CGCG (G8): Zwicky Catalog of Galaxy Clusters (19930515)
FILES: cgcg.g8 (9134 galaxy clusters)
CONTENTS: Zwicky Catalog of Galaxy Clusters
DATASET: This file comes in HyperSky dataset G-11, version 19930515. Availability on HyperSky CD-ROM: Vol. 1 (1993)
DESCRIPTION: This datafile contains 9134 galaxy clusters from the Catalog of Galaxies and Clusters of Galaxies (CGCG), by Fritz Zwicky, as prepared and by Marin Kalinkov of the Bulgarian Academy of Sciences. The HyperSky file is based on the machine-readable version of the catalog provided on the NASA/GSFC Astronomical Data Center (ADC) cd-rom "Selected Astronomical Catalogs", Volume 1.
The CGCG covers a subset of the fields of the Palomar Sky Survey, or POSS, extending from the north celestial pole south to the Sky Survey declination zone astride the celestial equator. The CGCG also excludes POSS fields north of the celestial equator which are primarily Milky Way fields. Zwicky identified clusters in 560 of the POSS fields. To determine cluster diameters, Zwicky drew isopleths at the level where the cluster density was twice that of the background density of galaxies. The number of cluster members was determined by counting all galaxies within the isopleth and within three magnitudes of the brightest member, subtracting the background count. All Zwicky clusters are rich clusters, having at least 50 members within 3 magnitudes of the brightest member.
Zwicky's method of defining galaxy clusters provides an interesting contrast to the method used to define Abell galaxy clusters (available in HyperSky dataset G-3). Abell considered all galaxy clusters to have the same intrinsic diameter (about 3 megaparsecs), and counted all galaxies within two magnitudes of the third brightest galaxy to determine the cluster size, again making a background correction. One result of this is that any cluster in both the Zwicky and Abell cluster catalogs will usually be classified as larger and more populous by Zwicky than it is by Abell. You will also find many Zwicky clusters which contain two or three Abell clusters. For example, the Zwicky Hercules galaxy cluster (Zwicky cluster 7763 in the ADC catalog) contains Abell clusters 2147, 2151 and 2152.
In the ADC catalog, Zwicky clusters are ordered in ascending Right Ascension, and are identified by an associated cluster number which runs from 1 to 9134. This numbering scheme was not used in the original published version of the CGCG. In the original version of the CGCG, Zwicky identified galaxy clusters using a combination of the CGCG field number and the cluster number within that field. Zwicky's identification scheme suffered from the problem that a cluster spanning several fields would have several identifications, one for each field. Thus the HyperSky catalog follows the ADC catalog in identifying Zwicky's clusters with the sequential numbers 1-9134. Zwicky's cluster identifications also appear, however, in the comments following the cluster identification line.
Note that this HyperSky dataset covers only CGCG galaxy clusters. Individual CGCG galaxies are available in HyperSky G-4. CGCG fields can be identified and outlined on your HyperSky field display with boundary data files available in HyperSky dataset A-1.
DATA: The HyperSky datafile contains the following data from the catalog:
MAG: No cluster-related magnitudes appear in the ADC catalog or in the derived HyperSky data file.
IDENTIFICATION: The sequential Zwicky cluster number, from 1 to 9134, prefixed by ZC (for "Zwicky Cluster"). Note that the ZC prefix is a HyperSky convention, and was not used in the ADC catalog.
COMMENTS: The comment lines contain the following additional information, provided if available for each galaxy cluster:
(a) One of the following three strings defining the cluster type:
1. "open"
2. "medium compact"
3. "compact"
(b) One of the following five strings defining cluster distance class:
1. "near"
2. "medium distant"
3. "distant"
4. "very distant"
5. "extremely distant"
The cluster type and distance class will appear together on one line, separated by a comma.
(c) The number of galaxies in the the cluster, given by a string of the form "NN gals", where NN is a number.
(d) The cluster diameter, given by a string beginning with "dia=NN" where NN is the number of arc minutes (if NN is followed by a single quote) or degrees (if NN is followed by the string "deg").
The number of galaxies and cluster diameter will appear together on one line, separated by a comma.
(e) All possible Zwicky cluster identifications, given on one or more lines each beginning with the string "P-C:" (for Plate-Cluster). Following this on each line will appear up to four comma-separated Zwicky identifications of the form PPP-CCC, where PPP is a CGCG plate number, and CCC is Zwicky's number for the cluster on that plate.
According to the ADC file descriptions there can be as many as twelve separate identifications for each cluster. Thus there may be as many as four "P-C:" lines for a Zwicky cluster in the HyperSky file. In practice, however, and the vast majority of clusters will have a single line with four or less identifications, and most clusters will have only one Zwicky identification.