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