By measuring the velocities of hundreds of clusters of galaxies distributed across the entire sky, scientists have discovered a bulk flow of over a million mph in the direction of the constellations Centaurus and Vela. Induced by gravity, the flow of clusters is independent of the expansion of the universe and extends over billions of light-years.
This result comes as a great surprise and could have profound implications for cosmological models of the very early universe. "We know about several large mass concentrations in the nearby universe," says University of Hawaii astronomer and team member Harald Ebeling, "and their gravitational pull on galaxy clusters in their vicinity is well documented. This flow is off the scale, though; it may go back to imbalances in the mass distribution that are now outside the observable universe."
The study's findings and their implications are presented in a paper to appear today in the electronic version of the Astrophysical Journal Letters (http://www.journals.uchicago.edu/doi/abs/10.1086/592947), and are also discussed in a NASA Press release (http://www.nasa.gov/centers/goddard/news/topstory/2008/dark_flow.html).
Team leader Alexander Kashlinsky of NASA's Goddard Space Flight Center and Fernando Atrio-Barandela from the University of Salamanca, Spain, are experts in the analysis of tiny variations in the temperature of the cosmic microwave background (CMB), the faint hiss of radio waves left over from the Big Bang. Most of the temperature fluctuations are caused by the passage of CMB photons through structure in our own galaxy, the Milky Way.
A tiny, additional shift in temperature, however, is caused by CMB photons being scattered by hot X-ray emitting gas in galaxy clusters, causing a telltale shift in the CMB spectrum that can be used to deduce the clusters' motion. In 2000, Kashlinsky and Atrio-Barandela predicted that this so-called kinematic Sunyaev-Zel'dovich effect should be measurable, but that it would require stacking the signal from many individual clusters.
UH co-author Harald Ebeling was able to help. He specializes in the compilation and scientific exploitation of large samples of galaxy clusters. Using X-ray satellites as well as the largest optical telescopes at Mauna Kea Observatories on the island of Hawaii to find clusters and measure their distances, he managed to identify distant galaxy clusters even in regions of the sky that are largely obscured by the Milky Way. Together, Ebeling and team member Dale Kocevski, then a UH graduate student and now a postdoctoral fellow at the University of California, Davis, created the first all-sky catalog of X-ray bright galaxy clusters that made this cosmic flow experiment possible.
As a next step, the team intends to test their result by extending their study to yet larger distances using an even larger cluster sample and the very latest CMB data. "If this flow is confirmed and found to continue to yet larger distances, it may offer us a fascinating opportunity to examine the state of the universe at a time that will never be accessible to direct observation," says Ebeling.
All-sky map: Hot gas in moving galaxy clusters (white) shifts the temperature of cosmic microwaves. Hundreds of distant clusters seem to be moving toward a 20-degree-wide patch of sky (pink ellipse). Credit: NASA/WMAP/A. Kashlinsky et al.
Inset: The galaxy cluster 1E 0657-56 lies 3.8 billion light-years away. It is one of hundreds that appear to be carried along by a mysterious cosmic flow. Credit: NASA/STScI/Magellan/U.Arizona/D.Clowe et al.
Founded in 1967, the Institute for Astronomy at the University of Hawaii at Manoa conducts research into galaxies, cosmology, stars, planets, and the sun. Its faculty and staff are also involved in astronomy education, deep space missions, and in the development and management of the observatories on Haleakala and Mauna Kea.
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