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Two Heads Are Better than One

Two telescopes, two young astronomers, one amazing discovery

PRINCETON, NJ — In 1972, a young astronomer predicted that moving clusters of galaxies would leave a subtle imprint on the cosmic microwave background radiation - but he had no way to check his prediction. Forty years later, another young astronomer has proved him right - by combining data from two huge international astronomy collaborations.

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Light from the cosmic microwave background radiation (illustrated as a colorful region at the top of the image) passes through galaxy clusters on its way to the Atacama Cosmology Telescope (photo at the bottom of the image). The light becomes redder when it passes through a galaxy cluster moving away from us, bluer when it passes through a galaxy cluster moving toward us.
As light from the cosmic microwave background radiation (top) travels from the early universe to Earth, it sometimes passes through a moving galaxy cluster. If the cluster is moving away from us (left), the light becomes slightly cooler and redder. If the cluster is moving toward us, the light becomes slightly hotter and bluer.

The radiation is detected by the ACT telescope in Chile (bottom). By combining microwave data from ACT with galaxy cluster locations and redshifts from SDSS-III, researchers have detected galaxy cluster motions and cosmic microwave background temperatures - leading to the first detection of the kinematic S-Z effect.

Figure Credit: Sudeep Das (UC-Berkeley)
ACT Telescope Photo Credit: Adam D. Hincks (Princeton University)

Tatar astrophysicist Rashid Sunyaev, working in Moscow with his advisor Yakov Zel'dovich, predicted in 1972 what would become known as the "kinematic Sunyaev-Zel'dovich (kSZ) effect." The tiny effect had eluded scientists for 40 years - until Nick Hand, as an undergraduate at Princeton University - worked with his advisor David Spergel to find it in combined data from the Atacama Cosmology Telescope (ACT) and the Sloan Digital Sky Survey (SDSS-III).

Hand, now a graduate student at the University of California Berkeley, was the first of 58 authors on a paper announcing the results, published today on the arXiv preprint server and submitted to the journal Physical Review Letters. Now that this incredibly small record of galaxy cluster motions has been seen, studying it further will help improve our understanding of the components of our universe and the formation of the largest clusters of galaxies.

"It's an exciting confirmation of an effect first theorized 40 years ago," Hand said. "The current generation of astronomy surveys is capable of detecting an effect that, even by astronomy standards, is very, very slight."

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Headshot photos of Rashid Sunyaev and Nick Hand
Rashid Sunyaev and Nick Hand

Credit for Sunyaev photo: Markus Marcetic/MOMENT, from the Crafoord Prize website

The kSZ effect can be seen in the "cosmic microwave background radiation (CMB)," the faint glow of light given off by the universe when it was just 380,000 years old. The effect was first proposed in 1972 by Moscow-based astrophysicists Rashid Sunyaev and Yakov Zel'dovich. The telescopes of the time could not make measurements of the CMB - they could barely see it at all - so Sunyaev and Zel'dovich turned to a tool they already had: their minds. Given that the CMB exists, they thought, and given what we know about the universe, what should we expect to see?

Sunyaev and Zel'dovich knew that the light of the CMB has been traveling across the universe to our eyes. Because space is big and mostly empty, most of the light from the CMB travels straight from there to here, so we see the light just as it was nearly 14 billion years ago. But, they realized, some of the CMB light passes through galaxy clusters on the way, and those clusters leave their mark.

As light from the CMB passes through a galaxy cluster, it interacts with the hot ionized gas between galaxies. The gas is "ionized" because it is so hot that the electrons get ripped away from their atoms, leaving them to float freely. As a microwave from the CMB passes through the cluster, every once in a while, it hits one of these free electrons. The collision between the microwave and the electron slightly increases the microwave's energy in the direction that the galaxy cluster is moving.

That means that when CMB light passes through a galaxy cluster moving toward Earth, it appears hotter by a few millionths of a degree; when the light passes through a galaxy cluster moving away from Earth, it appears slightly cooler. A few millionths of a degree is small even by the standards of astronomers. Sunyaev and Zel'dovich had no way to test their prediction, and neither did anyone else.

The kSZ effect is so tiny that, even forty years later, measuring it is still a Herculean task. "The kSZ signal is small because the odds of a microwave hitting an electron while passing through a galaxy cluster are low, and the change in the microwave's energy from this collision is slight," said David Spergel of Princeton, Hand's senior thesis advisor and an ACT collaborator. But by averaging ACT's maps of CMB temperature for thousands of clusters, Spergel said, the kSZ signal gets stronger in comparison to unrelated signals or the noise from measurement errors.

To find the CMB temperature near thousands of clusters, astronomers needed accurate maps of where the clusters were. ACT did not have such maps, but SDSS-III did. With ACT's temperature maps and SDSS-III's list of galaxy clusters, all that remained was to find someone with enough knowledge, time, and patience to lead the study.

Enter Nick Hand. At the time, Hand was a senior at Princeton University, looking for a senior thesis project. Working with Spergel, he began to look for ways to combine the datasets.

Following a suggestion from Arthur Kosowsky of the University of Pittsburgh, another ACT collaborator, Hand marked the locations of the 7500 brightest galaxy clusters from the SDSS-III catalog on ACT's CMB map, then measured the temperatures of the CMB at those locations. Sure enough, when he averaged all the measurements, he found that the average temperature difference between any two galaxies showed that they were slightly more likely to be moving towards each other than away from each other. That means that the temperature differences between galaxies reflect the motions of galaxy clusters — just as Sunyaev and Zel'dovich had predicted.

Naturally, one astronomer particularly excited about today's result is Rashid Sunyaev. He was in his 20s when he predicted the effect that now bears his name; today, at age 69, he is the director of the Max Planck Institute for Astrophysics in Garching, Germany. "Like any theoretical scientist proposing an observational effect, I was dreaming for almost 40 years that it would be discovered 'in the next several years,'" Sunyaev said. "It's extremely elegant that the authors were able to choose the most interesting groups of galaxies using the SDSS-III results."

Now that Sunyaev's dream has been realized, what's next? "As maps get better, we can better detect the kSZ effect signal. One of the main advantages of the kSZ effect is that its magnitude is independent of a cluster's distance from us," Hand said, "so we can use it to measure cosmic velocity on large scales." In fact, the kSZ effect can measure the velocities of the clusters much more precisely than the methods astronomers traditionally use. The ACT collaboration obtained evidence for clusters that are several billion light-years away moving at velocities of up to 600 kilometers per second (more than one million miles per hour).

The ability to measure galaxy cluster velocities so precisely could give astronomers insight into the strength of the gravitational forces that pull them together. Chief sources of the forces are dark energy and dark matter, so measurements of the kSZ effect could someday help to test theories of what those mysterious substances could be. In addition, because the strength of the kSZ effect depends on the distribution of electrons in clusters' ionized gas, the effect can also be used to study such gas in nearby clusters. Those studies could help reveal how galaxies first formed.

The new discovery also holds another important lesson. Given the tiny temperature differences that the kSZ effect produces, looking at just the SDSS-III data alone, or just the ACT data alone, would not have been enough.

"This result is a great example of an important scientific discovery relying on the rich data from large astronomy surveys," said Kosowsky. "These are big collaborations of scientists," added David Schlegel, an astronomer at Lawrence Berkeley National Laboratory and principal investigator of the Baryon Oscillation Spectroscopic Survey (BOSS), one of the four components of SDSS-III. "Working in only one of these collaborations is a full-time job. Pulling together data from both to measure such a tiny signal was quite a feat."


Published Results

Hand, N. et al. 2012, Detection of Galaxy Cluster Motions with the Kinematic Sunyaev-Zel'dovich Effect, submitted to Physical Review Letters and available on the arXiv preprint server as arXiv:1203.4219 .


Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the U.S. Department of Energy Office of Science. The SDSS-III web site is

SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration including the University of Arizona, the Brazilian Participation Group, Brookhaven National Laboratory, University of Cambridge, Carnegie Mellon University, University of Florida, the French Participation Group, the German Participation Group, Harvard University, the Instituto de Astrofisica de Canarias, the Michigan State/Notre Dame/JINA Participation Group, Johns Hopkins University, Lawrence Berkeley National Laboratory, Max Planck Institute for Astrophysics, Max Planck Institute for Extraterrestrial Physics, New Mexico State University, New York University, Ohio State University, Pennsylvania State University, University of Portsmouth, Princeton University, the Spanish Participation Group, University of Tokyo, University of Utah, Vanderbilt University, University of Virginia, University of Washington, and Yale University.

About the Atacama Cosmology Telescope (ACT) Project

The ACT collaboration involves a dozen universities, with leading contributions from Princeton and the University of Pennsylvania, and with important detector technology from NASA's Goddard Space Flight Center and the National Institute of Standards and Technology.

Support for ACT comes primarily from the National Science Foundation.

ACT operates in the Chajnantor Science Preserve in northern Chile under the auspices of the Commission Nacional de Investigación Cientifica y Tecnológica (CONICYT).