Der CFA Galaxien Katalog
Sheets and Voids
Maps of the galaxy distribution in the nearby universe reveal large
coherent structures. The extent of the largest features is limited only by the
size of the survey. Voids are present in every survey large enough to contain
them. Many galaxies lie in thin sheet-like structures. The largest sheet
detected so far is the Great Wall. The frequent occurrence of these structures
is one of several serious challenges to our current understanding of the origin
and evolution of the large-scale distribution of matter in the universe.
Geller and Huchra discovered the Great Wall on the first three sections of the CfA (Center for Astrophysics) redshift survey, which covered the region from eight to 16 hours of right ascension and from 26.5 to 42.5 degrees of (north) declination. Although the Leo and Hercules Clusters lie slightly south of this region, the authors make it clear that the same structure extends south of the survey boundaries. The wall of galaxies is skewed to our line of sight, with a mean redshift distance that varies from Z = 0.023 (~350 million light years) in Leo to Z = 0.033 (~500 million light years) in Hercules.
It measures 200 million by 600 million light years in area with a thickness of only 20 million light years. At 1016 solar masses the Coma-Hercules superclusters make up the bulk this wall. In the schematic below only part of the great wall--the Coma supercluster--is visible. The Hercules cluster (composed of Abell clusters A2147, A2151 and A2152) is not pictured in the schematic but it would be up above and adjacent to the coma supercluster.
Stellar maps, published in 1986, were a great surprise to the astrophysicists. They had expected to find relative uniformity above the scale of the already-familiar galaxy clusters. Instead, the first surveys showed--and subsequent surveys have confirmed--that great clusters of galaxies are arranged in thin sheets or long filaments. The longest sheet detected, called the "Great Wall," extends hundreds of millions of light years across the maps.
North and South, Sheets and Voids
(Courtesy: Margaret J. Geller and Emilio E. Falco,
Harvard-Smithsonian Center for Astrophysics.
Credits: Geller, da Costa, Huchra, and Falco.)
Large-scale structure in the universe in the northern and southern galactic hemispheres. Each of the 9,325 points in this image represents a galaxy. The Earth lies at the center; unmapped regions to the top and bottom are inaccessible because the plane of the Milky Way obscures them. Note the large-scale patterns in both hemispheres, like the Great Wall stretching across the northern hemisphere.
This 3D presentation of the CFA Galaxy Catalogue
shows where the main concentration of galaxy clusters are in our observable universe.
Our Milky Way galaxy is at the very center. On this plot only galaxies from the most dense regions are shown. The red circles show the location of galaxy clusters which were taken from the ACO Catalog. The Great Wall is the semi-circle of galaxies at the top right part of the graph. Note that it contains several red circles of clusters. Two of them are the Coma and Hercules superclusters.