2.4. Description of the Catalogue

The observed properties of 603 isolated pairs of galaxies are collected in Appendix 1. In its columns we present the following information:

(1) -- Ordinal numbers of the pairs in the catalogue. The letters a and b indicate the western and eastern components of the systems. Pairs in the catalogue and galaxies in each pair are presented in increasing order of right ascension. Among the 603 objects in the catalogue, 18 appear to be single galaxies, but such cases, e.g. no. 42, are not excluded from the catalogue in order to preserve the original numerical sequence of the objects.

(2) and (3) -- Equatorial coordinates of the galaxies for the epoch 1950.0. The basic positional data were taken from the CGCG (Zwicky et al. 1961-1968). Small corrections were performed in certain cases to distinguish the members of galaxies in close pairs.

(4) -- The upper and lower lines give respectively the galactic longitude and galactic latitude of the center of the pair to the nearest degree.

(5) -- Apparent magnitudes of the galaxies from the CGCG transformed to the Holmberg photometric system according to (2.14). For tight pairs for which the CGCG lists only integrated magnitudes, individual magnitudes of components were derived according to (2.11).

(6) and (7) -- Major and minor axis diameters of the galaxies in arc minutes. The diameters were measured on the blue charts of the Palomar Survey and transformed to the standard isophote of 25^m/sq.arc sec. using (2.12) and (2.13).

(8) -- Morphological types of the galaxies in the Hubble classification: E, S0, Sa, Sb, Sc, Sm. The last type includes irregular galaxies. In the original version of the catalogue (Karachentsev, 1972) we presented only a coarse classification of objects into elliptical and spiral. To refine the structural type we compared the appearance of the galaxies on the blue and red Palomar charts. Wherever possible, we have incorporated results from large-scale images of the galaxies.

(9) -- Angular separation between components in arcminutes.

(10) -- The isolation criterion satisfied by the pair, according to (2.6) through (2.9) and illustrated in Figure 1.

(11) -- Type of interaction between components. With type L we indicate pairs in which one or both galaxies have linear extended structure of the type tail (t), bridge (b), or combination (bt). Type A indicates an extended, common `atmosphere' around both components, with amorphous symmetry (am), or shell-like and irregular (sh). The last interaction class, D, indicates disturbances in the spiral structure or general form of one (1) or both (2) components. This classification scheme is shown schematically in Figure 3, along with the original, less abbreviated form of the notation. This scheme is sufficiently simple to be compared with the descriptions of interaction in the MCG or in the Arp atlas (1966). In its first approximation it depends only weakly on the orientation of a pair of galaxies with respect to the line-of-sight.

Figure 3.

(12) -- Spectral type of the galaxies on the Tifft classification (1982): S, M, W or A (see above), which depends on the number and intensity of emission lines. For each determination, we have incorporated our own spectrograms and results from the literature. A comparison with Tifft's classification was performed on 302 galaxies in common.

(13) -- Radial velocity of the components in kilometers per second, corrected for the solar motion from the standard formula, V = 300 sin l^IIcos b^II, where l^II and b^II are the galactic longitude and latitude of the galaxies. Nearly half of the pairs in the catalogue have multiple measures of the radial velocity, and in such cases we have adopted the data presented with the greatest accuracy, subject to the condition that the radial velocities for both components of a pair are taken from the same author.

(14) -- Internal errors of measurement of the radial velocity from the published sources. Tifft (1982) does not list individual errors but lists a general error, _V 65 km/s. For measurements by Tifft we adopted a value of _V = 70 km/s, independently of the spectral type of the galaxy or the conditions of its observation.

(15) -- An indication of the source of the radial velocity, divided into the following categories: K-measurements with the 6-m telescope (Karachentsev, 1980a, 1981a, 1981b, 1983); W-White et al. (1983); T-Tifft (1982); P-observations at the Palomar and Crimean Observatories (Karachentsev et al. 1974, 1975, 1976, 1979); R-data from the reference catalogue of de Vaucouleurs et al. (1976), with the addition of the Rood (1983) catalogue; A-references from additional sources (Gregory 1975, Khachikhian 1973, Huchra et al. 1983). Excluding the 18 catalogue objects which appear to be single galaxies, the distribution of the 585 pairs according to the source of data on radial velocities is as follows: K-362, T-118, W-42, P-38, R-22, A-3. For the single objects, Appendix 1 presents only the coordinates and radial velocities.

The overall distribution on the sky of the double galaxies in the catalogue is presented in equatorial coordinates in Figure 4. This distribution exhibits a zone of avoidance produced by absorption of light along the Milky Way and also weakly exhibits some non-uniformities produced by a variation of limiting magnitudes from volume to volume in the CGCG.

Figure 4.

In Table 2 we summarise the numbers of galaxy pairs falling into various categories. The first three lines present the number of double systems satisfying the isolation criteria for various degrees of strictness. Then, the pairs are grouped by morphological types, and by interaction class. At the end we give the number of objects in common with other catalogues and atlases.

Figures 1-30 of the insert present large-scale images of 43 pairs of the catalogue, all of which were obtained at the prime focus of the 6-m telescope at a scale of 8.6"/mm. Comparison of the structure of the objects on these photographs with a description of the pairs in Appendix 1 will indicate the types of interaction of the galaxies and the degree of isolation of the pairs from their nearest neighbours.

Figures 31 and 32 show reproductions of the spectrograms of double galaxies obtained with the 6-m telescope at a dispersion of approximately 100Å/mm. They exhibit a range in emission properties and illustrate the classification of spectral types.

Knowing the radial velocities allows a final reduction of apparent magnitude and angular diameter to a standard system, as well as a derivation of the luminosity and linear diameter. The differences in radial velocities and apparent linear separation between components are then used to estimate the mass for double systems. The values of these basic derived quantities are given in Appendix 2, whose columns present:

(1) -- Ordinal number of the pair from the catalogue. Apparently single objects are now omitted.

(2) -- Apparent magnitudes of the galaxies on the Holmberg system corrected for extinction effects according to (2.15). As already noted this is very close to the integrated magnitude B⁰_T of the RCBG.

(3) -- Surface brightness of the members of pairs in magnitudes per square arcsecond

where the angular diameter of the major axis of the galaxy, a₂₅, is measured in arcseconds. We remind the reader that for tight pairs the CGCG presents only integrated total magnitudes for the objects. Individual values for the components are derived from (2.11), assuming the same surface brightness for both galaxies. Such very tight pairs are indicated by an asterisk next to the surface brightness of the first component. Small differences in B for the components of some of these pairs arise after a reduction of the diameters to the standard isophotal system.

(4) -- Linear diameter of the major axis of the galaxy A₂₅ = a₂₅ V₀ / H in kiloparsecs. For determination of the distances we have taken the individual values of radial velocity for the components V₀. Here and in the following analysis we adopt a Hubble constant H = 75 km/s/Mpc.

(5) -- Absolute magnitude of a galaxy on the Holmberg system

(6) -- Linear separation between components of the pair, X = x₁₂(V₁ + V₂)/2H in kiloparsecs, projected on a plane normal to the line of sight.

(7) -- Total orbital mass of the pair

(2.17)

where G is the gravitational constant and the coefficient (32 / 3) incorporates projection factors based on the assumption of a random distribution of the orientation of pairs, and circular motion for the members of the pairs. Pair masses are given in units of solar mass.

(8) -- Ratio of the orbital mass of the pairs to the total luminosity of their components

(2.18)

in solar units for solar absolute magnitude M = + 5.40^m.

Note that the calculated masses and mass-to-light ratios contain statistically induced errors. Because of the quadratic dependence of mass on the velocity difference, errors in measures of radial velocity contribute an asymmetric distribution of errors in the mass. This will lead to a systematic overestimate of the orbital mass of double systems. An unbiased estimator of the mass-to-light ratio is given by

(2.19)

where the errors in the radial velocity components ₁ and ₂ are seen to be uncorrelated. The individual values f_c for pairs are not given here because they can easily be computed from the data in columns (13) and (14) of Appendix 1.