X-rays Reveal What Makes the Milky Way Move (Forwarded)

Institute for Astronomy
University of Hawaii
Honolulu, Hawaii

Contacts:
Mr. Dale Kocevski
Institute for Astronomy
University of Hawaii at Manoa
Honolulu, Hawaii 96822
1-808-783-7881

Mrs. Karen Rehbock
Assistant to the Director
Institute for Astronomy
University of Hawaii
1-808-956-6829

Embargoed until Wednesday, January 11, 2006, at 2:30 p.m. EST (9:30 a.m.
Hawaii Time)

X-rays Reveal What Makes the Milky Way Move

A new survey by University of Hawaii astronomers has 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.
This finding is being presented today by UH graduate student Dale D.
Kocevski and collaborators at the American Astronomical Society meeting
held in Washington, D.C.

The UH scientists 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
affected by much farther regions of the Universe than previously thought.

Kocevski worked with Dr. Harald Ebeling, Dr. R. Brent Tully, both also
with the UH's Institute for Astronomy, and Dr. Chris R. Mullis, a UH
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 report finding 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, the CIZA team identified a significant concentration of
galaxies behind the Great Attractor, near the Shapley Supercluster,
which lies 500 million light-years away or four times the distance to
the Great Attractor region. The Shapley Supercluster, first identified
in 1930 by Harlow Shapley, is the most massive association of galaxies
out of the 220 identified superclusters in the observable Universe. It
contains the equivalent of nearly 10,000 Milky Ways, or four times the
amount of mass currently observed in the Great Attractor region.

With the galaxy cluster population mapped over the entire sky for the
first time, Kocevski analyzed how all the clusters surrounding the Milky
Way would affect it and found that only 44% of our galaxy's motion
through space is due to the gravitational pull of galaxies in the nearby
Great Attractor region. The remaining portion 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.

The results confirm previous work, which suggested the Milky Way's
motion was influenced by structures more distant than the Great
Attractor, but this study is the first to reach this conclusion after
having fully mapped the Great Attractor and regions behind it.

The finding resolves one of the long-standing problems associated with
the Great Attractor. The presence of a massive overdensity relatively
close to the Milky Way suggested that extreme mass concentrations such
as the Great Attractor were fairly common in the Universe. This implied
that the Universe contained much more matter than was measured by other
means such as supernova Ia observations. The finding of a less massive
Great Attractor and the large distance to the Shapley supercluster
implies that extremely massive overdensities are rare in the Universe,
which brings the suggested density of the Universe in line with the
density established by independent means.

Preprints of papers submitted to The Astrophysical Journal on the Milky
Way's motion and the CIZA survey can be found at:

* http://arxiv.org/abs/astro-ph/0510106
* http://arxiv.org/abs/astro-ph/0512321

This work was supported by the NASA Graduate Student Research Program.

The Institute for Astronomy at the University of Hawaii 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.

Established in 1907 and fully accredited by the Western Association of
Schools and Colleges, the University of Hawaii is the state's sole
public system of higher education. The UH System provides an array of
undergraduate, graduate, and professional degrees and community programs
on 10 campuses and through educational, training, and research centers
across the state. UH enrolls more than 50,000 students from Hawaii, the
U.S. mainland, and around the world.

FIGURE CAPTIONS

[Figure 1:
http://www.ifa.hawaii.edu/info/press...cale300dpi.jpg
(1.5MB)]
This image of the core of the Shapley Supercluster shows a small portion
of the thousands of galaxies that comprise Abell 3558, the galaxy
cluster at the center of the largest mass concentration in the
observable Universe. Image taken with NASA's Hubble Space Telescope;
courtesy J. Blakeslee, Washington State University.

[Figure 2:
http://www.ifa.hawaii.edu/info/press...-1-06/Fig2.jpg (28KB)]
Two-dimensional projection of the cluster population within 800 million
light-years of the Milky Way. Each blue halo represents a cluster of
galaxies. Superclusters are located where multiple halos group together.
The Milky Way's motion through space is due to a combination of the
gravitational pull of the Great Attractor (small arrows) and the pull of
the Shapley Supercluster, which produces a large-scale flow in which
much of the Universe near our galaxy is streaming toward the more
massive supercluster (large arrows) Credit: IfA.