3. Revised classification. In the course of a survey of bright southern galaxies with the 30-inch Reynolds reflector at Mount Stromlo [80] a classification and notation system has been developed to include in a consistent scheme all or most of the recent revisions and additions to the standard classification. The classification follows as closely as possible the revised Mt. Wilson-Palomar scheme, but includes the additional types and sub-types suggested by the Harvard, Lick and Mt. Stromlo work; the notation is based on the original Mt. Wilson system, but has been modified and supplemented as required by the introduction of new types and sub-types and so as to permit a finer description of structural features. Classification and notation are illustrated in Fig. 2 which may be regarded as an extension of Hubble's tuning-fork diagram.

Fig. 3. A 3-dimensional representation of the revised classification scheme and notation system. From left to right are the four main classes: ellipticals E, lenticulars S0, spirals S and irregulars I. Above are the ordinary families SA, below the barred families SB; on the near side are the S-shaped varieties S (s), on the far side the ringed varieties S (r). The shape of the volume indicates that the separation between the various sequences SA (s), SA (r), SB (r), SB (s) is greatest at the transition stage S0/a between lenticulars and spirals and vanishes at E and Im. A central cross-section of the classification volume illustrates the relative location of the main types and the notation system. There is a continuous transition of mixed types between the main families and varieties across the classification volume and between stages along each sequence; each point in the classification volume represents potentially a possible combination of morphological characteristics. For classification purposes this infinite continuum of types is represented by a finite number of discrete ``cells''. Compare with Plates I and II.

Actually Fig. 2 may be considered as a plane projection of a three dimensional representation, the idea of which was first conceived during conversations with Dr. Sandage in August 1955. It is schematically illustrated in Fig. 3 which may be consulted in conjunction with Fig. 2 to follow the detailed description of the adopted classification and notation (compare also with Plates I and II). The three-dimensional classification appears necessary to represent objects of mixed characteristics in correct relation to the main sequences as well as the progressive divergence of the various sequences from E to S0/a through the S0 stages and their ulterior convergence from S0/a to I through the S stages.

The present classification scheme and notation system rest on the following principles:

Four classes are retained: ellipticals E, lenticulars S0, spirals S, irregulars I, which coincide with the main divisions introduced by Hubble.

The two families of lenticulars and spirals, originally denoted ``normal'' S and ``barred'' SB by Hubble, are now designated ``ordinary'' SA and ``barred'' SB, so as to permit the use of the compound symbol SAB for ``intermediate'' objects of mixed characteristics. The symbol S alone is used when a spiral object cannot be more accurately classified as either SA or SB because of poor resolution, unfavorable tilt, etc. The ``ordinary'' spirals are not more ``normal'' than those of the ``barred'' family which are at least as common (cf. Sect. 3c).

In the new class of lenticulars the two families are denoted SA0 and SB0, depending on the absence or presence of a bar structure across the central lens; ``intermediate'' objects with a very weak bar are noted SAB0. The symbol S0, used for SA0 in Hubble's notation, is now used for a lenticular object which cannot be more precisely classified as either SA0 or SB0; this is often the case for edgewise objects ¹.

Two main varieties are recognized in each of the lenticular and spiral families, the ``annular'' or ``ringed'' type, denoted (r), and the ``spiral'' or ``S-shaped'' type, denoted (s). Intermediate types are noted (rs). In the ``ringed'' variety the structure includes circular (sometimes elliptical) arcs or rings (S0) or consists of spiral arms or branches emerging tangentially from an inner circular ring (S). In the ``spiral'' variety two main arms start at right angles from a globular or little elongated nucleus (SA) or from an axial bar (SB).

The distinction between the two families A and B and between the two varieties (r) and (s) is most clearly marked at the transition stage S0/a between the S0 and S classes. It vanishes at the transition stage between E and S0 on the one hand, and at the transition stage between S and I on the other (cf. Fig. 3).

Four sub-divisions or stages are distinguished along each of the four spiral sequences SA (r), SA (s), SB (r), SB (s), viz. ``early'', ``intermediate'' and ``late`` denoted a, b, c as in the standard classification, with the addition of a ``very late'' stage, denoted d. Intermediate stages are noted Sab, Sbc, Scd. The transition stage towards the magellanic irregulars (whether barred or not) is noted Sm, e.g. the Large Magellanic Cloud is SB (s) m.

Along each of the non-spiral sequences the signs + and - are used to denote ``early'' and ``late'' subdivisions; thus E⁺ denotes a ``late'' E, the first stage of the transition towards the S0 class ². In both the SA0 and SB0 sub-classes three stages, noted S0 ^-, S0 °, S0 ⁺ are thus distinguished; the transition stage between S0 and Sa, noted S0/a by Hubble, may also be noted Sa^-. Notations such as Sa⁺, Sb^-, etc. may be used occasionally in the spiral sequences, but the distinction is so slight between, say, Sa⁺ and Sb^-, that for statistical purposes it is convenient to group them together as Sab, etc. Experience shows that this makes the transition subdivisions, Sab, Sbc, etc. as wide as the main sub-divisions, Sa, Sb, etc. ³.

The classification of ``irregulars'' is still somewhat in doubt; objects obviously related to the magellanic type, but which do not show clearly the characteristic spiral structure, are noted I (m); those with an elongated core and asymmetrical branches are probably later stages of the SB sequences, e.g. NGC 4449, NGC 6822, etc., while those more nearly symmetrical and without bar-like core are probably later stages of the SA sequences, e.g. IC 1613, IC 2574, etc. (see Plate VII) the distinction vanishes in the ultimate, chaotic dwarf stages of low surface brightness, e.g. the Wolf-Lundmark nebula or the Sextans system ⁴.

A faint, outer ring-like structure is often observed in lenticulars and early-type spirals which is not clearly related to a specific type, i.e. with some variants it appears about equally in all four sequences near the transition stage S0/a. This particularity which seems therefore more characteristic of a certain stage of evolution than of any definite line of evolution, is denoted by an (R) preceding the symbol of the class. Examples of (R) SA are NGC 1068 (M77), NGC 4736 (M94), NGC 7217, etc. of (R) SB: NGC 1291, NGC 1326, NGC 2859, etc. Some objects have both an outer (R) structure and an inner (r) pattern; a good example is NGC 6753, noted (R) SA (r) ab (22).

In the spiral sequences supplementary data of some interest are the multiplicity of the spiral pattern and the character, ``massive'' or ``filamentary'' of the spiral arms. These are included whenever possible as subscripts respectively after the family and variety symbols. Thus M33 is noted SA (S) ₂ c_m and described as ``an ordinary, late-type, S-shaped spiral with two main massive arms''. When one or more additional and weaker arms are present this is noted 2 + 1 (e.g. M99), 2 + 2 (e.g. M51), etc. An asymmetrical, regular spiral with one arm stronger than the other as is often the case among late-type barred spirals (e.g. NGC 7479) is noted 1 + 1 (not 2). Branching arms not clearly assignable to a definite multiplicity are noted 1⁺, 2⁺, etc. as the case may be; this is a frequent occurrence among late-type spirals. A complicated spiral pattern of multiplicity higher than 4, as often observed in late-type spirals, is simply noted n (e.g. NGC 2903, NGC 7793); this, however, occurs also among early-type spirals of the ringed variety (e.g. NGC 4736, NGC 7217).

Edgewise systems which can usually be classified only as E7, S0, S (a, b, c, d) or I are noted (sp) for ``spindle''. For ellipticals the traditional notation E0, E1,...E7 may be preserved, although it is not homogeneous with the rest of the classification scheme and notation system used for the other classes; it is really almost superfluous when measured dimensions and flattening are given.

An advantage of the present notation system is that it is possible to retain significant information on objects poorly resolved by dropping one or more symbols which from right to left refer to stages,varieties, families and classes of increasing generality. Thus S may be either SA or SB, S0 either SA0 or SB0, SA is either SA (r) or SA (s),..., Sa is either SAa or SBa,..., etc. Hence the nature and number of symbols used can be selected according to the amount of detail visible in any given object with any particular telescope. Conversely the degree of completeness in the symbolism gives an indication of the relative degree of resolution available; this, of course, depends on both the size of the telescope and the size of the nebula.

These are discussed in detail by Zwicky. It is nevertheless of some importance for the present discussion to determine the separation between neighboring objects within which their mutual interaction begins to cause significant structural distortion and thus sets a limit to the validity of the classification system established in the first instance for isolated galaxies. A rough classification of pairs is as follows:

P (a) : pairs of widely separated objects showing no clear sign of interaction (occasional optical pairs are included in this group);

P (b) : close pairs showing clear signs of interaction, such as tidal distortion of their outlying parts;

P (c) : colliding pairs where the ``main bodies'' are in actual contact and show extensive disruption of their internal structure.

Many parameters are obviously involved, such as separation, diameters, masses, velocities, etc. in the interaction of two galaxies. Only the first two are more less directly accessible in most cases and more precisely the ratio x' of the projected separations s'_1,2 between the nuclei G₁, G₂ of a pair to the sum of their apparent photographic radii r₁, r₂. The true separation s is not known in individual cases, but for a random distribution of orientations of mean (G₁ G₂) with respect to the line of sight the mean value of s' / s is / 4 0.79. For 40 pairs south of - 35° observed with the Reynolds reflector on Mt. Stromlo ⁵ the following mean values were obtained:

Allowing for scale factors the average projected and actual separations are:

on a distance scale in which Hubble's expansion parameter is 180 km/sec per Megaparsec. The mean critical separation (x' 1.6, x 2.0) within which tidal distortion becomes notable is then of the order of 10 kpc. Among the bright galaxies south of -35° only 10 out of more than 400, or less than 2.5%, are within this limit. Hence, the revised classification should be applicable to at least 97 or 98% of the external galaxies, of which about 95% can be included in the normal scheme of Fig. 2 and the remaining 2 or 3% must still be regarded as special or ``peculiar'' cases reserved for further analysis.

A classification of over 200 southern galaxies in the revised system [80] yields the following apparent relative frequencies for a sample essentially selected according to apparent magnitude and surface brightness (all objects listed in the Shapley-Ames Catalogue south of - 35°).

The regular spirals SA, SB make up just over fifty percent of the sample, ellipticals and lenticulars close to a quarter each. The discrepancies between earlier statistics (cf. Sect. 1b) may therefore have hinged mainly on whether the lenticulars were counted as ellipticals or spirals the ratio S/E changing correspondingly from 1 / 1 to 3 / 1. Note also that the ordinary spirals appear no longer more abundant than the barred spirals despite the inclusion of intermediate types (SAB) with the SA group in the table.

The detailed frequency in each of the sub-divisions along the regular classification sequence is given in Table 2, in which E / S0 = E ⁺, S0 ^- and S0/a = S0 ⁺, Sa ^-.

A plot of the frequencies (Fig. 4) gives a fairly smooth curve indicating maxima at Sc and E separated by a minimum at Sa. The low apparent frequency of the Sd, Sm stages is certainly due to a large extent to the low absolute luminosity and surface brightness of galaxies at these stages, but it is not yet possible to correct the apparent frequencies for such selection effects. On the other hand the high frequency of ellipticals cannot be due in general to an abnormally high absolute luminosity, so that it probably reflects the actual great abundance of this class ⁶.

Fig. 4. Frequency distribution of revised types in the Mount Stromlo survey.