Super Clusters: Definition

Superclusters are very large groupings of smaller galactic clusters. Superclusters are the largest known structures in the Universe. They are not gravitationally bound to each other; The Milky Way Galaxy is located in the Local Group, which in turn is located in the Virgo Supercluster.

Local Super-Clusters

Super Clusters: Distribution

A consistent picture regarding the phenomenology of large scale structure in the universe is emerging from observations using different tracers: galaxies, clusters, pencil-beam surveys, quasars, and radio galaxies.

Large-scale superclusters are observed to scales of 150 Mpc in the distribution of galaxies, clusters of galaxies, and probably quasars and AGNs. The same superclusters are traced well by galaxies and by rich clusters. The superclusters appear to be flattened systems, with dimensions of up to = 15exp02 x 20 Mpc3; their mean space density is low, 5 x 10exp-6 Mpc-3; and their mean separation is = 1.00 Mpc.

Great Walls, Great Attractors, and the generic superclusters are all similar structures with different names. They appear to surround large underdense regions. These super-clusters are the main origin of the galaxy peaks observed at 100- to 150-Mpc intervals in narrow pencil-beam surveys. The peaks originate when the narrow beam crosses the large-scale superclusters. It is suggested that superclusters are not randomly distributed in space but rather are weakly correlated on large scales. A network system of superclusters is suggested by the data; a cellular, or Zeldovich pancaketype, model may provide an approximate representation of the observations. Understanding the detailed topology of the structure will require considerably larger redshift samples of galaxies and clusters than are currently available.

A richness-dependent cluster correlation function and a universal dimensionless cluster correlation appear to represent well the available data for galaxies, groups, and clusters, as well as quasars and radio galaxies. The predictive power of these relations has succeeded, since automated cluster surveys (APM, EDCC) have yielded results that are consistent with these predictions.

The observations of the spatial distribution of galaxies and clusters of galaxies, as well as the mass function of clusters, suggest that the standard fQ = 1 CDM model is inconsistent with the data. A low-density CDM model, with Qk 0.2-0.3 (with or without a cosmological constant such that fQ + A = 1), yields results that are consistent with the observations of galaxy and cluster correlations and with the mass function of clusters.

Super Clusters: History

In the 19th century the idea of an infinite universe was abandoned, mostly for reasons later to be found incorrect. This change in philosophy continued. Between 1908 and 1922 Charlier showed that an infinite universe could be built of a hierarchical structure as long as there were certain inequalities relating the masses and luminosities to the ratio of its units and sub-units of successive order. He attempted to detect these “second order” structures by counting nebula in different zones of the sky.

No real attention was paid to this line of thought with the exception of Charlier’s students. In 1937 and 1941 in student thesis the distribution of galaxies was discussed. The conclusion was that our galaxy was situated within a large mega-galactic cloud or super-cluster somewhat away from the center, the center being in the general direction in the north galactic pole. The far edge of the cloud was about 100 million light-years away.

Most astronomers ignored these results, even though new data was available showing a definite non-random spacing of galaxies in our neighborhood. In the 20’s and 30’s English astronomers repeatedly called attention to the strange distribution of the brighter spiral galaxies along a greater circle that was almost perpendicular to the galactic plane.

In 1932 Harlow Shapley published a catalog of 1250 brighter galaxies strongly confirming the strange distribution noted by the English astronomers. Shapely noted that because of the vast extent of the super-cluster the local cluster of galaxies could be a part of it. He did not ever suggest the possibility of a local super-cluster and was a vocal opponent of the idea till his death.

Between 1918 and 1924 the distances to the “spiral nebula” were finally established. It was clear that these nebula were far outside the Milky Way galaxy. They were clearly galaxies in their own right. This was a move from a universe of stars to that of a universe of galaxies. With an exceptional group or cluster notwithstanding the extragalactic universe was taken to be populated by individual galaxies in a very homogeneous layout, as long a large enough area were chosen.

Swiss astronomer Fritz Zwicky, working at Palomar Observatory in the 1930’s, demonstrated that single galaxies homogeneously dispersed was not the structure of the universe. The universe was actually made up of large clusters of galaxies, millions of light years in diameter. Harlow Shapely discovered vast galaxy clouds stretching over 10’s and perhaps 100’s of light years of space. Not only is the universe not uniform on a “small” scale of groups of clusters but also on the much larger scale of galaxy clouds.

In 1938 Zwicky advanced the idea that galaxies were not the basic building blocks of the universe but rather the galaxy clusters themselves, which he postulated as being homogenous throughout the universe.He visualized space as being divided into unequal “cluster cells” filling the universe as suds divide a volume of suds. The cluster at the center of each bubble was thought to have a sphere of influence” defined by the bubble.

In the early 1950’s the modern studies of super-clustering and the local “Supergalaxy” began. A revision of the Shapley-Ames catalog that finally included the neglected southern sky showed once again the strange distribution that the English astronomers found in the 20’s and 30’s, that circled the sky in both hemispheres, while being most conspicuous in the northern hemisphere.

This was clearly evidence of a local “supergalaxy”, with the local group rather far from the center which seemed centered in the Virgo cluster, acting as a nucleus for the local “supergalaxy”. It was clear that ours was not the only supergalaxy. Other supergalaxies were discovered centered in the constellations of: Fornax, Hydra, Pavo-Indus, Perseus, and elsevware. Data was accumulating that superclustering occurred throughout the universe and not just locally.

In 1958 Abell demonstrated that clusters are not distributed at random but have clustering that indicates a second order structuring. This was demonstrated repeatedly for the next twenty years but it was not until 1976 that the supergalaxy concept became generally accepted in astronomy. This occurred, in part, because of work done by Tully and Fisher.

In an interesting unrelated development George Gamow noted that rotation is a universal phenomenon and suggested that astronomers search for differential rotation effects in the radial velocities of nearby galaxies. He thought that such an effect should be present if the galaxies were a part of a universal super-system. Gamow did not consider intimidate-scale systems.

Several attempts at such analysis were implimented in the early 1950’s. In 1956 a large enough data base was available and in 1958 it was found that the expansion of the universe is neither linear nor isotropic in our vicinity, particularly within the local supercluster.

In an irregular and clumpy universe the local value of the Hubble ratio must fluctuate with the average local mater density from zero in regions of high density (galaxies) to asymptotic values in regions with very low density between independent superclusters.

Mapping the velocity field of the local supercluster has gotten a great deal of attention. There have yet to be any useful models developed as the data and its analysis have been conflicting, at best. We do have good values on the motion of our Local Group relative to the Virgo cluster at about 150 m/s. The direction is not directly at Virgo but is at the center of mass of galaxies nearby (20-30 degrees away).

Super Clusters: Morphology

Several of the nearer superclusters, including the local one appear on the sky as elongated structures. As more structures were discovered, the initial assumption that they were flat disks seen edge on became untenable. Many must be string or sheet-like structures.

There is overwhelming evidence that the clusters and superclusters are found mainly on the faces and, especially, along the intersections of the faces of polyhedral cell structures, with very few galaxies within the cells. Plotting redshift vs. direction in the sky one gets a map of the space distribution of the galaxies showing the extreme irregularity of the space distribution of galaxies and their concentration in long filamentary structures separated by great voids that contain few or no galaxies.

The current view of the spatial distribution of galaxies is the opposite of that originally offered by zwicky. While it still can be likened to a “volume of suds”, but instead of finding the clusters and superclusters in the center of the “cluster cells” we now view them as existing on the walls and intersections of the bubbles, much as the soapy water in the suds. The origin of this strange structure has been argued but no current theory is accepted as definitive.

There is currently no evidence of higher-order clustering above the third order and therefor clustering does not go on forever with a limit about 1 billion light years. The Carpenter Relation says that the larger the cluster the lower its mean density, possibly the effect of a balance between the maximum mass and kinetic energy content packed into a given volume of space.

In 1967 and 1969 Russian astronomers demonstrated that the contrast between systems of different orders decreases steadily as the size (order) of the system increases. This means that even if there were clustering of higher than the third order the contrast between orders would not be sufficient for differentiation.