The present Catalogue is based mainly on the Palomar Sky Survey
prints. It is designed to be essentially complete to the limiting
diameter 1'.0 on the blue prints; it also includes all galaxies to the
limiting apparent magnitude 14.5 (even those smaller than 1`.0) in the
Catalogue of Galaxies and of Clusters of Galaxies (CGCG) by Zwicky et
al. It covers the sky to the north of
= -02°30'. The
galaxies are
ordered according to their right ascensions; the epoch used is 1950.0.
Besides descriptions of the galaxies and their surrounding areas, the Catalogue contains classifications in conventional systems. Both as concerns the use of a limiting diameter and in the classification and description technique, the present Catalogue differs from the Morfologiceskij Katalog Galaktik (MCG) by Voroncov-Vel'yaminov et al., which was also based on the Sky Survey. Unlike most modern catalogues of galaxies, this one contains position angles for flattened galaxies. The revived interest during recent years in the orientation of galaxies in space makes it important to have such data listed for a large number of fairly bright galaxies.
For details concerning the Sky Survey, see Wilson (1952), California Institute of Technology (1954), Abell (1958) and Minkowski and Abell (1963).
Diameters have been measured on both the blue and red Sky Survey prints. The measuring procedure is described in section 2b. The classifications and descriptions have been designed to give as complete an account as possible of the appearance of the objects on the prints. Eighty-two plates from the Uppsala Observatory 40-in. Schmidt telescope at Kvistaberg have been used for tests of the classifications; the plates cover about 480 square degrees in areas rich in galaxies. Magnitudes are listed from CGCG or estimated from the blue prints for objects fainter than 15.7.
Special care has been taken to record peculiarities, interaction, distortion, or the occurrence of bridges, jets, plumes and other non-regular phenomena. A fixed vocabulary has been developed to describe properties not covered by the classification symbols. When possible, different classification systems have been used in a complementary way, to fully exploit the information content of the images on the prints.
The application of a limiting diameter results in the inclusion of a fairly large number of "dwarf" objects with an extremely low surface brightness. The difficulties of discovering such objects and of measuring their diameters make their inclusion in the Catalogue somewhat arbitrary. Of course, the concept of a "limiting diameter" has different significance for different types of galaxies; it is naturally the less meaningful, the lower the surface brightness of the objects.
In order to make the Catalogue as complete as possible and to avoid large inhomogeneities in the measurements, the Sky Survey prints have been searched through three separate times. The Catalogue is based mainiy on the last two surveys. Each survey continued for approximately one year. It is, of course, impossible to keep a fixed limiting diameter strictly, as the outer parts of the galaxies are usually very faint and an apparent "edge" is difficult to define. Moreover, the Sky Survey prints themselves cannot be expected to be quite homogeneous. Variations in the actual limiting diameter are thus to be expected, and, consequently, the total number of galaxies down to the estimated limiting diameter is for different areas not certainiy significant for the description of the distribution of galaxies: this number should, in any case, be treated with care. An analysis of data from the Catalogue will appear shortly.
The original data were recorded in a card catalogue, including approximately 15,000 cards and containing all separate measurements and classifications. Listed diameters are mean values of the last two measurements. Classifications have, in the final Catalogue, been listed in such a way as to give an account of the considerable uncertainties which are often present. Attention should be paid to the definition of the "information parameter", which may be interpreted as an estimate of this uncertainty.
All survey work, measurements, identifications and classifications were made by the author, who also prepared the final manuscript for printing by the offset method.
A catalogue of this kind may be expected to be used in three different ways: pro primo, as a reference work, giving elementary data for a large number of galaxies in the northern sky; pro secundo, as a fairly homogeneous set of data which may be used in statistical investigations; pro tertio, as a guide to the selection of objects in several kinds of observation programs, for example, studies of interacting galaxies.
Peter Nilson
Astronomical Observatory, Uppsala
March, 1973