COMA (G0): Coma Cluster Galaxy Catalog (19940313)
FILE: coma.g0 6724 Coma cluster field galaxies
CONTENTS: 1983 Coma Cluster Galaxy Catalog
DATASET: These files come in HyperSky dataset G-18, version 19940313. Availability on HyperSky CD-ROM: Vol. 2 (1995)
DESCRIPTION: This datafile contains a total of 6724 galaxies in the field of the Coma galaxy cluster, as described in the paper "The Coma Cluster - I. A Catalog of Magnitudes, Colours, Ellipticities and Postion Angels for 6724 Galaxies in the Field of the Coma Cluster" by Godwin, Metcalfe and Peach in 1983 (see reference below).
The above catalog contains measurements of isophotal magnitudes, radii, (b-r) colors, ellipticities and position angles for 6724 galaxies and probable galaxies with b(26.5) <= 21.0 in a 2.63-degree-square are centered on NGC 4874 (12h 59.6m, +27 57.6' equinox 2000).
The list of 6724 galaxies includes 6205 galaxies, 146 double galaxies, 2 triple galaxies, 350 probable galaxies and 21 probable double galaxies.
The HyperSky file for this catalog is a "small-scale" datafile. That is, the file covers only a small area of the sky centered on the Coma galaxy cluster. HyperSky treats such small-scale files in a special way: it will display only the outline of the catalog area, NOT its contents, until you reduce the HyperSky field scale appropriately. For this file, you must reduce your HyperSky vertical field scale to 7.0 degrees or less before you can see the catalog galaxies. A half-degree HyperSky vertical field scale provides a reasonable separation of catalog member galaxies; larger scales will result in overcrowding unless the HyperSky faint magnitude limit for galaxies is set a value brighter than the 21st magnitude catalog limit.
DATA:
The HyperSky datafile contains the following data from the catalog:
POSITION: Positions in the source Coma catalog are given as Cartesian offsets from an origin (12h 57.3m, +28 14.4' equinox 1950). Offset units are in arc seconds, extending west and north from the origin. When these raw offsets are converted to standard RA and Dec, and the resulting positions are displayed in a HyperSky field relative to Guide Star Catalog and RC3 galaxy catalog positions (at least for the brighter Coma catalog galaxies), a westward (RA) offset of 2.4 seconds is seen uniformly across the Coma catalog field. It is ASSUMED this 2.4 second offset results from rounding in the above origin: specifying RA to 0.1 minute does NOT provide equivalent accuracy to specifying Dec to 0.1'. Hence, in generating the HyperSky file, this 2.4 second offset was added as a correction to all Coma catalog RA values. As a result, in a 30' scale HyperSky field all brighter Coma catalog galaxy positions match almost exactly RC3 and GSC positions for the same galaxies. It is assumed that all faint Coma catalog galaxy positions are now equally accurate. If however you wish to use Coma catalog positions based precisely on the above-cited origin, you must remember to subtract 2.4 seconds of RA from values displayed in HyperSky.
MAG: The b-band (blue) magnitude, integrated within the 26.5 b mag per square-arcsecond isophote. This limiting isophote, about 3% over the background sky brightness, was chosen for optimum signal-to-noise ratio. The rms deviation, based on repeatability of multiple scans of the same plates, is 0.13 mag for b < 20, rising to 0.2 mag for 20 < b < 21. External errors are likely about 0.15 mag for images brighter than b=20.
IDENTIFICATION: The Coma cluster catalog identification number, increasing from 1 to 6724. The prefix "Com" has been added strictly for the HyperSky file.
COMMENTS: The comment lines contain the following information, provided if available for each galaxy:
* A line having the phrase "probable galaxy" if identification as a galaxy isn't definite. Metcalfe and Peach subjectively discriminated between galaxies and stars by examination of 14270 images brighter than b-band magnitude 21.0. A discussion of the completeness and purity of the search is given in Section 5 of the reference below.
* A line giving the blue diameter of the galaxy, using the isophotal radius based on the area enclosed within the isophote used to determine the blue magnitude, above.
* A line giving the r-band (red) magnitude and dimensions of the galaxy. The magnitude is integrated within the 24.75 r mag per square-arcsecond isophote. This isophote, like the b-band isophote used above, is about 3% over the background sky brightness, and was chosen for optimum signal-to-noise ratio. The r-band and b-band magnitudes are separated by roughly the mean galaxy color, so that they refer to roughly equal metric areas in a given galaxy. The rms deviation is 0.2 mag for r < 18.
R-band dimensions are based on the red diameter of the galaxy, using the isophotal radius based on the area enclosed within the r-band isophote described above. Where dimensions are provided, this diameter is the larger (major axis). The minor axis value is derived by applying the r-band ellipticity to the major axis value. The r-band ellipticity is the axial ratio of the ellipse best approximating the 24 r mag per square-arcsecond isophote.
* A line starting with "(b-r)=" giving the color. The external error in the color is probably about 0.15 mag for images brighter than 20.
* A line including alternate identifications (mainly NGC & IC values) and other short comments. In cases where the object is noted as "double" or "triple", the photometric data above refer to the combined image of its components.
* A line starting with "pa=" giving position angle in degrees of the above ellipse, as measured counterclockwise in the sky from north. The angle is accurate to within a few degrees for most images.
REFERENCE: Godwin, J.G., Metcalf, N., Peach, J.V., 1983, M.N.R.A.S., 202, 113.