2.2. The Observational Data and Their Reduction

The selection of isolated pairs of galaxies, the measurement of various characteristics of the objects, and their reduction to a single standard system, are accomplished in several stages. Each of the 27,841 galaxies of the CGCG catalogue was considered for inclusion in a double system with one or another of its neighbouring galaxies brighter than magnitude 15.7. On the blue charts of the Palomar Survey we measured the angular diameters of possible pair members and also the diameters of any neighbouring galaxies satisfying (2.7) with the adopted dimensionless coefficients (2.8). Pairs were included in the catalogue if the angular distance of the neighbour, in some cases galaxies fainter than the limit magnitude 15.7, exceeded the isolation expression (2.6). Naturally, the catalogue also includes many pairs with isolation according to the stronger criteria (2.9).

Formally, such a procedure would require examination of an extremely large number of galaxies. In the actual process a test for isolation with respect to the i-th neighbour went rather quickly. In selecting double galaxies we considered only angular diameters, apparent magnitudes, and relative separations of the galaxies. Existing information on radial velocities or signs of interaction between the galaxies was not considered, in order to exclude any possibility of subjective bias.

In the second stage of compilation of the catalogue we measured various characteristics of the double galaxy candidates: apparent inclination of the pair members; position angle of the major axis of the galaxies and of the line connecting their centers; morphological type according to the Hubble classification; and estimates of the types of interaction. All of these data were included in the Catalogue of Isolated Pairs of Galaxies in the northern sky (Karachentsev, 1972) which includes 603 objects with declination north of -3°. The catalogue also includes the NGC numbers of the galaxies and references to the galaxy's number in the Holmberg (1937) catalogue and in the atlases of interacting systems by Vorontsov-Vel'yaminov (1959, 1977) and Arp (1966).

Numerous notes to the catalogue contain additional information about the double galaxies. Here are described cases of especially interesting structure in the components. Compact galaxies or Markarian objects of blue colour are noted, along with supposed optical pairs or possible single objects. Cases of questionable double nature arise from the fact that the structure of the bright parts of galaxies are practically invisible on the Palomar charts due to the high contrast with which the prints were produced. Therefore, very compact pairs of galaxies may sometimes appear as single ones containing disturbed spiral structure or dark lanes apparently dividing the galaxy into two parts. We note that some apparent mistakes arise from the terminology: there is no sharp boundary separating dwarf galaxies and very luminous stellar complexes or super-associations.

About 30% of the double systems in the catalogue are very close pairs, for which the CGCG lists only a total apparent magnitude m. In these cases the individual magnitudes of the galaxies m₁ and m₂ were assigned according to

(2.11)

where a and b are the angular diameters of the pair members measured along the major and minor axes. Using (2.11) presupposes a common surface brightness for the galaxies of a single pair. Note that for some fraction of such very close pairs, the apparent magnitude of the individual components becomes fainter than the limit of 15.7.

In the next stage of our reduction the apparent magnitudes were reduced to a standard system. For this we adopted the reduction scheme proposed by Paturel (1977, 1979). The diameters of the galaxies were referred to a standard isophote having surface brightness 25^m/sq.arc sec., and the apparent magnitudes of galaxies in the CGCG were reduced to the Holmberg system.

Comparison of our own measurements of diameters with the diameters from the UGC catalogue (Nilson, 1973) exhibit an excellent correlation.

Notable departures between the diameters measured by Nilson, a_N, and our own measurements, a_K, occur for galaxies with strong signs of interaction. In cases of tails and bridges, Nilson includes such features as part of the galaxy while we consider them to be details distinguished from the basic structure of the galaxy. Excluding such objects we measure for 235 galaxies in common without strong signs of interaction, the following relation: a_N = 1.07a_K - 0.07, where the diameters are expressed in minutes of arc. The rms dispersion about this regression line is _a = 0.28'.

The transformation from measured diameters to the standard system incorporates the Holmberg effect (an incorrect increase in the major axis of a galaxy seen nearly edge-on) and the shrinking of isophotes due to foreground extinction according to Paturel's formula (1979):

(2.12)

Here, m_A = 0.20 cosec | b^II| is the expression for extinction at galactic latitude b^II and m_K is the correction for redshift which comes from the radial velocity V₀ ( km/s), and the morphological type of the galaxy. This is normally referred to as the K-correction. According to Pence (1976),

The coefficient c_T in (2.12) accounts for the brightness gradients of galaxies according to their morphological types:

The measured axial ratio of galaxies b/a is reduced to the standard isophote 25^m/sq.arc sec. according to

(2.13)

The magnitudes in the CGCG catalogue were estimated with the aid of a moving plate-holder, which results in a dependence of the magnitude on angular diameter and galaxy structure. According to the data of Paturel (1979) the magnitudes presented in the CGCG, m, may be transformed to the Holmberg system, m_Ho, by

(2.14)

where a₂₅ (in minutes of arc) and (b/a)₂₅ were taken from (2.12), (2.13), and the last term contains a correction to the zero point of the CGCG from volume to volume: c_vol = 1.07 for volume 1, 1.23 for volume 5, and 1.45 for volumes 2, 3, 4, and 6. Finally, the magnitude m_Ho is reduced for the effects of obscuration

(2.15)

where the symbols m_A and m_K were explained earlier, and

(2.16)

accounts for internal obscuration of light in the galaxy ⁽¹⁾.

The reduction formulae (2.12) to (2.16) are intended to exclude the basic systematic limitations in the apparent magnitudes and diameter scales and produce sufficiently accurate material for further statistical analysis.

¹ It is useful to consider the transformation between our system (m_Ho^c, a₂₅) and the system (B⁰_T, D₀) of the Reference Catalogue of Bright Galaxies (RCBG). There are 71 pair members with values of B_t⁰ and 118 galaxies with measures of D₀. The mean relation for them is <m_Ho^c - B⁰_T> = - 0.002^m ± 0.035^m and <log a₂₅> = 0.96 <log D₀> + 0.02. The differences between the two systems are very small, so that hereinafter we can simply deal with either quantity. Back.