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MPG/ESO 2.2-m Telescope,
La Silla Observatory, ESO
Larger false-color image of the ACO or Abell 3627
supercluster of galaxies, which lies near the core
of the Great Attractor.
A huge volume of space that includes the Milky Way
and superclusters of galaxies is no longer thought
to be flowing towards a largely unseen mass called
the Great Attractor, but rather is flowing past it
towards the Shapley Supercluster behind it (more).
In late 2005, a team of astronomers engaged in a X-ray survey called the Clusters in the Zone of Avoidance (CIZA) project revealed that the Milky Way is not being drawn towards a concentration of mass called the Great Attractor but to an even more massive region behind it called the Shapley Supercluster, which lies around 500 million light-years away or four times the distance to the Great Attractor. Over the two decades since the discovery of the Great Attractor, subsequent observations at infrared wavelengths indicated that the Milky Way not, in fact, bring drawn towards the Great Attractor. Indeed, the CIZA team reported that the Great Attractor actually has only about a tenth the mass that was originally estimated (IFA press release; Maggie McKee, New Scientist, December 15, 2005; Kocevski and Ebeling, 2005; and Kocevski et al, 2005).
Kocevski et al,
2005a and 2005b,
The Shapley Supercluster lies
behind the Great Attractor and
is much more massive, with
the equivalent of nearly
10,000 Milky ways or four
times the currently observed
of the Great Attractor (more).
Region around the Mostly Unseen Mass
In the 1980s, a group of astronomers known as the "Seven Samurai" (David Burstein, Roger Davies, Alan Dressler, Sandra Faber, Donald Lynden-Bell, Roberto J. Terlevich, and Gary Wegner) found that galaxies are very unevenly distributed in space, with galactic superclusters separated by incredibly huge voids of visible ordinary matter. The Great Attractor is one such structure, a diffuse concentration of matter some 400 million light-years in size located around 250 million light-years (ly) away in the direction of the southern Constellation Centaurus, about seven degrees off the plane of the Milky Way -- at a redshift-distance of 4,350 kilometers (or around 2,700 miles) per second. It lies in the so-called Zone of Avoidance, where the dust and stars of the Milky Way's disk obscures as much as a quarter of the Earth's visible sky.
C. Kraan-Korteweg and
Ofer Lahav, Scientific American
(October 1998): pp. 50-57
(Permission being sought)
The Great Attractor is located
in a region of the universe that
is obscured from observers in
the Solar System by the dust of
the Milky Way's disk (more).
The Great Attractor is apparently pulling in millions of galaxies in a region of the universe that includes the Milky Way, the surrounding Local Group of 15 to 16 nearby galaxies and larger Virgo Supercluster, and the nearby Hydra-Centaurus Supercluster, at velocities of around 600 (in the Local Group) to thousands of kilometers (or miles) per second (Lynden-Bell et al, 1988; and Dressler et al, 1987). Based on the observed galactic velocities, the unseen mass inhabiting the voids between the galaxies and clusters of galaxies is estimated to total around 10 times more than the visible matter in this region of the universe and so must be composed of mostly dark matter. Calculations of the mass of the Great Attractor originally set at around 5.4 times 1016 to Solar-masses have been revised sharply downward with further study dur to subsequent infrared and x-ray studies. Galaxies located on the other side of the Great Attractor are no longer thought to be pulled in its direction (; Kocevski and Ebeling, 2005; Kocevski et al, 2005; and Renée C. Kraan-Korteweg, 2000). [More discussion of how the sheets, filaments, and stars of the early universe developed from quantum fluctuations and the gravitational condensation of dark and ordinary matter can be found in First Stars.]
C. Kraan-Korteweg and
Scientific American (October 1998): pp. 50-57
(Permission being sought)
Millions of galaxies may be moving towards
the Great Attractor, including the Virgo and
the Hydra-Centaurus superclusters of
Core of the Great Attractor
The core of the Great Attractor lies within the so-called "Centaurus Wall" of galaxies. From the perspective of observers in the Solar System, this Great Wall-like structure is viewed edge-on (Woudt and Kraan-Korteweg, 2000). The intersection of the Centaurus Wall and the Great Attractor includes the Norma Cluster or Supercluster -- ACO 3627, Abell 3627, or A3627 (Woudt et al, 2000, 1999a, and 1999b). Indeed, the Milky Way, the Local Group, and the surrounding Virgo Supercluster, as well as the Hydra-Centaurus Supercluster, appear to be part (or at least "appendages") of the sheet of ordinary and dark matter that forms the Centaurus Wall. [More discussion and color images of the Virgo Supercluster and the Centaurus Wall are available from Professor Anthony P. Fairall's lecture on "Large-Scale Structures in the Universe.")
© Anthony P. Fairall,
An Atlas of Nearby Large-scale
Structures, from Large-scale
Structures in the Universe
(Used with permission)
The core of the Great Attractor
("A3627" at left) lies within the
"Centaurus Wall" of galaxies
(more maps and discussion
on Large-scale Structures in
The Great Attractor's core region appears to be dominated by the Norma Supercluster, a highly obscured, nearby, and massive group of galaxies close to the plane of the Milky Way (Woudt et al, 1999a and 1999b; Kraan-Korteweg et al, 1996; and Patrick Alan Woudt, 1998 PhD thesis). In the absence of the obscuring effects of the Milky Way, the Norma Supercluster would have appeared as prominent as the well-known Coma Cluster or Supercluster, but nearer in redshift-space. Indeed, spectroscopic observations support the idea that the Norma Supercluster is the dominant component of a "Great Wall"-type of structure and is comparable in size, richness and mass similar to Coma in the northern part of the Great Wall (Woudt et al, 2000; and 1997). [The Norma Supercluster is also associated with a "Finger of God effect" in plots of galactic redshift velocities when viewed from the perspective of the Solar System.]
and Patricia A. Henning, 1997 (PASA, 14:1)
(Permission being sought)
Larger image with x-ray contours.
The core of the Great Attractor appears to include
the Norma Supercluster of galaxies (more).
The overdensity of galaxies in the region of the Norma Supercluster was first detected in the 1980s. Although astronomers have since observed a large excess of galaxies with optical and infrared telescopes in this region, no dominant cluster or central peak has been identified. This strongly suggests that a significant fraction of the Great Attractor's overdensity could still be obscured by the Milky Way, possibly in another rich cluster of galaxies around the strong radio-source PKS1343-601 (Woudt and Kraan-Korteweg, 2000).
Up-to-date technical summaries on the Great Attractor may be available at: NASA's ADS Abstract Service for the Astrophysics Data System; the SIMBAD Astronomical Database mirrored from CDS, which may require an account to access; and the NSF-funded, arXiv.org Physics e-Print archive's search interface.
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