Morphological properties of the galaxies have been deduced from the visual inspection of (a) deep images obtained from digital co-addition of the images from all measured plates, (b) digitised images obtained from single high quality plates taken under good seeing conditions, (c) radial intensity profiles from surface photometry, and (d) the radial run of the position angle.
Co-addition of Schmidt-plates is known to be a powerful method for the evaluation of faint features in galaxies (e.g. Malin & Carter [1993]; Schweizer & Seitzer [1988]; Malin [1994]; Kemp & Meaburn [1995]). Up to 37 images have been digitally centred and co-added for every galaxy. Due to the significantly increased signal-to-noise ratio, faint features down to a surface brightness of 27magarcsec-2 can be recognized. On the other hand, the bright inner parts of galaxies as well as any features affected by bright foreground stars are not well studied on the stacked images, primarily owing to the rather poor seeing conditions for the majority of the plates. Therefore, a combination of the deep, co-added image of a certain galaxy with its image on a single high-quality plate (seeing < 1'') is the most appropriate way to analyse the information on structure details.
Three independent inspection runs were performed including all 660 galaxies in every run. For each galaxy a page of six different pictures was inspected containing the direct images from the co-added plates and the one from the best single plate, respectively, each at two different magnifications and combined with the corresponding contour plots from these both images. Artifacts like plate flaws or artificial Earth satellites are, in general, strongly suppressed in the co-added image due to plate stacking. With regard to very faint structure details in the galaxy images however, one has to ensure that unusual features detected are real. In these cases, all single images involved in the co-added image had to be inspected. The whole procedure of evaluating morphology was rather time-consuming but proves to provide quite reliable results.
One of the main aims of the study of morphology is to derive estimates of the morphological types. It should be emphasized that the galaxy images have been observed in the B band where most existing schemes for morphological classification of galaxies have been defined. Unfortunately, only a rough discrimination between E/S0, S and Irr types is possible for most galaxies on Schmidt plates at the distance of the Perseus cluster. (The notation Irr is used as well for normal disk-dominated late-type galaxies Sdm, Sm, Im as for galaxies which cannot be assigned Hubble-types.)
The presence of an apparent spiral structure is taken as the main criterion for the classification of spiral galaxies. In this context, the use of blue-sensitive plates is important to ensure the detection of features related to recent star formation, like spiral arms. We searched for indications of spiral arms on (a) the direct images, (b) the contour plots, and in (c) the radial run of the position angles. Unfortunately, the appearance of spiral structure can be mimicked by triaxiality in early type galaxies or even by a superposition of faint foreground stars. In addition, radial luminosity profiles have been analysed for 79 sufficiently extended galaxies. For most galaxies the types estimated from these different methods were in good agreement.
For many galaxies of the sample it is difficult to decide clearly between E, S0 and S types. In these cases, two types are given in the catalogue with the more likely one at first position. If an alternative type is less likely, but cannot be excluded, it is quoted with a question-mark. Classification has been completely impossible for less than 10% of the galaxies. This concerns small and faint objects, often located close to bright foreground stars. For spiral galaxies it is sometimes possible to dicriminate between early- and late-type, mainly from the bulge-to-disk ratio.
For the galaxies with isophotal analysis by Poulin et al. (1992), the agreement with regard to the three basic morphological types (E/S0, S, Irr/Pec) is very good: among the 43 galaxies contained in both lists and unambiguously classified by one single type, there are 39 galaxies (i.e. 91%) where the estimated types from both lists are in agreement; for two galaxies no classification has been possible in our study. For the 35 galaxies where two possible types are given in at least one of the two lists, 33 classifications (94%) have at least one type in common. Furthermore, for the 30 galaxies from our catalogue contained in the lists of Perseus galaxies by Andreon et al. ([1997a]) we find a similar result: among the galaxies classified as early types by Andreon et al. (their Table 5) we have 3 out of 16 objects in our catalogue with differing classification (S in our catalogue, S0 in Andreon et al.). For 14 galaxies classified as S by Andreon et al. (their Table 3) we find a disagreement only for one (No. 120), which we classify as S0. As reliable estimates of morphological types have been the main goal of these both studies, we conclude from this comparison that our classification as E+S0, S, and Irr types should be correct for about 90% of the brighter galaxies (B < 16). The uncertainties increase for fainter and/or smaller galaxies.
Many inspected galaxies show faint morphological peculiarities in the co-added images. Despite different attempts (e.g. Naim et al. [1997]), quantifying peculiarity is still a challenge. During a "training phase'', we studied the appearance of catalogued peculiar galaxies (Arp & Madore [1987]; Whitmore et al. [1990]), of various individual interacting galaxies, and of typical features appearing in numerical simulations of galaxy encounters (e.g. Barnes [1992]; Howard et al. [1993]). After a first inspection run of our images of the A426 galaxies, we decided to use a classification scheme for detected peculiar structures which is essentially a combination of the schemes by Karachentsev ([1972]), Arp & Madore ([1987]), and Whitmore et al. ([1990]) with four basic types:
IN: systems with signs of strong interaction in the form of bridges (br) and/or tails (ta).
DIS: systems with signs of distortions, which can show one or several of the following features coded by a number: peculiar spiral arms (1), three-armed spirals (2), M51 type (3), unusual dust absorption (4), elliptical galaxy with isophote twist (5), non-concentric core (6), loops/shells (7), jet-like structures (8), fuzzy structure in the outer regions (9), radial spikes (10).
CH(n): n systems (n>1) within a common halo.
PL: polar ring galaxies or related objects (see Whitmore et al. 1990).
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