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a. Definition of Categories
The basic approach adopted in the compilation of our Catalogue was that as we scanned survey plates we inspected all galaxies down to slightly less than 0.2 mm (i.e., about ten seconds of arc at the scale of the Schmidt plates). Of course the majority of these galaxies were more or less regular and approximately symmetric. Only when a galaxy departed substantially from the norm was it catalogued as peculiar. In addition, because we initially scanned survey plates at low (5x) magnification we were aware when galaxies occurred in clusters, groups, and loose associations. When we encountered galaxies which were unusually close together compared to the average, we catalogued them as a pair, group, rich cluster, or galaxy with apparent companions. Of course in not every instance will these galaxies be physically associated, but the association categories furnish a homogeneous selection of those most likely to be associated. They can later be tested statistically or with individual observations.

Within the individual categories there is always some indefinite range where the noteworthiness of the peculiarities becomes rather subjective. While objects in this twilight zone of the Catalogue are undoubtedly incomplete, down to the size limit adopted here we feel that we have produced a catalogue which, by anyone's standards, includes all of the most peculiar galaxies.

At the same time, distance-dependent biases do exist in our selections and these will vary from Category to Category. For instance, galaxies showing jets and filaments will be lost from the Catalogue at a larger apparent size than the galaxies having interacting companions; galaxies with apparent companions will be selected from a smaller volume of space than apparent doubles, etc. The reader is therefore warned against making sweeping comparisons between relative frequencies of peculiarities unless this very important selection effect is taken into account.

It is important to stress that we did not start this project with a preconceived set of categories. Rather, we developed the categories empirically by simply listing all unusual objects from a preliminary analysis of a set of thirty-six plates. We then noted what categories these objects themselves defined, and thereby established the categories listed in Table 1. It is remarkable, looking back now, to realize that such a large number of peculiar galaxies could be fitted into so few categories of peculiarity. In the end, surprisingly few galaxies needed to be put into Category 22 as miscellaneous objects.

One of the important uses of the Catalogue is to enable astronomers, interested in a certain class of objects, to obtain a sufficiently large list of candidates with similar characteristics as a basis for further investigation of specific phenomena.

In the photographic section of the Catalogue the objects are ordered carefully within each Category. It is hoped that this conveys, to some extent, an empirical continuity of certain characteristics which may help us understand the cause of some peculiarities. As shown in the previous table of codes, the few words that describe each Category furnish a definition of the kinds of objects which comprise each Category. In addition, there are amplifications of the definitions and brief comments at the beginning of each Category when it is introduced. More detailed remarks on the possible significance of the Categories and their relation to each other are given below.

b. Statistical Frequency of the Various Categories.
The present Catalogue was designed to provide some additional information which has been previously unavailable. Specifically, we have counted all galaxies in each field down to the same size limit to which we have catalogued the peculiar galaxies. This means that we can calculate how frequent certain types of peculiarities are, down to a given limiting apparent diameter. For example, we can now say with what absolute frequency we find M51-types, or ring forms, or galaxies in chains. Similarly, it is also possible to calculate the relative frequency with which one Category is likely to be found as compared to another; for instance, how frequently does one encounter interacting doubles as compared to interacting triples? As noted before, however, all such intercomparisons should be done with care.

We have inspected and counted all galaxies to an estimated size limit. There is of course some imprecision inherent in this approach when attempted by visual means. Exposure limits and personal judgement vary from plate to plate. Additionally, some of our fields are obscured by galactic absorption. This led to the expected result that we found only a few galaxies in some fields, and undoubtedly missed some connections and interactions among those galaxies we did look at in the obscured fields. Nevertheless, these cases are relatively few in number and probably do not significantly alter our results. Knowing the total counts of galaxies in each of our fields, we can give, with fair accuracy, what percentage of all the galaxies we looked at actually fall into the various Categories.

In the Appendix, we have listed the number of galaxies that we have inspected and counted in each of the 537 fields which comprise the survey area of the Catalogue. It is clear that the counted number of galaxies varies strongly with the galactic latitude of the field. In fact, the numbers can be taken as indicative of the average obscuration for each field. Of course it should be realized that the obscuration can vary dramatically within a single 6.5 degree by 6.5 degree field and that the counts can also be dramatically affected by the presence of galaxy clusters within a given field.

At the same time it should be noted that because of the limited dynamic range of the photographic plates and because we were working with deep exposures, we were especially sensitive to peculiarities in the faint outer reaches of most galaxies; the inner disk often being severely over-exposed. However, in fields obscured by foreground galactic absorption, galaxies can appear ``ghost-like'', with much lower apparent surface brightnesses. For these objects at low galactic latitudes our basis for judging peculiarities would necessarily shift from the outer to the inner regions. This should be borne in mind by the reader, since this particular surface-brightness effect makes the discovery of ``dwarf galaxies'' at low galactic latitudes suspect, unless there is some additional property that makes the identification compelling.

The total number of galaxies which are classified into each Category is obtained by counting the number of times each code number appears in the positions and descriptions part of the Catalogue (Volume I). As noted previously, the same galaxy could be classified under more than one Category. For example, one galaxy could have an interacting companion and several apparent companions, and some of these companions in turn could have jets or tails or other peculiarities. By summing the number of times each Category appears, we obtain the total number of galaxies exhibiting that particular peculiarity. These statistics, expressed as percentages, are listed in the final column of Table 1. For the 6,445 individual entries there are 8,954 classification codings.

A further quantity of interest is a measure of the frequency with which galaxies in a diameter-limited sample are classified as peculiar. This statistic can be readily obtained by dividing the total number of entries in the Catalogue by the total count of galaxies examined, appropriately scaled for plate overlap. The total number of galaxies found on each survey plate is given in the Appendix. These are the raw counts obtained by us as we scanned each plate. In order to correct these for overlap the numbers listed should be decreased by a factor of 0.83 determined by us empirically from the numbers of peculiar galaxies discovered more than once in the course of the survey. In total we inspected 93,556 galaxies (77,838 when corrected for overlap) out of which we classified 6,445 as peculiar, indicating that eight percent of all galaxies looked at are peculiar.

c. The Atlas of Peculiar Galaxies and other Catalogues
Although the present Catalogue was developed without specific reference to the earlier Atlas of Peculiar Galaxies (Arp 1966), looking back now we can see many of the present Categories illustrated in that Atlas. Some Categories were given slightly different names in the Atlas. For example, the Atlas had an important class of objects called ``Companions on Arms'', which is now Category 9: M51-Types. But many Categories turned out to be essentially the same; some examples from the earlier Atlas being ``galaxies with associated rings'', ``galaxies with jets'', ``disturbed galaxies with interior absorption'', and ``chains of galaxies''. New categories were able to be added to the present Catalogue because of the large number of galaxies included and because we took advantage of the ability to compare surrounding sky areas. The new Categories include the apparent associations (Categories 1, 8, 23, 24), and the galaxies of very high or very low surface brightness (Categories 13 and 20).

In addition to furnishing examples of galaxies which illustrate the standard classification of types of galaxies, the Hubble Atlas (Sandage 1961) presents some well-known examples of peculiar galaxies. The Hubble Atlas also contains some objects from the southern hemisphere. Modern systematic surveys of southern hemisphere galaxies start with de Vaucouleurs (1956), who gives types and descriptions for the listed galaxies. Classification information is also given in Evans (1957) and Shobbrook (1966), while Sersic (1968) has published an atlas which includes the most peculiar bright galaxies in the southern hemisphere.

The most recent morphological classifications of southern galaxies are given in the Las Campanas Survey (Dressler and Sandage 1978; Sandage and Brucato 1979). Of course, the extensive classification of both normal and peculiar galaxies is available from the ESO/Uppsala Survey of the ESO (B) Atlas of the Southern Sky (for recent references in this series, see Holmberg et al. (1980) and the final compilation by Lauberts (1982).

d. General Introduction to Discussion of Individual Categories of Peculiarity
One of the reasons for the interest in peculiar galaxies arises from the possibility that the perturbation of a galaxy can be treated as a departure from dynamic equilibrium. Physical properties of the galaxy can then be derived by analyzing the event as an experiment performed on the galaxy. This is particularly true if we can assemble sufficient examples of the phenomenon to separate the various parameters which differ between systems, as for example the types of initial impulses and the time since the last disturbance. Perhaps an even more important question is what causes the peculiarity in the first place. Is it a disturbance due to an interaction with a nearby object or is it an internally generated disturbance such as an explosion or ejection?

The Categories which are empirically assembled in the present Catalogue point to the existence of various kinds of events and forces. First, there are the interaction events where two galaxies are found close to each other and deformed, presumably by gravitational or other long-range forces. Second, there are apparently isolated galaxies where the disturbance appears to arise from within the galaxy itself, possibly due to internal dynamical instabilities, nuclear activity, or other explosive events including supernovae or extensive star formation. Third, there are objects which appear to differ from the norm because they reflect differences in the wider environment in which they formed and evolved; for instance, differences noted in two galaxies could be attributed to the effect of a hot gaseous medium, higher mean velocity dispersion or a rapidly fluctuating gravitational potential for a galaxy in a cluster, as compared to a similar object in the field. Peculiarities could then be ascribed to three main causes, immediate neighbourhood, internal activity and general environment. Of course, the assigning of probable causes for peculiarity can only be tentative when one is dealing with the appearance alone. What appears as a single peculiar galaxy, for example, might be an accidental superposition of two galaxies along the line of sight. In other instances an apparently isolated but disturbed galaxy might be the physical merging of two systems. Alternatively, what appears to be a gravitational interaction might be enhanced internal activity (such as the fueling of an active nucleus) only triggered, but not caused, by the proximity of another galaxy. Or internal activity may be simultaneously occurring in two or more galaxies which are only coincidentally close together. Correlation and cause are rarely easy to disentangle in astronomy. As a starting point, however, the Categories which most suggest certain causes are listed below:

Interaction Candidates: Categories 1-6, 9.
Possible Internal Activity: Categories 6-16.
Initial Conditions or Evolution: Categories 8, 13, 14, 16-20, 23, 24.

Like the Atlas which preceded it, the present Catalogue poses intriguing questions as to whether some of the different Categories are physically related to each other, and if so, how. The following detailed discussion of each Category is meant to high-light some of the important peculiarities in galaxies and suggest ways in which possible relationships between the Categories might help us to make progress towards explaining these systems.

Category 1: Galaxies With Interacting Companions

The first examples of this Category in Volume II are spirals with well-defined spiral arms. The reason for starting with these cases is to insure that the effects of the interaction on the arms can be well seen. The experiments which are thus presented to us deal with the phenomenon of spiral structure. The nature and cause of spiral arms has long been debated, with the density wave theories of Lin (Lin and Shu 1964), the magnetic tube concepts of Piddington (1970), the ejection track theories of Ambartsumian (1958), Arp (1963, 1969, 1978), and Oort (1975), and most recently, the self-propagating star formation theories of Mueller and Arnett (1976), all attempting to explain spiral structure. One way of making progress on this problem is to examine spiral arms under physical stress. Of particular interest in this regard are new cases shown here where the spiral arm appears retarded in rotation by the nearby companion as illustrated by AM 2011-705 and AM 2203-281. In the present Catalogue, there is also one case where the spiral arm seems to flow around the companion (i.e., AM 0244-302).

Later in Category 1, we encounter sub-category 1b, also illustrated Volume II. Here the companions are interacting strongly with the whole body of the main galaxy. They furnish some of the most certain physical interactions and the companions might be considered in the role of test probes. As we go still later into this category we encounter companions relatively more widely separated and central galaxies relatively less disturbed. Among these later sub-categories, we encounter some very unusual shapes in the companions themselves (e. g., AM 0036-432). Some of these suggest that the companions are internally active in their own right, although that internal activity may or may not have been triggered by an interaction.

By the end of the category, the evidence for interaction is more subtle. This arrangement emphasizes the important point that the present category is perfectly continuous with Category 8: Galaxies with Apparent Companions. Whatever the important considerations in Category 8 are, be they initial conditions, evolution, or interaction, some of the same forces may be acting here.

Category 2: Interacting Doubles

The continuity of this Category with the preceding is made empirically clear by considering examples of galaxies with interacting companions; when the companion is larger than approximately half the size of the main galaxy, we classed it as an interacting double. It is by no means certain, however, that all such systems arise from the same causes or initial conditions as those in Category 1. It is interesting to note that some galaxies with small companions can appear to be just as violently disturbed as those interacting with galaxies of much larger size. This raises questions about how the interaction takes place: Is it strictly a gravitational interaction, primarily between stars or do plasma, gas, dust or magnetic fields also play a role? It also raises questions concerning time-scales and frequencies: Over what fraction of their life-times are interacting galaxies in contact? And what fraction of all galaxies interact at all?

By consulting the statistics in Table 1, it is seen that this Category is the most frequently used classification in the whole Catalogue. There are simply a very large number of galaxies encountered where the two galaxies are roughly the same size, a few diameters apart, and apparently disturbed. The forms of these disturbances are endlessly varied and Volume II attempts to give a feeling for these forms.

The E + E sub-category, which leads off the interacting doubles, contains examples of systems where galaxies without much apparent gas and dust are interacting. There the forms of the interaction suggest primarily gravitational perturbations of stars in orbits. Undoubtedly, many of these cases can be treated by the classic precepts of gravitational interaction, as most extensively studied by Toomre and Toomre (1972).

The E + S sub-category, which follows, shows spirals in the gravitational field of a companion E. Presumably here we can see spirals deform in a purely gravitational perturbing field. This sub-category is interesting also from the standpoint that E galaxies have a tendency to occur in clusters and groups with other E or lenticular galaxies. The question is, how do these E + S associations come about? There is also a puzzling suggestion in the literature that, when ellipticals and spirals are in physical pairs, the spirals have systematically higher redshifts (Tifft 1972; Tifft and Tarenghi 1975; Jaakkola 1971, 1973; Arp 1976a; Sulentic 1977).

The final photographic sub-category shows a selection of S + S interactions. These are usually the most spectacular because deviations from equilibrium can be most easily seen in spirals. Also, in many cases, the gas and dust, which have stronger interactive properties than stars, are involved. Study of this sub-category should yield some new information on the physical differences between different types of spirals. Moreover, the relation between this interacting sub-set of spirals and the apparently non-interacting double spirals in Category 23 needs to be clarified. This is especially interesting in light of the strong correlation of Hubble type (Page 1975, Noerdlinger 1979) and colour (Tomov 1978) between pairs of spirals in binary systems. This correlation could be an initial condition of the system, but it is also possible that in these cases the Hubble type and colours are determined by the on-going interaction rather than the internal dynamics of the individual galaxies (Madore 1980).

A whole sub-category of interacting doubles is not included in Category 2. It consists of those interacting doubles which are apparent companions of spirals (Sulentic, Arp and di Tullio 1978). This sub-category is illustrated in Category 8. There are also examples of interacting doubles in Category 6: Ring Galaxies, and in other Categories as well.

In general, there remains an enormous amount of observational work to be done on these interacting doubles. Large numbers of accurate redshifts need to be obtained. Morphologies need to be specified more precisely. Sizes and apparent magnitudes need to be measured. Perhaps new detailed information on this large Category of doubles in the present Catalogue will help to interpret the data which has already been gathered on the Karachentsev doubles in the northern hemisphere (Karachentsev 1974, 1980; Peterson 1979; Tifft 1980) and those of Schweizer (1979) in the south.

Throughout the examples of interacting galaxies runs the question of whether the galaxies are suffering temporary encounters or are merging, fissioning, or ejecting. Considerable study will be needed to decide eventually whether one or more of these processes is working, and if there is more than one, whether any one of them is dominant. One statistic of importance, which is derived in the present Catalogue, is the percentage of all galaxies that are interacting doubles. This must be an important number in constructing theories of how the interactions occur in terms of initial and present conditions. Moreover, in the multiply interacting systems, we frequently encounter linear arrangements. This surprising feature of the Catalogue implies that something more than random encounters of galaxies is taking place. Comparisons with similar samples at higher redshift will be especially interesting.

Categories 3 through 5: Interacting Triples to Quintets

When three galaxies fall within a few diameters of each other on the sky and are disturbed, they have been classed as an interacting triple. When the number of apparently interacting galaxies is greater, they have been classified as interacting quartets and quintets. It is significant that no unequivocal examples of six, or more, interacting galaxies of roughly equal size were encountered in our study. Any candidates for interacting galaxies in greater number than five will have to be sought in the present Catalogue in Categories 17 and 18, Chains and Groups of Galaxies. Sometimes there are smaller galaxies present in interacting systems, which would take the group into a higher order interacting classification if they were considered of dynamical importance in the group. The classification here, however, has to be made on the subjective impression as to what the main components of the group are.

Considering that the interacting double category is by far the largest category in the Catalogue, it is perhaps surprising that there are so few interacting triples. The reduction in numbers as we proceed to interacting quartets and higher orders, is even more drastic. Theories explaining the frequency of interacting doubles should also lead to predictions of the numbers of interacting triples, quadruples and higher-order interactions; the present data should help to select between such theories.

Among the triples, there are two sub-categories distinguished in Volume II: the first, where three galaxies of any type which are roughly the same size seem to be interacting; and the second, where there is rather striking alignment. This tendency towards alignment persists also into the quartets and quintets: AM 1303-372, which consists of a group almost in contact, shows linear elongation. In the northern hemisphere, interacting or disturbed groups have already been shown to have a tendency to be aligned (Arp 1973). In the well-known Stephan's Quintet, for example, the four high-redshift members are aligned very well, extending even to a fifth high-redshift member to the east. The tendency for the groupings of disturbed galaxies to align is clearly related to Category 17: Chains, because many of the galaxies in chains are also disturbed.

In some groups of disturbed galaxies ring forms appear. Since rings are one of the most striking non-equilibrium forms found, the ring classification generally took precedence over other Categories. Therefore, some apparently interacting groups can also be found in Category 6: Ring Galaxies, which will now be discussed.

Category 6: Ring Galaxies

Any galaxy with a well-defined ring around it, and any galaxies apparently related to this form, have been included in this Category. We have tried to exclude galaxies where spiral arms have become wound up tightly into a circle or near circle (they are in Category 10: Galaxies with Peculiar Spiral Arms) but there is some question as to whether the latter forms are related. There may be some overlap or there may simply be an ambiguity in the classifications.

There have been three major attempts to explain the origin of ring galaxies (Freeman and de Vaucouleurs 1974; Theys and Spiegel 1976; Lynds and Toomre 1976). All of these approaches involve the collision of a cloud or galaxy with another galaxy in a special way, usually through the center of the galaxy which becomes the ring galaxy. The resulting, nearly symmetrical disturbance is proposed to account for the formation of the ring. Naturally, the presence of nearby companions is of great importance to these kinds of theories, and the present examples of ring galaxies with companions furnish many cases where redshifts, magnitudes, and spectra can be obtained in order to check these theories. Conversely, it will be very interesting to uncover examples where there are no apparent companions in the vicinity of the ring. Some examples which deserve further study are shown in Volume II under this Category. Perhaps they have an independent origin.

The first sub-category starts with examples of empty rings, that is rings lacking any apparent nucleus. These are particularly interesting because it seems difficult, although by no means impossible (see Theys and Spiegel 1976), to conceive of a collision which would remove a nucleus entirely. Alternatively one could postulate that the target galaxy had no dominant nucleus to begin with. A further alternative is to postulate that the radial perturbation which formed the ring was caused by internal activity of the nucleus (Saslaw, private communication 1978; Williams, in preparation 1985). Later in this sub-category, we encounter some galaxies with nuclei which are quite off-center, while toward the end of this sub-category the nuclei become fairly well centered. In the examples showing moderately off-center nuclei, we encounter the spectacular rings such as AM 0644-741 and AM 1006-380 and the remarkable ``Cartwheel'' (previous discussions of some of these objects can be found in Graham (1974), Fosbury and Hawarden (1977), Few, Madore and Arp (1980)). Morphologically, the objects toward the end of these illustrations lead more or less continuously into the second sub-category.

The second sub-category has rings with centrally located nuclei. Some resemble spiral galaxies in which the arms have become wound up by differential rotation, as arms would unless sustained by whatever the spiral arm generating mechanism is. From the pictures available to us, we put objects into the present Category if their rings are more or less unbroken circles with no hint of spiral connections to the central nucleus. The existence of this sub-category, however, raises the interesting question of whether some rings can originate from the winding up of spiral arms or luminous ejected filaments by differential rotation.

Finally, there is a sub-category consisting predominantly of barred spirals which have a high-surface-brightness inner ring and a faint outer ring as well. These ``bulls-eye'' shapes, to our knowledge, have received no analytical attention to date. Kormendy (1979) discusses some ring-form galaxies which may have some relation to ``bull's-eye'' forms as well as to rings in spirals in general.

One rather interesting object in this Ring Galaxy Category, is AM 0226-320. It represents the best example of either a prolate galaxy with a fine, well-delineated equatorial ring, or an edge-on galaxy with a ring circling the poles. For further discussion of this class of objects and references to previous work see Schweizer, Whitmore and Rubin (1983).

Category 7: Galaxies with Jets

We consider jets here as straight or nearly straight linear features. Because of the importance and relative rarity of jets, classification into this Category tended to override classification into other Categories. This Category perforce includes many objects which may only possibly have jets. Accidentally superposed companions or stars can fool the eye, so higher resolution or photography to lower surface brightness will be needed to confirm whether many of these cases are actually jets or not. In general, any linear feature that might be a jet was included in this Category, with the expectation that further confirmatory work would be done.

It should be remembered that one of the most famous cases of jets from a galaxy, NGC 1097, was discovered initially as an uncertain possibility on a very deep Schmidt plate, and only confirmed after special efforts (Wolstencroft and Zealey 1975). Such information-stacked plates taken with the CTIO 4m reflector, eventually revealed a total of 4 jets emerging from the nucleus of this galaxy (Arp 1976b; Lorre 1978). The picture of NGC 1097 = AM 0244-302, shown in Volume II, is from a standard IIIa-J survey plate and the print here does not show even the brightest jet.

In general, counterparts to high-surface-brightness jets known in the northern hemisphere, such as in M87 and in 3C273, were not found in our southern hemisphere survey. Because the deep exposure IIIa-J plates we surveyed were saturated over a moderately large fraction of the central galaxy images, there may be some such jets that we have missed in interior regions. But results from the ESO Quick Blue Survey, which was made using shorter exposures, turned up no outstanding new cases (Holmberg et al. 1980), so we can conclude that such strong jets are very rare and unusual (see also the Proceedings of Second ESO/ESA Workshop, Munich, 18-19 Feb. 1981, ESA Sp-162, March 1981).

Remarks on some of the individual candidates for jets which we have illustrated, are made in the introduction to the category in Volume II. We should note that Category 15: Galaxies with Tails, Loops of Material or Debris, may be allied closely in some cases with Category 7: Galaxies with Jets. If we consider an initially straight jet deforming under the action of rotation, then some tails or loops could have been jets. Conversely, it could be that some filaments are being viewed in the plane of their orbits, and therefore appear like straight jets.

Category 8: Galaxies with Apparent Companions

The galaxies that are nearest in space to us, and that we therefore know the most about, are almost all grouped together in units of one large, dominant galaxy, and a retinue of smaller companions. Examples are the Local Group (dominated by M31) and the M81 and M101 groups. It is important to find out how general a phenomenon this kind of grouping is over large regions of the universe. The present Catalogue, including all of the sky south of declination -22 degrees, presents extensive lists of the most conspicuous, apparent groupings. These are the cases most likely to turn out to be physical associations, when more detailed observations become available.

The forms of the companions, and their distribution around the central galaxy, present many puzzling problems. Holmberg (1940, 1969) shows that fainter galaxies, many of them presumably physical companions, tend to concentrate in the vicinity of the minor axes of large spiral galaxies. Later, Sulentic, Arp, and de Tullio (1978) showed that disturbed or non-equilibrium galaxies group preferentially around large spirals. A striking local example of this phenomenon is seen in the two closest companions to M81, (i.e., M82 and NGC 3077). Lynds and Sandage (1963), Sandage (1961), Bertola (1981b), and Arp (1980) established the extremely peculiar nature of these companions. Arp (1981a, 1981b, 1982) has shown that companion galaxies, including many of the ones shown in Volume II, tend to have early spectral types and a high incidence of emission. What causes these peculiarities: Is it the interaction of the companions with the main galaxy? Or is there some more fundamental reason for the differences in the companions? Are they formed at a later epoch than the main galaxy? Or are they formed in some basically different process?

Some clues to these puzzles may be obtained by further studies of this Category. It is the one which is illustrated with the largest number of examples in Volume II, and the reason for this is to supply observers with many more candidate groups from which they can select new cases for study. Of particular interest is the tendency for many companions to be interacting doubles, as shown in the third sub-category. What is the connection of interacting double companions to Category 2: Interacting Doubles? Are some of the interacting doubles in Category 2 companions to larger galaxies which are projected some appreciable distance away on the sky?

In the second sub-category, a number of examples are shown where the companions are aligned across the central galaxy. If this is a significant trend, it may be telling us something about the origin or dynamical evolution of the companions and their orbits. Whatever the reasons for the alignment, it is tempting to suppose that there is a common explanation for the alignments observed in the interacting triples, quartets, and quintets, and also, for the alignments observed in Category 17: Chains.

Category 9: M51-Types

In the Atlas of Peculiar Galaxies (Arp 1966), the M51-type galaxies were called ``Companions on the ends of spiral arms.'' They made up one of the most important sections of the original Atlas, and were ordered from the smallest to the largest companions. The purpose was to show a sequence of forms. One interpretation of this sequence was made by Arp (1969) in which he postulated that compact bodies were ejected from the nucleus of the parent galaxy. During their passage outward these compact objects formed the spiral arms in the underlying galaxy and then evolved into larger, companion-sized galaxies.

Subsequently Toomre and Toomre (1972) published calculations which showed that certain kinds of encounters between galaxies could gravitationally draw out filaments of stars which would connect them. The Toomres' simulations modelled some of the observed objects so well that there could be little doubt that such a mechanism was acting to some degree. One of the cases most successfully modelled was M51. In recognition of that analysis and its most famous prototype, we renamed the Category, M51-Types.

It is not necessarily true, however, that in every case companions on the ends of spirals arms have to be the result of gravitational encounters (note particularly the effects of small companions; see discussion by Vorontsov-Velyaminov (1957, 1975a, 1975b)). Is a companion originating in the nucleus of a galaxy distinguishable dynamically from a collisional encounter where the companion passes very close to the nucleus? Again, reliable masses and densities of the objects involved are needed to make further analysis. We present many new cases of M51-types, where the properties of a large number of members of this Category can be studied.

Finally, there is the question of the spiral arms themselves. The implication from the Toomre work was that the spiral arms were caused by the encounter (although, in the theoretical calculations, there tended to be appreciable differences between the tidal arm and the counter-tidal arm which are not generally typical of the observations). Nevertheless, if some spiral arms were formed by encounters, there is an obvious temptation to relate all spiral formation to encounters with companions; otherwise there would have to be two or more separate mechanisms for the formation of spiral arms! The all-collision origin for spiral arms is contra-indicated, however, by the existence of spiral galaxies which do not have any companions. It is, of course, difficult to exclude completely the possibility that there might always be companions around a galaxy, postulating that they are of sufficiently high mass-to-light ratio so that they are not conspicuous optically. But we are disinclined to invoke the unobservable too soon.

So we return to the question of the nature of spiral arms touched upon briefly in Category 1: Are spiral arms of such a nature that they are only formed by gravitational encounters? Are they even generally formed by gravitational encounters? Are they formed by perturbations of an internal origin? Or are there a variety of physical effects all of which can lead to the formation of spiral arms? The further study of the nature of spirals will be assisted by examples in the next two Categories. There we illustrate both the variety of arms that can be found, and the conditions under which they exist.

Category 10: Galaxies with Peculiar Spiral Arms

This Category deals with galaxies where the major peculiarity is in the spiral arms and is to be distinguished from Category 12: Peculiar Disks, where any arm peculiarity seems to be just a general consequence of the disturbance of the main body of the galaxy.

The largest sub-category illustrated here is that of one-armed spirals. In one portion of this sub-category, one arm is simply much stronger than the other arm; but, in other cases, there seems to be truly only a single arm. It would seem very difficult, in the density- wave theory, for spiral arms to be formed by an asymmetrical density perturbation which extended on only one side of the nucleus. If these cases, on further examination, turn out to be truly one-armed spirals, they would seem to pose a formidable challenge for any theory of spiral-arm formation. The stability and persistence of the phenomenon in any one example is, of course, an open question.

In Volume II, examples of spirals are given both with very narrow arms and with very broad arms. It is possible that by studying the different conditions in the disks of these galaxies, some clue as to the determining causes of spiral-arm width may emerge.

Another puzzling phenomenon, of which three examples are given in the photographs, is that of barred spirals where the arms are double as they come off one end of the bar. In some cases, they remain closely parallel for a considerable distance.

Finally, there is a sub-category of what are essentially elliptical galaxies with spiral arms around them. Since these spiral arms (rings?) are usually only discovered with deep photography, there may be further examples of this sub-category just below the level of usual detection. (It should be noted that these features are not the plateaus in surface brightness or ``ringing'' structures originally described in the Atlas of Peculiar Galaxies (Arp 1966) and later popularized by Malin and Carter (1980)).

This is a rather amusing Category to classify because the terminology ``E galaxy with spiral arms'' would seem to be a contradiction in terms. Accordingly, we have placed such objects in Category 10: Galaxies with Peculiar Spiral Arms by using the rationale that if arm regeneration were to stop in a spiral, the arms would grow old, fade, and possibly wind up. We then might be seeing examples of large-bulge systems whose arm formation has been discontinued within the last few hundred million years. The spectrum of the nucleus of AM 0148-472 (Arp 1981a), for example, shows an old stellar population; but the nuclear-bulge population of an early-type spiral could not easily be distinguished from that of an E galaxy. Detailed surface photometry and/or kinematic studies will help to decide between the elliptical versus bulge hypotheses. But these objects possibly represent transitional systems between elliptical and spiral galaxies, and are therefore very interesting in their own right.

Category 11: Three-Armed Spirals and Multiple-Armed Spirals

Three-armed spirals are a surprising Category to encounter because the majority of spiral galaxies appear to have some kind of two-armed, grand design, while the remainder have highly bifurcated or patchy arms. The density-wave theory, as mentioned in the discussion of one-armed spirals, usually gives symmetric density perturbations that are long-lived. Radio ejection is usually in the form of jet and counter jet, or lobe and counter lobe. There are, however, examples of short-lived minor density asymmetries and of triple ejections. In any case, as shown in Volume II we find a number of spiral galaxies with three, fairly equal arms, often symmetrically arranged. We expect they should be important tests for theories of spiral structure and in fact some work in the direction of modal analysis of spiral arms, from the point of density wave theory, has already beens begun (e.g., Haas 1983).

It is to be noted that often the clearest examples of three-armed spirals are the more distant objects where resolution does not reveal details of the arms, but rather emphasizes their general aspects. Some of these galaxies are illustrated in the photographs, as well as cases where more resolution in the arms is possible.

One commonly noticed property of spiral arms is their tendency to bifurcate somewhere along their length. We show some cases here where the bifurcation is so strong that the galaxy has the appearance of being a three-armed spiral. This may have some implications for the interpretation of all three-armed spirals and undoubtedly makes the challenge of supplying a general mechanism for spiral-arm formation even more difficult.

We also encounter some multiple-armed spirals and give some examples of these in Volume II. They may or may not form a significant transition to flocculent-arm spirals. The case of AM 0545-341 is shown here as an example of the latter kind of spiral. The subject of grand design and patchy-armed spirals is treated in Gerola and Seiden (1978), while the role of dust in spiral arms is discussed in Elmegreen (1980a), and the subject of spurs on the arms of spiral galaxies is treated in Elmegreen (1980b).

Finally, we should note that spiral arms usually remain fairly distinct, but sometimes they can apparently interact or even cross over each other. In this Catalogue AM 0102-274 is an example where some arms seem to cross each other and seem almost braided or intertwined. Other northern hemisphere examples of galaxies with crossing arms are discussed in Arp (1969).

Category 12: Peculiar Spirals

This Category is meant to illustrate spirals with peculiarities other than in their arms. The classification is made when the main body of the spiral shows unusual asymmetries or global distortions. There are no obvious sub-categories. The pecularities tend to be quite individual and not readily allied to other kinds of peculiarities. An example is the rectangular nucleus of AM 0902-680, similar to the peculiar northern hemisphere galaxy NGC 128 (Bertola and Cappacioli 1977).

The significance of this Category may lie in illuminating what happens to the arms of a galaxy when the main body of the system is disturbed. If the inner regions of the galaxy become irregular, presumably there are irregular gravitational perturbations which are transmitted outward and should effect the spiral arms if the arms are a result of gravitational density waves. By studying this correlation (or lack of correlation) between the behaviour of the spiral arms and the structure of the interior, some essential properties of spiral galaxies may eventually be derived.

It is also true that strong disturbances are more apparent in spiral galaxies than ellipticals because of the gas, dust and rotation in spirals. Some of the mysterious internal activity which takes place in the centers of galaxies, therefore, can best be studied in the Catalogue examples, given here, of peculiar spirals.

Category 13: Compact Galaxies

Even on a survey taken under the uniform conditions of the SRC Southern Schmidt Survey, inevitable differences in registered density of sky background make it difficult to estimate surface brightness of galaxies by inspection. Occasionally, however, the surface brightness of an object becomes so high that it burns out an area of the image. In this case, galaxies can be recognized if they are either a) bigger in diameter than stars, b) appear like bright stellar objects without diffraction spikes, or c) if there is a slight fuzz around the edge of a burned-out image. The above definition of compact objects also fits, rather well, one class of plate defects which can be encountered; therefore, particular effort has been made to check that our candidates are real, by assuring that they are present on more than one plate.

In general, we have not listed faint images which are only suspected of being non-stellar. Such faint objects generally comprise the interesting class of compact objects and groups of compact objects investigated by Zwicky (1971) and Shakbazian (1978). Our objects are more like the brighter objects catalogued by Zwicky. Some investigators (Massey 1975; Kormendy 1977; Fairall 1978) claim that compact galaxies are not different from ordinary galaxies. From our experience with the brighter compacts, this seems difficult to accept because, for the same diameter, our compact galaxies have surface brightnesses which burn out or saturate the plate, whereas the vast majority of galaxies do not have this effect. In any case, the present Catalogue lists galaxies that represent the largest, highest-surface-brightness compact galaxies encountered in the southern hemisphere.

An initial investigation of the spectroscopic properties of thirty-one compact galaxies, selected from U.K. Schmidt Survey plates, is given in Allen et al. (1978) and also Martin, Penfold and Glass (1978). Spectroscopy of southern compact and bright-nucleus galaxies has been reported by Fairall (1977, 1979, 1980, 1981, 1983, 1984). Galaxies with high-surface-brightness ``hot spots'' in their nuclei have been catalogued by Sersic and Pastoriza (1965), but in general these are not a sub-set of our compact galaxy Category which usually implicates the entire galaxy. Some factors which might be involved in producing the high surface brightness are: strong gaseous emission lines, high mass density, extensive non-thermal emission, or recent, high-mass star formation. Like most extragalactic objects, the present compact objects tend to be associated with other galaxies. Further investigations will have to be made to determine more accurately if these apparent associations are real, and what they might tell us about the cause of the high surface brightness. It would be particularly interesting to answer the inverse question: Are there any compact galaxies which are truly isolated in space?

Category 14: Galaxies with Prominent or Unusual Dust Absorption

Dust absorption is usually most conspicuous in edge-on spirals. In general, this kind of object has not been catalogued here, unless the dust appears to deviate from a flat, planar distribution. When dissipative particles like dust accumulate in a thin rotational plane, as is presumed to be the case where rotating spirals are concerned, the phenomenon is not especially a curiosity. But when we encounter irregular or asymmetric absorption features, we have good reason to believe that something must have perturbed them. Moreover, when we see filaments or tubes of dust, such as in the famous northern hemisphere galaxy NGC 4753, the puzzling question arises as to what mechanism could have formed such features. As in the case of stellar configurations, it is also true that non-equilibrium dust features should be differentially wound up, twisted, or dissipated by the normal motions within a galaxy. Therefore their presence would appear to indicate recent formation.

Another kind of object, included in Category 14, are galaxies where the dust marks an unexpected orientation of the rotation plane. Most elongated galaxies have been assumed to be oblate spheroids, by analogy to the many flattened spirals seen at varying degrees of edge-on projection. A few rare cases of elliptical galaxies are known, however, where the dust indicates a rotational plane that suggests that the object is a prolate spheroid. One famous case is that of the giant elliptical radio galaxy, NGC 5128 Cen A. Some additional cases like this are noted in the present Catalogue. The possibility of triaxial shapes for galaxies has also received recent theoretical and observational attention; an overview of work on the three-dimensional shapes of elliptical galaxies can be found in Bertola (1981a, 1981b).

Category 15: Galaxies with Tails, Loops of Material or Debris

Galaxies with luminous filaments which are more or less straight have been classified into Category 7: Galaxies with Jets. Luminous features which are appreciably curved have been called tails and when there are any signs of re-entrant behaviour, that is arms returning in the direction from which they originated, they have been called loops. These latter types are put into the present Category 15: Galaxies with Tails, Loops of Material or Debris.

Many doubles, and galaxies with interacting companions, show tails or loops. Presumably this is because of their mutual gravitational interaction, as can be seen in Categories 1 through 5. However, there is a category of galaxies whose members appear single, but also show tails or loops. This is an important Category because it implies that if these galaxies are truly single, then internal activity is a good candidate for producing the phenomenon. In this regard it is useful to consider that an initially straight jet could drift, fall back or be perturbed by other objects in the vicinity, and, with time, evolve into a tail or loop.

Of course there is always the possibility that what looks like a single galaxy may, in fact, be a merged, double galaxy. This could possibly enable the tails and loops to be accounted for with the usual gravitational scenario. In the Catalogue, if there was any chance that the filament could be emanating from a single galaxy, the object was put into the present Category.

Category 16: Irregular or Disturbed Galaxies

In this Category, as in the previous one, we have tried to select galaxies that have a good chance of being only one entity. But in this case we require more than exterior peculiar features, like arms or filaments. Rather we require that the entire galaxy must be asymmetrical, irregular, or of an unusual nature. Again the implication is that single irregular galaxies may have been subjected to irregular conditions of formation or that the more disturbed cases may be reacting to some recent internal perturbation.

Single galaxies are defined here as galaxies which have no comparably-sized galaxy in the immediate neighbourhood which appears to be affecting them. These Category 16 galaxies can, however, be companions to larger galaxies. The prototype for a Category 16: Irregular or Disturbed Galaxies, is the famous northern hemisphere irregular, M82 (see The Hubble Atlas, Sandage 1961; Atlas of Peculiar Galaxies, No. 337, Arp 1966). An interpretation of M82 is that it is not its proximity to M81, but rather its own internal activity, which accounts for its fractured, asymmetrical appearance. As emphasized in previous Catagories of this Catalogue and elsewhere (Arp 1973, 1982; Sulentic, Arp and di Tullio 1978), companion galaxies tend often to occur in non-equilibrium forms.

As a future step in the study of the various disturbed categories in this Catalogue, it will be of interest to investigate more quantitatively the relative occurrence of disturbances, as a function of the galaxy's proximity to larger galaxies, as a function of membership in groups, in clusters, etc. Similarly, it will be of importance to know whether any disturbed galaxies are completely isolated in space. While such a study was beyond the scope of this investigation, some feeling for the trends can be had by consulting the relative frequencies of the various Categories as listed in Table 1, since the classification codes themselves (as well as the comments in the main Catalogue) do incorporate some statement as to the local environment defined by other nearby galaxies.

Category 17: Chains

For some investigations this is possibly one of the most significant and least expected categories in the Catalogue. It was noted some time ago that chaining or linear alignments of galaxies appeared to be a frequent occurrence (Arp 1968; Oort 1970). But until we started to list such configurations for the present Catalogue it was not realized just how prevalent these a chains of galaxies were. As Table 1 shows, they are almost as frequent as apparently unaligned groups of four or more galaxies. These groups of unaligned galaxies serve as a control in the sense that we assume four galaxies in a group will be equally recognizable, whether they are aligned or not aligned. Although we feel that the tendency for chaining is very striking visually, we must leave for future statistical analysis the problem of deriving the exact frequency and accuracy of alignment which is in excess of that expected from random arrangements of unaligned groups. Of course, some opinion has already been expressed that chains are accidental projections without significance (Turner and Sargent 1973).

The other striking aspect of this Category is how frequently the galaxies in these chains seem to be disturbed or in non-equilibrium forms. The empirical fact of the disturbance supports the conclusion that the chains are not accidental configurations, because the chance of finding successive random galaxies in such disturbed forms is small. Another result, which bolsters the physical reality of the chains, is the tendency for interacting triples and quadruples to be aligned, as was discussed earlier under those Categories. Finally, the diametric companions of Category 8, many of which are themselves non-equilibrium galaxies, form aligned chains in the sense that the large galaxy is at the center of the chain. In Arp (1968) it was shown that chains of galaxies often originate on either side of radio galaxies, which tended to be the largest and/or the most active galaxies in the chain.

The reason why we feel that the general existence of chains of galaxies, if validated, is so important is because of the following argument: If chains were formed on the order of ten billion years ago, their presently indicated redshift dispersions should have dissipated the chains long ago. So, either the indicated redshift dispersions are wrong, or the groupings are accidental, or the chains are considerably younger than ten billion years.

To have galaxies recently formed is a radical departure from conventional theories of an epoch of galaxy formation in the far past. As for those chains which contain disturbed galaxies, they could have been collisionally disturbed by interactions on plunging orbits through the center. If such a circumstance seems unlikely, we are left with galaxies which are disturbed because they are internally active; this in turn may or may not be further evidence for recent formation.

One possible method to form galaxies in chains is to have the pregalactic material collapse through linear instabilities in the pregalactic medium (Oort 1970). Perhaps magnetic flux tubes could guide such collapses. However we again encounter the age difficulty, in that with the measured redshift dispersions the chains would have to have been formed recently and, as far as we know, we see no evidence for such recent condensations out of the intergalactic medium. An alternative is to eject protogalaxies from central parent galaxies (Arp 1976a). But this also would have to be a recent activity, unless the velocity dispersion along the chain was incorrectly inferred from the measured redshifts. If the physical significance of the candidate chains given in this Catalogue is confirmed, they will perhaps force profound changes on our ideas about the formation, evolution, and nature of galaxies.

Finally, it should be remarked that there is some evidence (Arp 1973) that non-equilibrium chains like that in Stephan's Quintet, for example, fall close to and appear to be associated with larger nearby, low-redshift spirals. If this line of argument is valid, it would mark the non-equilibrium chains as having large components of non-velocity redshifts.

Category 18: Groups

Objects classified into this Category fit the definition of being four or more galaxies which are not aligned. Additionally they are galaxies which stand out conspicuously by virtue of their density contrast with respect to the surrounding field density of similar sized objects. The distinction between a large group and a small cluster is somewhat arbitrary, but is made along the following lines: a cluster is regarded as a populous, dense grouping of galaxies the membership of which generally shows a gradation in galaxy sizes from large to small; a group tends to be a small aggregate of galaxies each more or less the same size. Of course, some group members may have additional small companions, but they are not a necessary or dominant component.

We have distinguished three broad types of galaxy aggregates in the present Catalogue: galaxies with relatively small companions, galaxies in groups with each member being about the same size, and populous clusters which have a broad range of sizes. We are making a rough statement about the luminosity function of galaxies within these aggregates; saying that there appear to be distinctly different luminosity functions. The challenge now becomes that of systematizing this impression and discovering the reasons why different aggregates of galaxies have different luminosity functions, if such is the case. Our definitions may of course force these correlations, but one must then wonder why the missing categories, such as populous, high-density groupings of low-luminosity galaxies are not found.

Some conspicuous groups of galaxies in the southern hemisphere have been previously listed by Klemola (1969) and by Sersic (1968). Rose (1976), Snow (1970), and Duus and Newall (1977) have each catalogued groups and clusters of galaxies. Some of their poorer clusters appear in our catalogue under Category 8: Galaxies with Apparent Companions; this difference in point of view is interesting to note.

Category 19: Clusters

Over the area of the southern sky which we catalogued, we naturally saw many clusters of galaxies. We have made no systematic attempt to record all of these since a complete catalogue of southern galaxy clusters has been made by Abell and Corwin (Clusters of Galaxies in the Southern Hemisphere, in preparation) and we refer the reader to that source. However, in the course of inspecting the southern sky, we did encounter some clusters that were outstanding. In the spirit of including all very unusual objects in the present Catalogue, we have listed all these most conspicuous galaxy clusters. A number of these are clusters dominated by cD galaxies and double, interacting, giant elliptical galaxies. Not surprisingly, many of the identifications of Parkes radio sources with objects in our Catalogue are with these clusters. Some overlap is also found with the previous brief catalogues published by Snow (1970), Rose (1976), and Duus and Newall (1977).

Category 20: Dwarf Galaxies (Low Surface Brightness)

The characteristic of low surface brightness is usually taken as a sufficient condition for low intrinsic luminosity. It is apparently not a necessary condition for low luminosity because there are intrinsically faint galaxies known with high surface brightness (Arp 1965; Sargent and Searle 1970). The dwarfs which are large in angular extent usually reveal a very irregular morphology. It is difficult to identify specific classes of irregularities, however, and although such objects are completely listed in the descriptive part of the Catalogue Volume I, only a few are illustrated in Volume II.

It should be noted at the outset that there is a technical problem in discovering small, low-surface-brightness objects: many times plate defects can look very much like real objects, and especially like dwarf galaxies. We have made every effort to check such images on two independent plates in order to minimize this error.

The outstanding problem in dwarf galaxies seems to be to explain why the grand spiral design becomes bore and more disorganized and irregular as the dwarfs become lower in surface brightness, lower in luminosity, and presumably lower in mass. The examples of dwarfs given here, it is hoped, will furnish a varied enough list of objects so that this problem can begin to be investigated.

Some analyses of dwarfs from U.K. Schmidt Survey plates have already been made. Webster et al. (1979) have searched for dwarfs belonging to NGC 5128. Longmore et al. (1982) have presented a list of optical and neutral hydrogen data for 151 low-surface-brightness galaxies. An example of a low-surface-brightness resolved galaxy, which turned out to be a member of the Local Group, is AM 0640-505, given in Cannon, Hawarden, and Tritton (1977) and now known as the ``Carina dwarf''. Other possibilities have been cited by Longmore et at. (1978) and by Madore and Arp (1979). Examples of dwarf galaxies found by ESO astronomers can be found in the general compilation by Lauberts (1982) as well as in selected announcements by Schuster and West (1976), Lausten et at. (1977) and Cesarsky et at. (1977).

The low-surface-brightness objects of small apparent diameter are particularly interesting. Is there a minimum absolute diameter below which dwarf galaxies do not form (or survive)? To answer this, distances are needed. Since much less morphological detail is visible, it is not clear from the appearance alone whether any particular object is a small, extremely low-luminosity object seen fairly nearby (i.e., loose, inter-galactic star clusters) or whether it is a more typical low-luminosity extragalactic system seen at greater distances (i.e., dwarf galaxies at two to ten megaparsecs, say). How many star clusters are there in the Local Group large enough to be seen as fuzzy patches, but far enough away to be poorly resolved? The statistics are poor, but the searches are far from complete. One such object is AM-1 (Madore and Arp 1979). Since such star clusters are of particular interest in filling out the census of objects belonging to the Milky Way and other Local Group galaxies, they were especially looked for in the course of our survey. However, the very faintest and smallest could easily be missed, even in a careful search (e.g., AM-4, Madore and Arp 1982); so we are probably complete only to the level of AM-1, which is fairly bright. Nevertheless, some of the ``dwarfs'' listed in our Catalogue may prove to be Local Group globular clusters.

Category 21: Stellar Objects with Associated Nebulosity

Although it became apparent that the vast majority of the members of this Category which we encountered were simply reflection nebulae (of which there are a great many of varied form at low galactic latitudes) we have retained the Category for the few high galactic latitude objects which may be of future interest to others. Some of these objects may yet prove to be more than their morphology suggests. One reflection nebula is illustrated in Category 22 in Volume II.

Category 22: Miscellaneous

The number of objects necessarily classified into the miscellaneous Category are very few. This is a simple demonstration that the other twenty-three Categories encompassed most of the peculiar and unusual aspects of galaxies. It is surprising that so many different peculiar objects can be classified into so few Categories. Perhaps this does mean that the subject is less complicated than it could be, and that only a few fundamental factors are causing the non-equilibrium forms with which we have to deal.

There are two examples of galaxies with ``ringing'' or stepped, concentric rings illustrated in the miscellaneous Category in Volume II. Objects of this kind were first shown in the Atlas of Peculiar Galaxies (Arp 1966); for example, No. 227 through 231. Later, Malin and Carter (1980) printed further high-contrast examples of this type of system. Generally, the rings imply some shocked star formation, but the cause of shock, whether it be internal explosion, merger, or encounter, still needs to be explored. It might be argued that these belong in Category 6: Ring Galaxies or even in Category 15: Galaxies with Loops, Tails of Material or Debris, but they are, we believe, distinct from these Categories, yet sufficiently rare that they do not demand a separate Category to themselves. Thus their present location in the Catalogue.

Category 23 and 24: Close Pairs and Triples

Sometimes large galaxies are encountered which are strikingly close together, relative to comparable sized galaxies in the general vicinity, and the supposition is strong that they are close together in space and probably related. If they are large enough for details to be resolved, and do not show signs of deformation or interaction, this is interesting in itself: Are they only temporarily close with insufficient time to effect gravitational perturbation? Or are all these cases separated much further in the line-of-sight dimension than in their projection on the sky? In any case, it was felt useful to have examples of galaxies which seem to belong together, but which are apparently not perturbed. This Category has also been included to complete the reporting of all galaxies which, from their distribution on the sky, appear to be associated together. For groups of galaxies greater than three in number, one should consult Categories 17, 18, and 19.

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