forming any new stars at all.

Graduate student Kate Whitaker is lead author of a paper that describes this research, “The NEWFIRM Medium-band Survey: Photometric Catalogs, Redshifts, and the Bimodal Color Distribution of Galaxies out to z ~ 3," published in the June 20 online edition of the Astrophysical Journal. Her team’s survey of the distant universe shows that even very young galaxies 12 billion light years away are either awake or asleep, indicating that galaxies have behaved this way for more than 85 percent of the history of the universe. Kate and her colleagues fabricated a set of filters, each one sensitive to different wavelengths of light, which they used on a four-meter Kitt Peak telescope in Arizona. They spent 75 nights peering into the distant universe and collecting light from 40,000 galaxies, creating the deepest and most complete survey of its kind ever made.

stars and galaxies

Hubble Space Telescope image showing red and blue galaxies. The blue galaxies with spiral structure are “awake“; the red galaxies are “sleeping.” (Photo credit NASA)

Kate and her colleagues deciphered the galaxies’ dual behavior based on the color of the light they emit. Because of the physics of star formation, active, wakeful galaxies appear bluer, while the light emitted by passive, sleepy galaxies tends toward the redder end of the spectrum. The researchers found that there are many more active galaxies than passive ones, which agrees with the current thinking that galaxies start out actively forming stars before eventually shutting down.

“We don’t see many galaxies in the in-between state,” says Professor Pieter van Dokkum, Kate’s adviser and one of the paper’s authors. “This discovery shows how quickly galaxies go from one state to the other, from actively forming stars to shutting off.” Whether the sleeping galaxies have completely shut down remains an open question, Whitaker says. However, the new study suggests the active galaxies are forming stars at rates about 50 times greater than their sleepy counterparts.

“Next, we hope to determine whether galaxies go back and forth between waking and sleeping or whether they fall asleep and never wake up again,” van Dokkum says. “We’re also interested in how long it takes galaxies to fall asleep and whether we can catch one in the act of dozing off.”

“I started to get interested in physics in high school and have generally enjoyed problem-solving throughout my academic life,” Kate says. “On the opposite end of the spectrum, I have always enjoyed the arts. Somehow, the beauty and mystery of astronomy seemed to connect my analytic and artistic sides perfectly.”

This project is at the core of her dissertation, tentatively titled "The Properties of Massive, Quiescent Galaxies at 1.5 < z < 3.5." She was present in the control room of the telescope dome for roughly half of the nights that data were collected, from just before sunset until sunrise the following morning. Most of the time, she and her colleagues “sat in front of several parallel computer monitors, checking the data as it was taken and telling the telescope operator where to move the telescope next,” she explains. The observations were restricted to when the target fields were visible, so even though the observation runs were spread over 16 months from March 2008 through June of 2009, “we lost many nights of data due to winter storms and rainy weather.”

Other graduate students participated in the study, including Rachel Bezanson, Erica Nelson, and Tomer Tal. Additional Yale authors on the article include Gabriel Brammer, Adam Muzzin, Kyoung-Soo Lee, Britt Lundgren, and David Wake. Further assistance was provided by astronomers from Leiden University, Princeton, the Harvard-Smithsonian Center for Astrophysics, Tufts, Carnegie Observatories, UCO/Lick Observatory, and the University of Kansas.