4. Observation procedure

The survey work refers to altogether 174 spiral galaxies of types So-Sa-Sb-Sc, both normal and barred spirals. The material comprises all systems of these types in my catalogue (Holmberg 1958) that have major diameters 5'.0 (and distance moduli be low certain limits). In order to get a more complete material, 38 galaxies with diameters above 5'.0 have been added from the. Reference Catalogue by de Vaucouleurs et al. (1964; the diameters of this cat. have been reduced to the Holmberg diameter scale).

With respect to the following analyses it seemed advisable to divide the spiral systems into three different classes, A, B and C.

Class A. Systems with an edgewise orientation (app. diam. ratio 0.53) that are not seriously disturbed by nearby large galaxies.

Class B. Systems with a face-on orientation (app. diam. ratio > 0.53) that are not seriously disturbed by nearby large galaxies.

Class C. Systems that may be disturbed by nearby large galaxies.
In the writer's diameter system, a diameter ratio of 0.53 corresponds to an inclination of the principal plane to the line of sight of about 30°. As regards the disturbance from another galaxy, we have to consider not only the gravitational action on the satellites of the primary galaxy but also a possible mix-up of the satellites belonging to the two systems. In an attempt to pick out spirals that are not seriously disturbed, we have tried to apply the following working rule, based on the separation of the second galaxy from the primary system, as compared to the radius, r, of the survey area. Separation > 2r: no disturbance; r < sep. < 2r: no disturbance if the estimated mass of the second galaxy is less than one-fifth of the mass of the primary system; sep. < r: no disturbance if the mass is less than one-twenty-fifth. The masses are estimated from the luminosities and morphological types (or integrated colors). Following this rule, the number of spiral systems in the classes A, B and C are found to be 62, 64 and 48, respectively.

The distance moduli of the spiral galaxies have been taken from the writer's catalogue (1964), which lists absolute moduli, as derived from both photometric data and redshifts (H = 80); the arithmetical mean has been accepted (for four objects the moduli have been revised). In the case of the 38 galaxies from the Reference Catalogue, the distances are based only on the redshifts (all redshifts larger than 500 km/sec). As a limiting distance modulus we have adopted 31.2 for spirals with an edgewise orientation, and 30.9 for the remaining systems. The difference is due to the fact that the disturbance from optical companions is less pronounced for the former group, the physical companions apparently being confined to a position-angle interval of 30° -90° (cf. sect. 7). It may be noted that the number of optical companions in a survey area is, on an average, proportional to the distance.

The adopted absolute distance moduli, and the classes to which the galaxies have been assigned, are listed in Table 7.

It should be pointed out that the material includes some spiral systems from the Virgo cluster. As regards number of satellites, these spirals seem to be comparable to galaxies in the general field. The number of optical companions, as found from the comparison areas, also seems to be the same, which is explained by the fact that the great majority of the background objects are located far behind the cluster.

For the circular survey areas around the selected spirals we have, as was explained in the preceding section, chosen a radius of 50 kpc. The apparent radius ranges from 251' (NGC 224) to 9'.9 (dist. mod. = 31.2). The two comparison areas, of the same size, have been placed at standard distances of 100 mm east and west of the survey area, the distances being measured parallel to the edges of the Sky Atlas prints. For some nearby galaxies with very large survey areas the distances have been increased to ± 200 mm; in the few cases in which a comparison area happens to include a prominent galaxy, it has also been moved to twice the distance. For NGC 224 and 598, which for the sake of completeness have been included in the material, no comparison areas have been used, the number of optical companions in these cases presumably being = 0.

Although the actual survey work on the Sky Atlas is based on the 103a-O prints a careful comparison has always been made between the blue and the red prints; the standing rule has been that no object is to be included unless it is recognizable as a galaxy on both prints. The work has been performed with an eyepiece having a magnification of 10 times, and equipped with a precision scale divided into 0.1-mm intervals.

The first part of the work was aimed at picking up all galaxies in the survey areas and the comparison areas that have major diameters 1.0 kpc (as referred to the distance adopted for the central spiral system). For a few very distant spirals this corresponds to an apparent diameter as small as 0.'20; on an average, the limiting diameter is 0.'44. For all galaxies above this limit the major diameters have been measured. In the survey areas the polar co-ordinates have also been determined, that is, the separation of the object from the central system and the position angle, as measured from the direction of the major axis of the system. Attempts to estimate the morphological types have been successful only for galaxies larger than about 1'.0, and then only by a careful comparison of the blue and red atlas prints.

In the second part of the work, an attempt was made to extend the survey down to a limiting diameter of 0.61 kpc. Although the disturbance by the background-foreground field now becomes rather serious, it was found possible to do this for spiral systems with distance moduli 30.0. The limiting apparent diameter is in this case about the same as above or 0'21. The results of the second part will be used only to determine the statistical distribution of the absolute diameters of galaxies (cf. sect. 10).