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{{Short description|Astronomical catalogue of galaxy clusters}}

The '''Abell catalog of rich clusters of galaxies''' is an all-sky catalog of 4,073 rich [[galaxy clusters]] of nominal [[redshift]] ''z'' = 0.2. This catalog supplements a revision of [[George Ogden Abell]]’s original “Northern Survey” of 1958, which had only 2,712 clusters, with a further 1,361 clusters – the “Southern Survey” of 1989 – from those parts of the south celestial hemisphere that had been omitted from the earlier survey.

{{About|galaxy clusters|nebulae|Abell Catalog of Planetary Nebulae}}

{{Infobox astronomical survey}}

[[File:Heic1401a-Abell2744-20140107.jpg|thumb|right|200px|[[Abell 2744]] [[galaxy cluster]]{{snd}}[[Hubble Space Telescope#Frontier Fields program|Hubble Frontier Fields view]] (7 January 2014).<ref name="NASA-20140107">{{cite news |last1=Clavin |first1=Whitney |last2=Jenkins |first2=Ann |last3=Villard |first3=Ray |title=NASA's Hubble and Spitzer Team up to Probe Faraway Galaxies |url=http://www.jpl.nasa.gov/news/news.php?release=2014-007 |date=7 January 2014 |work=[[NASA]] |access-date=8 January 2014 }}</ref>]]

The '''Abell catalog of rich clusters of galaxies''' is an all-sky catalog of 4,073 rich [[galaxy clusterscluster]]s of nominal [[redshift]] &nbsp;''z'' = &nbsp;≤&nbsp;0.2. This catalog supplements a revision of [[George OgdenO. Abell]]’s's original “Northern"Northern Survey”Survey" of 1958, which had only 2,712 clusters, with a further 1,361 clusters – {{snd}}the “Southern"Southern Survey”Survey" of 1989, published after Abell's death by co-authors Harold G. Corwin and Ronald P. –Olowin from those parts of the south celestial hemisphere that had been omitted from the earlier survey.

The Abell catalog, and especially its clusters, are of interest to amateur astronomers as challenge objects to be viewed in dark locations on large aperture amateur telescopes.

[[File:ABELL 1132 2013-06-26 20-43.jpg|thumbnail|right|150px|Abell 1132]]

==The Northern Survey==

The original catalog of 2,712 rich clusters of galaxies was published in 1958 by George OgdenO. Abell (1927-831927–1983),<ref>{{cite journal|author=Abell, George O.|title=The distribution of rich clusters of galaxies. A catalog of 2712 rich clusters found on the National Geographic Society Palomar Observatory Sky Survey|journal=The Astrophysical Journal Supplement Series|volume=3|pages=211–88|date=1958|doi=10.1086/190036|bibcode=1958ApJS....3..211A|url=https://thesis.library.caltech.edu/1581/1/Abell_GO_1957.pdf|archive-date=2018-07-21|access-date=2020-08-19|archive-url=https://web.archive.org/web/20180721190654/https://thesis.library.caltech.edu/1581/1/Abell_GO_1957.pdf|url-status=dead}}</ref> who was then studying at the [[California Institute of Technology]]. The catalog, which formed part of Abell’sAbell's PhD thesis, was prepared by means of a visual inspection of the red 103a-E plates of the [[Palomar Observatory Sky Survey]] (POSS), for which Abell was one of the principal observers. [[Albert George Wilson|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 3.5x5× 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 third 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

*'''Richness:'''{{Anchor|Richness}} A cluster must have a minimum population of 50 members within a [[Magnitude (astronomy)|magnitude]] range of m3m₃ to m3m₃+2 (where m3m₃ is the magnitude of the third 3rd-brightest member of the cluster). To ensure a healthy margin orof 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’sAbell's accompanying statistical study). Abell divided the clusters into six “richness"'''richness groups'''",” depending on the number of galaxies in a given cluster that lie within the magnitude range m3m₃ to m3m₃+2 (the average number of galaxies per cluster for the entire catalog was 64):

*'''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 1.5''h''^-1 Mpc, where the [[Hubble constant]] is assumed to be''H''₀ = 100 km&nbsp;s^-1&nbsp;Mpc^-1, and ''h'' is a dimensionless scale parameter which usually takes value between 0.5 and 1. ''h'' = ''H''₀/100. The precise value of the Abell radius depends on the value taken for that parameter ''h''. For ''h'' = 0.75 (same as ''H''₀ = 75 km&nbsp;s^-1&nbsp;Mpc^-1), the Abell radius is 2 megaparsecs. This is more than twice the estimate Abell gave in 1958, when ''H''₀ was thought to be as high as 180 km&nbsp;s^-1&nbsp;Mpc^-1.

**'''Group 0:''' 30-4930–49 galaxies

**'''Group 1:''' 50-7950–79 galaxies

*'''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 ''H''₀ = 180 km&nbsp;s^-1&nbsp;Mpc^-1, these values correspond to distances of about 33 and 330 Mpc respectively; but using today’s estimate for H₀ (about 71 km&nbsp;s^-1&nbsp;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 12:''' mag80–129 13.3-14.0galaxies

**'''Group 23:''' mag130–199 14.1-14.8galaxies

**'''Group 34:''' mag200–299 14.9-15.6galaxies

**'''Group 45:''' magmore 15.7-16.4than 299 galaxies

*'''Compactness:'''{{Anchor|Compactness}} A cluster must be sufficiently compact that its fifty or more members lie within one “counting"counting radius”radius" of the cluster’scluster's centre. This radius, now known as the “Abell"Abell radius",” may be defined as 1.72/''z'' arcminutes, where ''z'' is the cluster’scluster's redshift, or as 1.5''h''^-1−1 &nbsp;Mpc, where the [[Hubble's law|Hubble constant]] is assumed to be''H''₀ = 100 &nbsp;km&nbsp;s^-1−1&nbsp;Mpc^-1−1, and ''h'' is a dimensionless scale parameter which usually takes value between 0.5 and 1. ''h'' = ''H''₀/100. The precise value of the Abell radius depends on the value taken for that parameter ''h''. For ''h'' = 0.75 (same as ''H''₀ = 75 &nbsp;km&nbsp;s^-1−1&nbsp;Mpc^-1−1), the Abell radius is 2 megaparsecs. This is more than twice the estimate Abell gave in 1958, when ''H''₀ was thought to be as high as 180 &nbsp;km&nbsp;s^-1−1&nbsp;Mpc^-1−1.

**'''Group 5:''' mag 16.5-17.2

*'''Distance:'''{{Anchor|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 &nbsp;km/s). Assuming ''H''₀ = 180 &nbsp;km&nbsp;s^-1−1&nbsp;Mpc^-1−1, these values correspond to distances of about 33 and 330 &nbsp;Mpc respectively; but using today’stoday's estimate for H₀ (about 71 &nbsp;km&nbsp;s^-1−1&nbsp;Mpc^-1−1) Abell’sAbell's upper and lower limits are actually set at about 85 and 850 &nbsp;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 &nbsp;(about 1,700 &nbsp;Mpc). Abell divided the clusters into seven “distance"distance groups”groups" according to the magnitudes of their tenth -brightest members:

**'''Group 6:''' mag 17.3-18.0

**'''Group 51:''' mag 1613.5-173–14.20

**'''Group 2:''' 80-129mag galaxies14.1–14.8

**'''Group 3:''' 130-199mag galaxies14.9–15.6

**'''Group 4:''' 200-299mag galaxies15.7–16.4

**'''Group 5:''' moremag than 299 galaxies16.5–17.2

**'''Group 6:''' mag 17.3-183–18.0

**'''Group 7:''' mag > 18.0

*'''Galactic-Latitude latitude:'''{{Anchor|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&mdash;{{snd}}not to mention interstellar obscuration&mdash;{{snd}}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 coordinate system|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 coordinate system|Equatorial coordinates]] ([[right ascension]] and [[declination]]) were given for the [[equinox of 1855]] (the [[epoch (astronomy)|epoch]] of the [[Bonner Durchmusterung]]) and [[Galactic coordinate system|galactic coordinates]] for [[1900]].

*'''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&mdash;not to mention interstellar obscuration&mdash;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 coordinate system|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 coordinate system|Equatorial coordinates]] ([[right ascension]] and [[declination]]) were given for the [[equinox of 1855]] (the [[epoch (astronomy)|epoch]] of the [[Bonner Durchmusterung]]) and [[Galactic coordinate system|galactic coordinates]] for [[1900]].

Also listed for each cluster were the following:

*The cluster's [[precession]] rate

*Thethe cluster's [[precession]] rate
*the [[Apparent magnitude|magnitude]] of the cluster's tenth -brightest member

*Thethe distance group of the cluster

*Thethe richness group of the cluster

==The Southern Survey==

[[File:Northern part of Abell 1758.jpg|thumb|Northern part of galaxy cluster [[Abell 1758]].<ref>{{cite web|title=Northern part of Abell 1758|url=https://www.spacetelescope.org/images/heic1801a/|website=www.spacetelescope.org|access-date=19 January 2018}}</ref>]]

The sky-coverage of the 1958 catalog was limited to declinations north of –27°, the original southern limit of POSS. To rectify this and other shortcomings, the original catalog was later revised and supplemented with an additional catalog – {{snd}}the “Southern Survey” –"Southern Survey"{{snd}}of rich galaxy clusters from those parts of the south celestial hemisphere that had been omitted from the original catalog.

The Southern Survey added a further 1,361 rich clusters to Abell’sAbell's original Northern Survey. The deep IIIa-J plates of the [[Southern Sky Survey]] (SSS) were used in the survey. These photographic plates were taken with the [[United Kingdom]]’s's 1.2-metre [[Schmidt Telescope]] at [[Siding Spring Observatory]], [[Australia]], in the 1970s. Abell began the survey during a sabbatical year in [[Edinburgh]] in 1976. There he enlisted the assistance of [[Harold G Corwin]] of the [[University of Edinburgh]], who continued to work on the catalog until 1981, at which time he joined the Department of Astronomy at the [[University of Texas at Austin|University of Texas]]. By then about half the survey had been completed. An interim paper on the Southern Survey was read at a symposium in [[1983]], about one month before Abell’sAbell's death; the catalog was completed by [[Ronald P Olowin]] of the [[University of Oklahoma]], and published in [[1989]].

Abell and Corwin worked from original plates stored at the [[Royal Observatory, Edinburgh|Royal Observatory]] in [[Edinburgh]], scanning the plates visually with a 3x wide-angle magnifier; Olowin used high-quality film copies, which he scanned both visually with a 7x magnifying lens and automatically with a [[backlit digitizer]].

The criteria for inclusion in Abell’sAbell's Northern Survey were retained, as were Abell’sAbell's “richness”"richness" and “distance”"distance" classifications – {{snd}}but with the distance classes now being defined in terms of redshift rather than magnitude. As before, clusters were included if they had at least thirty bright galaxies, as it was estimated that this would all but eliminate the possibility of genuinely rich clusters (i.e. clusters with at least fifty bright members) being omitted. The Southern Survey retains the system of designation devised by Abell for his original catalog, with the numbers running from 2713 to 4076. (The catalog contains three duplicate entries: A3208 = A3207, A3833 = A3832, and A3897 = A2462.) The equatorial co-ordinates are for the equinoxes 1950 and 2000, while the galactic co-ordinates are calculated from the 1950 equatorial co-ordinates.

Abell’sAbell's original catalog – {{snd}}revised, corrected and updated – {{snd}}was included in the 1989 paper, as was the Abell Supplement, a supplementary catalog of 1,174 clusters from the Southern Survey which were not rich enough or were too distant to be included in the main catalog.

==Format==

The standard format used to refer to Abell clusters is: Abell X, where {{nowrap|1=X = 1 to 4076}}. E.g. Abell 1656.

Alternative formats include: ABCG 1656; AC 1656; ACO 1656; A 1656, and A1656. Abell himself preferred the latter, but in recent years ACO 1656 has become the preferred format among professional astronomers and is the one recommended by the ''[[Centre de donnéesDonnées astronomiques de Strasbourg]]'' (see [[SIMBAD]]).

==Members==

{{mainMain|List of Abell clusters}}

Some interestingnotable members of Abell’sAbell's catalog include:

{{main|List of Abell clusters}}

*Abell S373, the [[Fornax Cluster]]

Some interesting members of Abell’s catalog include:

*[[Abell 426|A426]], the [[Perseus Cluster]]

*[[Abell 1367|A1367]], the [[Leo Cluster]]

*[[Abell 1656|A1656]], the [[Coma Cluster]]

*[[Abell 2151|A2151]], the [[Hercules Cluster]]

*[[Abell 3526|A35262744]], the [[CentaurusPandora's Cluster]]

*Abell 3526, the [[Centaurus Cluster]]

About 10% of Abell clusters at redshift z < 0.1 are not genuine rich clusters but, rather, the result of the superposition of sparser groupings. The extremely large and extremely rich [[Virgo Cluster]] was excluded from the Abell catalog because it covered too large an area of the sky to appear on a single photographic plate.<ref name="atlas">

{{cite web

| title = The Fornax and Eridanus Clusters

| publisher = An Atlas of The Universe

| url = http://www.atlasoftheuniverse.com/nearsc.html

| access-date = 2012-02-25

}}</ref>

==See also==

* [[Catalogue of Galaxies and of Clusters of Galaxies]]

* [[Hickson Compact Group]]

* [[List of galaxy groups and clusters]]

==References==

*{{cite book|author=Abell, G O; |author2=Corwin, H G; |author3=Olowin, R P|title=A Catalog of Rich Clusters of Galaxies|others=The Astrophysical Journal Supplement Series, 70 | pages=1-1381–138|yeardate=1989}}

{{Reflist}}

==External links==

*[http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1958ApJS....3..211A&amp;db_key=AST&amp;high=4190a9787218548 Abell's 1958 paper and catalog]

*[http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1989ApJS...70....1A&amp;db_key=AST&amp;high=4190a9787218916 Abell, Corwin and Olowin's 1989 paper and catalog]

*[https://web.archive.org/web/20000610231202/http://cdsweb.u-strasbg.fr/cgi-bin/VizieR-2?-source=VII/110A%2F110A Electronic form of the Abell catalog]

{{Authority control}}

==References==

*{{cite book|author=

Abell, G O|title=The distribution of rich clusters of galaxies. A catalogue of 2712 rich clusters found on the National Geographic Society Palomar Observatory Sky Survey|others=The Astrophysical Journal Supplement Series, 3 | pages=211-288 | year=1958}}

*{{cite book|author=Abell, G O; Corwin, H G; Olowin, R P|title=A Catalog of Rich Clusters of Galaxies|others=The Astrophysical Journal Supplement Series, 70 | pages=1-138|year=1989}}

==See also==

* [[List of Abell clusters]]

* [[List of galaxy clusters]]

[[Category:Astronomical catalogues of galaxy clusters]]

[[Category:Abell objects|*]]

[[de:Abell-Katalog]]

[[lb:Abell-Katalog]]