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X-rays Reveal What Makes the Milky Way Move

by Dale Kocevski

A two-dimensional projection of  galaxy cluster populations within 800 million light-years of the Milky Way. Many of the clusters were discovered using the UH 2.2-meter telescope on Mauna Kea. Credit: Dale Kocevski, UH IfA, supported by NASA Graduate Research Program.

IfA astronomers have found that, in a tug-of-war of cosmic proportions, our Milky Way galaxy is being pulled toward the largest concentration of matter in the observable Universe. They used a new X-ray survey to determine what region is winning the tug-of-war: a massive association of galaxies over 500 million light-years away. The study shows that our galaxy's journey through space is not entirely due to the pull of nearby galaxies, but is also affected by much farther regions of the Universe than previously thought.

IfA graduate student Dale D. Kocevski spearheaded the study. He worked with IfA astronomers Harald Ebeling and Brent Tully, and with Chris Mullis, an IfA alumnus who is now a research fellow at the University of Michigan.

Astronomers have long known that the Milky Way is moving toward the constellation Centaurus at a speed of 1.4 million mph, but the reason for the movement remained a topic of debate. Over 20 years ago, it was suggested that the motion was due to the gravitational pull of a nearby large concentration of matter dubbed the Great Attractor. The Great Attractor is what is known as a supercluster, that is, a group of clusters of galaxies, and was estimated to contain matter equal to more than over 10 million billion times the mass of the Sun.

Until now, efforts to find the Great Attractor were hampered by its location in the "zone of avoidance," an area behind the plane of the Milky Way where gas and dust within our galaxy block much of the visible light from objects outside it. The new survey, Clusters in the Zone of Avoidance (CIZA), is the first to search for the X-ray signatures of galaxy clusters behind the Milky Way and investigate the nature of the Great Attractor. Due to the difficulty of observing through the Milky Way, this region was the final portion of the sky in which the cluster population had yet to be mapped.

"X-rays can penetrate even regions that are extremely obscured by gas and dust, and galaxy clusters are sources of X-rays. This is what prompted us to attempt to map the distribution of galaxy clusters behind the plane of the Milky Way using X-ray observations," explains Ebeling, who initiated the survey in 1998.

Kocevski and collaborators found far fewer massive cluster systems near the Great Attractor than would be expected given the region's proposed mass. "One of our goals was to uncover the true mass of the Great Attractor. What we found is that it is not that great after all," says Kocevski.

Instead, a significant concentration of galaxies behind the Great Attractor is tugging on our galaxy. This distant area, which is four times more distant than the Great Attractor region, is dominated by the Shapley Supercluster, the most massive association of galaxies in the observable Universe. Kocevski has determined that only 44 percent of our galaxy's motion through space is due to the pull of galaxies in the Great Attractor region. The rest is the result of a large-scale flow in which much of the local Universe, including perhaps the Great Attractor itself, is being pulled toward the Shapley Supercluster.

http://www.ifa.hawaii.edu/~kocevski/astro/research/index.htm