2. EXPLANATIONS

(a) Arrangement of the Catalogue pages

The Catalogue is divided into an Index Section, containing lists of data for 12,921 galaxies, and a Notes Section. In the Index Section, the data for each galaxy are written on two lines (hereafter denoted a and b). Each page is divided into 12 columns. The listed data are as follows:

Column	Line	Data
1	a	Number of the galaxy in this Catalogue
2	a	, for the epoch 1950.0
2	b	1II, bII
3	a	MCG numbers (Morfologiceskij Katalog Galaktik by Voronco-Vel'jaminov et al.)
3	b	NGC and IC numbers; Messier numbers; Markarian's numbers; in a few cases, other identifications
4	a	Number of the Palomar Sky Survey field, in which the galaxy is best visible
4	b	Number of cluster in CGCG (Catalogue of Galaxies and of Clusters of Galaxies by Zwicky et al.) classified by Zwicky as "near", if the galaxy is located inside the contour line of such a cluster (according to lists on pages 445-447)
5	a	Diameters measured on the blue Palomar Sky Survey prints
5	b	Diameters measured on the red prints
6	a	Position angle (from north eastwards)
6	b	Estimated inclination to the line of sight of flattened galaxies (scale 1-7; 7 = edge-on)
7	a	Information parameter, estimating the uncertainty in the classification (scale 0-5; 0 = most uncertain case)
8	a	Classification in the Hubble system
8	b	Luminosity class, according to van den Bergh's system (only IV, IV-V and V)
9	a	Classification, according to de Vaucouleurs's revised Hubble system
9	b	Classification, according to Holmberg's revised Hubble system Columns 8 and 9 are also frequently used for short descriptions (cf. sections 3 and 4)
10	a	apparent photographic magnitude from CGCG (Zwicky) or estimated from the blue prints if fainter than 15.7
11	a	Radial velocity referred to the sun
11	b	Radial velocity corrected for solar motion
12		References to the Notes Section (*) or to other catalogues (cf. section 2b)

(b) Further discussion of the Catalogue data

Column 2, line a: Co-ordinates have been adopted from CGCG (with a few corrections accounted for in the Notes Section), to facilitate the joint use of CGCG and the present Catalogue. For galaxies fainter than 15.7 or located at low galactic latitudes not covered by CGCG, positions have been determined in relations to GC stars; the accuracy is only what is necessary for identification purposes (to tenths of minutes in right ascension and to minutes of arc in declination).

Column 2, line b: Galactic co-ordinates have been computed from listed and in an HP 9100A calculator. Two decimals are given in all values, regardless the accuracy in the position determinations. The north galactic pole is situated at

₀ = 12^h 49^m

₀ = + 27° 24' (1950.0)

and the north celestial pole (1950.0) at galactic longitude

₀ = 123° 00' (cf. Blaauw et al. 1960).

Column 3, line a: Numbers in MCG. For typographical reasons, these numbers are written, for instance, 03-01-018 instead of 3-1-18. The figures in the first position give the number of the Palomar Sky Survey zone, each zone covering 6° in declination, from the equator (No. 1) to the north celestial pole (No. 15). The figures in the second position give the number of the Sky Survey field along the zone; the figures in the last position represent the number of the galaxy in this field in MCG. Galaxies not included in MCG are listed with appropriate figures in the first two positions (the field number in MCG) but have 000 in the last position.

Column 3, line b: IC numbers are - if repeated or written in connection with NGC numbers - indicated by an asterisk (*) Mark = Markarian (numbers from the lists of galaxies with ultraviolet continuum: Markarian (1967, 1969a, b), Markarian and Lipovetsky (1971, 1972)). Ho = Holmberg (nine dwarf galaxies discovered by him and denoted by Roman numerals: Holmberg (1958, 1969)).

Column 4, line b: Clusters of galaxies classified as "near" in CGCG are listed in order of right ascension on pages 445-447. A number in this position gives the number of such a cluster if the galaxy is located inside its contour line; however, this location is no guarantee of membership in individual cases. Obvious foreground objects are denoted by having their cluster numbers placed within brackets.

Column 5: Diameters (major axis x minor axis) were measured directly on the Sky Survey prints by using a Leitz magnifier with a magnification of X 6 and a built-in measuring scale (graded to 0.1 mm). Listed values refer to maximum diameters on the prints, i.e. the maximum extent of features which may confidently be described as belonging to the galaxy. As these values are very difficult to obtain for early-type systems (E, SO and early Sa), the diameters for such objects have been placed within brackets. Late-type objects are usually somewhat patchy in the outskirts, where the surface brightness is low, which introduces uncertainties of a different kind; uncertain values for such objects have been indicated by a colon or, for great uncertainties, a double colon.

The galaxies have always been searched for on the blue prints. The prints have been surveyed through an S.0.C. "Visolett" magnifier with a magnification of X 1.6 and a diameter of 3.5 cm. The procedure was to survey first the right-hand half of the prints from the bottom upwards, and the lens was moved in "sweeps", with so much overlap that every object was expected to enter in the field of view at least three times. The same procedure was then applied to the left-hand part of the print, with sufficient overlap between the two halves. Every galaxy expected to be close to 1'.0 or larger was measured with the Leitz magnifier. At least two separate settings were made for each diameter, with the measuring scale in opposite orientations. Approximate positions were determined by using a coordinate grid.

Diameter values inside square brackets denote total diameters for pairs, triplets or multiple systems. Such objects were included in the Catalogue if they were in some way connected with each other (contact or connection by bridge or other intergalactic features), and the total maximum diameter was found to exceed 0'.9. For such objects, usually only diameters from the blue prints are listed.

Blue and red diameters were measured at such a long interval (usually several hours) that they may be regarded as essentially independent of each other. For practical reasons, it was impossible to make separate surveys on the blue and red prints.

Column 6, line a: Position angles were measured from the north eastwards by using a protractor 25.5 cm in diameter, in combination with a triangle giving the direction perpendicular to the upper and lower edges of the prints. Corrections for the inclination of the meridian lines were computed for different zones and indicated on the co-ordinate grid. Uncertain values are denoted by a colon. Values in brackets refer to almost face-on spirals or other galaxies with small ellipticity. Square brackets denote asymmetric systems, for which the position angle gives less significant information as regards the orientation in space.

At least two separate settings were made at each measurement, with the position of the protractor reversed in order to avoid effects from small asymmetries in the galaxy. In order to avoid selection effects in position angles (Öpik 1963, 1968), several prints were re-surveyed in different orientations. No systematic effects were noticed.

Column 6. line b: The inclination to the line of sight for spirals is estimated on a scale from 1 (face-on) to 7 (edge-on). For galaxies with a high inclination to the line of sight, the parameter was calculated from the Hubble formula n = 10 (a - b) / a, where a = major axis and b = minor axis. The value 7 is used for objects inclined not more than a few degrees to the line of sight.

Column 7, line a: Owing to the small scale and the high contrast, classifications on the Sky Survey prints are usually regarded as difficult. However, it is the author's experience that classifications (at least approximate ones) are possible for most galaxies to the limiting diameter 1'.0, provided that the estimated uncertainties are accounted for in an appropriate way. The information parameter (0-5) is to be interpreted as follows:

Columns 8 and 9: The Hubble class corresponds approximately to the well-known "tuning-fork" classification, with the class S B O added (Hubble 1926, 1936); it agrees fairly well with Sandage's revised Hubble system (1961), although Sandage's subtypes are omitted. For spiral galaxies, the class is estimated essentially from the texture of the arms or the texture of the outer parts of the objects. It appears to be generally recognized that even early spiral systems may (in contradiction to Hubble's criterion) sometimes be very open and that late spirals may have tightly coiled arms. Colours, which are helpful in the classification work, have been estimated by comparing the blue and red prints.

It has been argued (Voroncov-Vel'yaminov 1960, 1967) that traditional classification systems are not adequate to account for the variety of structures among galaxies. In fact, it seems likely that no existing classification system alone is able to represent the varieties, as concerns morphology, colour and surface brightness; thus, it appeared reasonable to use in this Catalogue several systems alongside each other in a complementary way. Voroncov-Vel'jaminov (1967) has also suggested that many "peculiar" galaxies form definite morphological types. Even if this statement is true, it is clearly impossible to base a more appropriate classification system, as concerns the morphology of galaxies, on the Sky Survey alone. The observed "peculiarities" have thus been accounted for mainly by a fixed description system.

Complementary classifications have been made essentially according to the descriptions by van den Bergh (1960a), de Vaucouleurs (1959) and Holmberg (1958). Very late spirals (de Vaucouleurs's stages dm and m) are not classifled in the Hubble system. The notation "dwarf" is used for objects with (1) very low surface brightness and (2) litlle or no central concentration of light on the red prints (cf. van den Bergh 1959).

In this context, the standard Hubble system (as concerns the spirals) may be interpreted as a division of galaxies according to colour (or texture in the outer parts). On the other hand, de Vaucouleurs's revised system accounts mainly for the morphological structure of the objects; the finer classification in "stages" makes the location of the object along the Hubble sequence (the "third dimension" in this system) more uncertain. Owing to the over-exposure on the prints, the "varieties" (central ring or spiral structure) are in many cases difficult or impossible to determine.

For the definition of the notations "singular", "peculiar" and other terms used in columns 8 and 9, see sections 3 and 4.

No classification system can be expected to be completely independent of the observer's interpretation of it. Thus, the classifications listed in the present Catalogue (UGC) may show certain individual features which distinguish them from results obtained by other observers. The aim has been, of course, to make the classifications as homogeneous as possible within the Catalogue itself.

Column 10, line a: Magnitudes adopted from CGCG. If the galaxy is a member of a pair or a system of higher order, and the total magnitude is given in CGCG, this magnitude value has been placed within brackets. For objects fainter than 15.7, the magnitudes have been estimated from the blue prints according to the following scale:

For brighter objects at low galactic latitudes not covered by CGCG, the magnitudes have been estimated to ± 0^m.5.

Column 11, line a: Radial velocities listed in the Reference Catalogue of Bright Galaxies (BG) are adopted in the present Catalogue, taking into account the corrections listed in Publ. astr. Soc. Pacific 78, 262 and 84, 461. Supplementary radial velocities have been taken from Arakelian et al. (1970a, b; 1971; 1972a, b), Arp (1972), Chincarini and Rood (1971), DuPuy (1970), Fairall (1971), Rudnicki and Tarraro (1969), Sargent (1970b), de Vaucouleurs and de Vaucouleurs (1967), Weedman and Khachikian (1968, 1969), and Zwicky (1971). Among other publications concerning radial velocities, attention is called particularly to Chincarini and Rood (1972), Ford et al. (1971), Page (1970), Sargent (1970a, 1972) and Ulrich (1971).

Column 11, line b: Radial velocity corrected for solar motion relative the Local Group according to the formula V₀ = +300 cos A, where A is the distance to the conventional solar apex at l^I = 55°, b^I = 0°, or l^II = 87°, b^II = +1 (cf. de Vaucouleurs and de Vaucouleurs 1964, p. 3; Humason et al. 1956; Humason and Wahlquist 1955).

In some cases, only corrected velocities have been available. Attention is called to the fact that some observers (in this context Sargent and Ulrich) correct the velocities to the centre of the Galaxy, using the value 250 km s^-1 for the solar motion (the current IAU value of galactic rotation). (Considering the uncertainties present in most redshift determinations, this difference is, in fact, usually of minor importance).

If the uncertainties in the measurements of the radial velocity were considered too large to make any correction meaningful, usually only the uncorrected value was listed.

Column 12: References according to the following system:

(c) The Notes Section

The galaxies are divided into groups of ten, according to their UGC numbers. Thus, every tenth UGC number is to be found in the left-hand margin. The UGC numbers in the notes are preceded by an "U" and followed by a bracket.

A note in the form "comp 1.5, 15, 0.4 × 0.3" is to be interpreted "companion at 1'.5 from centre, in position angle 15°, with diameters 0'.4 × 0'.3". The notation "p w comp at ..." ("pair with companion at ...") is used only if the companion is comparable in size and/or magnitude with the listed galaxy. "Companion" galaxies are accounted for to the limiting diameter 0'.3 and to a distance of 5'.0 from the centre of the galaxy (however, "companions" are not recorded in crowded areas). Listed co-ordinates and magnitudes refer to CGCG. For the sources of radial velocities, see section 2b.

Two UGC galaxies were described as a "pair" whenever there were no other nearby objects of similar size or brightness and it was deemed possible that they might form a physical pair.

Data concerning supernovae have been taken from Karpowicz and Rudnicki (1968) and from Kowal and Sargent (1971).

The terminology of the Notes Section is explained under Abbreviations and Terminology.