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
"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
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