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.