Abell Catalogue - The Northern Survey

The Northern Survey

The original catalog of 2,712 rich clusters of galaxies was published in 1958 by George O. Abell (1927–83), who was then studying at the California Institute of Technology. The catalog, which formed part of Abell’s PhD thesis, was prepared by means of a visual inspection of the red 103a-E plates of the Palomar Sky Survey (POSS), for which Abell was one of the principal observers. A. G. Wilson, another of the principal observers, assisted Abell in the initial stages of the survey by routinely inspecting the plates as they were produced. After the completion of the survey, Abell went over the plates again and carried out a more detailed inspection. In both cases inspection was made with a 3.5x magnifying lens.

To qualify for inclusion in the catalog, a cluster had to satisfy four criteria:

• Richness: A cluster must have a minimum population of 50 members within a magnitude range of m₃ to m₃+2 (where m₃ is the magnitude of the 3rd brightest member of the cluster). To ensure a healthy margin or error, this criterion was not applied rigorously, and the final catalog included many clusters with fewer than fifty members (though these were excluded from Abell’s accompanying statistical study). Abell divided the clusters into six “richness groups,” depending on the number of galaxies in a given cluster that lie within the magnitude range m₃ to m₃+2 (the average number of galaxies per cluster for the entire catalog was 64):
  • Group 0: 30-49 galaxies
  • Group 1: 50-79 galaxies
  • Group 2: 80-129 galaxies
  • Group 3: 130-199 galaxies
  • Group 4: 200-299 galaxies
  • Group 5: more than 299 galaxies

• Compactness: A cluster must be sufficiently compact that its fifty or more members lie within one “counting radius” of the cluster’s centre. This radius, now known as the “Abell radius,” may be defined as 1.72/z arcminutes, where z is the cluster’s redshift, or as 1.5h−1 Mpc, where the Hubble constant is assumed to beH₀ = 100 km s−1 Mpc−1, and h is a dimensionless scale parameter which usually takes value between 0.5 and 1. h = H₀/100. The precise value of the Abell radius depends on the value taken for that parameter h. For h = 0.75 (same as H₀ = 75 km s−1 Mpc−1), the Abell radius is 2 megaparsecs. This is more than twice the estimate Abell gave in 1958, when H₀ was thought to be as high as 180 km s−1 Mpc−1.

• Distance: A cluster should have a nominal redshift of between 0.02 and 0.2 (i.e. a recessional velocity of between 6,000 and 60,000 km/s). Assuming H₀ = 180 km s−1 Mpc−1, these values correspond to distances of about 33 and 330 Mpc respectively; but using today’s estimate for H₀ (about 71 km s−1 Mpc−1) Abell’s upper and lower limits are actually set at about 85 and 850 Mpc. It has since been shown than many of the clusters in the catalog are more remote even than this, some being as far away as z = 0.4 (about 1,700 Mpc). Abell divided the clusters into seven “distance groups” according to the magnitudes of their tenth brightest members:
  • Group 1: mag 13.3-14.0
  • Group 2: mag 14.1-14.8
  • Group 3: mag 14.9-15.6
  • Group 4: mag 15.7-16.4
  • Group 5: mag 16.5-17.2
  • Group 6: mag 17.3-18.0
  • Group 7: mag > 18.0

• Galactic-Latitude: Areas of the sky in the neighbourhood of the Milky Way were excluded from the study because the density of stars in those fields—not to mention interstellar obscuration—made it difficult to positively identify galaxy clusters. Like the richness criterion, this one was not applied rigorously, several clusters in or close to the Galactic Plane being included in the catalog where Abell was satisfied that they were genuine clusters that met the other criteria.

In the catalog as originally published the clusters were listed in increasing order of right ascension. Equatorial coordinates (right ascension and declination) were given for the equinox of 1855 (the epoch of the Bonner Durchmusterung) and galactic coordinates for 1900.

Also listed for each cluster were the following:

• The cluster's precession rate
• The magnitude of the cluster's tenth brightest member
• The distance group of the cluster
• The richness group of the cluster