Abell clusters are large clusters of galaxies, catalogued by George O. Abell for his PhD dissertation in 1958. Abell used the National Geographic Society Palomar Observatory Sky Survey (or POSS) photographic plates to assemble a catalogue of 2712 clusters of galaxies, within a redshift of approximately z = 0.2. The clusters are ordered in increasing right ascension of the apparent cluster center. It should be noted that Abell clusters make up only a small fraction of the galaxy clusters known, and are merely some of the closest -- and therefore most noticeable -- examples.

Abell defined his clusters as any group of galaxies containing at least 50 galaxies no more than two magnitudes fainter than the third-brightest member of the cluster. He used the third brightest member rather than the very brightest to avoid counting foreground, field galaxies which might lie along our line of sight to the cluster. However, it has the added benefit of avoiding comparing "normal" cluster galaxies to the brightest one. The brightest galaxies in clusters, often classified as cD galaxies, can be very luminous, and are often the product of mergers of many galaxies at the center of the cluster gravitational potential well. He broke these clusters down into five "richness" classes, namely

  • Class 0 -- 30-49 galaxies (not really "clusters")
  • Class 1 -- 50-79 galaxies
  • Class 2 -- 80-129 galaxies
  • Class 3 -- 130-199 galaxies
  • Class 4 -- 200-299 galaxies
  • Class 5 -- 300+ galaxies

To be counted, the galaxies of a candidate cluster are required to lie within a certain radial distance of the cluster center to be classified as cluster members. In the 1958 paper, Abell estimated the physical scale to be roughly 830 kiloparsecs, but this was based on an (incorrect) estimate of the Hubble constant as 180 kilometers per second per megaparsec. (The correct value of the Hubble constant is probably 65 plus or minus 10.)

What is surprising about these clusters is not that the galaxies within them represent a lot of mass, but that a lot of the mass in clusters lies not within the galaxies themselves but within the intracluster medium. X-Ray images of these clusters revealed that they are filled with extremely hot gas (at tens of millions of degrees Kelvin), mostly hydrogen and helium, with a sprinkling of heavier elements stripped from the interstellar media of member galaxies. The gas is extremely thin, with average densities much less than one atom per cubic centimeter, but it takes up a huge volume -- several cubic megaparsecs (or more than 1073 cubic centimeters). This yields gas masses comparable to the sum of the masses of visible galaxies in the clusters. Further dynamical studies of these clusters showed that most of the matter in clusters is, in fact, dark matter, with studies of galaxy kinematics suggesting dark matter mass fractions of 80 percent.

Some Abell clusters are now found to be produced by mergers of smaller clusters. One example of this is Abell 1656, the Coma cluster, which shows significant substructure both in its X-Ray and kinematic distributions. This shows that galaxy clusters are evolving structures, and are most likely products of primordial density peaks in the early universe, gradually accreting mass as the universe evolves.

Along those lines, Abell later classified the clusters as being regular (R) or irregular (I) depending upon the apparent shape and symmetry of the cluster, and the distribution of galaxy types (early or late) found within it. These types were later refined, and the intermediate RI and IR classes were added.

The original paper by Abell is entitled "The distribution of rich clusters of galaxies", Astrophysical Journal Supplement Series, 3, 211 (1958). A revised catalogue including southern clusters was published (six years after Abell's death in 1983) by Abell, Corwin, and Olowin, in "A catalog of rich clusters of galaxies", Astrophysical Journal Supplement Series, 70, 1 (1989). The latter contains 4073 clusters, including southern sources. They are both available from adsabs.harvard.edu

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