Family Traits of Galaxies: From the Tuning Fork to a Physical Classification in a Multi-Wavelength Context

Abstract

At the time of the Great Debate nebulæ where recognized to have different morphologies and first classifications, sometimes only descriptive, have been attempted. A review of these early classification systems are well documented by the Allan Sandage’s review in 2005 [107]. This review emphasized the debt, in term of continuity of forms of spiral galaxies, due by the Hubble’s classification scheme to the Reynold’s systems proposed in 1920 [97]. In The Realm of the Nebulæ [64] Edwin Hubble was first of all convinced about the need of a classification scheme to properly understand the nature of galaxies. ”The first step is obviously a study of the apparent features of the systems under investigations. The nebulæ might be members of a single family or they might represent a mixture of utterly different kinds of objects. The questions is very important for all investigations of a general nature. The nebulæ are so common that cannot all be studied individually. Therefore, it is necessary to know whether a fair sample can be assembled from the more conspicuous objects and, if so, the size of the sample required. The answer to this question, and to many others, is sought in the classification of nebulæ.” Hubble described his classification procedure as follows “sort out the nebulæ, by inspection of photographs, into groups of objects showing similar features. The more conspicuous members of each group can then be studied in detail and the results used for comparison of the groups themselves. The degree of success attained by the method depends largely upon the significance of the features selected as the basis of the classification. ... The features must be significant– they must indicate physical properties of the nebulæ themselves and not a chance effect of orientation– and also they must conspicuously enough to be seen in a large numbers of nebulæ.” In the 1936 Hubble’s classification scheme, nebulæ are divided into “two very unequal groups. The great majority are called regular nebulae, since they exhibit a common pattern, conspicuous evidence of rotational symmetry about dominating, central nuclei. The remaining objects, about 2 or 3 per cent of the total number, are called irregular, because they lack both rotational symmetry and, in general, dominating nuclei.” The pillars of the classification of the regular nebulæ are ”either elliptical or spirals. Objects in each group fall naturally into ordered sequences of structural forms ... The progression throughout the complete sequence thus runs from the most compact of the elliptical nebulæ to the most open of the spirals. ... The terms early and late are used to denote relative position in the empirical sequence without regard to their temporal implications.” Hubble concluded that his classification scheme, known today as the Hubble sequence (HS) and/or the tuning fork, is purely empirical in nature and emphasized that the above consideration “is important because the sequence closely resembles the line of development indicated by the current theory of nebular evolution as developed by Sir James Jeans.” So an empirical approach is needed according to Hubble to build a robust classification system, although the morphologist has to keep an eye on the theory! Since the HS definition, the galaxy morphological classification systems had an uninterrupted evolution whose main objective is to account for the plethora of substructures emerging from galaxy photographic observations. Up to the end of the 1980’s astronomers concentrated on organizing such variety of morphologies (in the optical band) underlying the smooth continuity between classification bins (e.g. [35]). At odds, classification systems taking into account the galaxy absolute magnitude, break the HS at high, where only cluster Dominators (cDs) are found, and at low luminosities (see e.g. the 3D classification system proposed by van den Bergh [127]), where dwarf galaxies display their variety . For decades the morphology has been considered the hollmark of a galaxy and the classification process should be made by a “specialist”. All galaxy catalogues report a classification, even if purely descriptive, we mention the Morphologicheskji Katalog Galaktik by Boris Vorontsov-Veliaminov (1962-1974), but very few have been considered a “must-see” catalog to refer in term of classification. We mention The Hubble Atlas of Galaxies by Allan Sandage (1961), A Revised Shapley-Ames Catalog of Bright Galaxies, by Sandage and Gustav Tammann (1981–1989) and multiple versions of The Reference Catalogue of Bright Galaxies by Gerard de Vaucouleurs and collaborators (1981-1989). Today the process of morphological classification of galaxies is going through the stress-tests of high-resolution, wide-field, deep imaging and multi wavelength observations through digital devices. Galaxies are today scrutinized with an unprecedented detail. The digital imaging provides the possibility to elaborate images, modeling galaxies, evidence asymmetries as well as identify sub-structures from the very center out to the extreme periphery of the galaxy. Different morphological structures can be investigated at several wavelengths and compared. Galaxies change, often dramatically, their morphology when observed at different wavelengths, still the basic HS is used to select galaxy samples over which to infer their global properties. We may synthesize the question about galaxy classification using two sentences which express two opposite views. From one side, the sentence of Halton Arp in the Introduction to the Atlas of Peculiar Galaxies comes to mind ”... But far from all galaxies fit the Hubble sequence of nebular forms. In fact, when looked at closely enough, every galaxy is peculiar.” Peculiar features tell us a story, maybe of that “unique” galaxy. On the other side, Ron Buta (1992), collaborator of Gearard and Antoinette de Vaucouleurs, resumed the need of a morphological classification in the following sentence ”As long as only a few criteria define a system, and if image material of a similar quality to that which formed the basis of the system is used, then there will be a greater ease of applicability and reproducibility of that system by independent observers. If one later finds correlations between fundamental observables and classifications, then the system could lead to physical insight ...” This Chapter focuses on the evolution of the galaxy classification schemes throughout this century and wishes to introduce the debate about the power entrusted, since the introduction of the HS, to the morphology in disentangling the galaxy history, i.e. in identifying the formation/evolutionary mechanisms that are believed to be at the origin of the observed morphology. In this context, the debate today enumerates very different positions from those suggesting that the galaxy morphology is the basic parameter, sufficient to identify an evolutionary path, to those who believe necessary to “isolate few ” additional galaxy physical parameters to that purpose, up to researchers that uses sophisticated statistical approaches in which the morphology is simply one of the parameters that come into play. In Section 3.2 Debra Elmegreen starts discussing the manifold of spiral galaxies that, since the daybreak of the extragalactic astronomy, charmed astronomers. In Section 3.3 the class of S0s is considered. In the original Hubble classification S0s are viewed as transition objects between Spiral and ellipticals at odds with van den Berg, more recent classification, which considers them a sequence parallel to that of normal and barred spirals. Gary Welch in 1999 [134] used colourful words to describe the S0 class “To span the abyss between the two main classes is the job of S0s or lenticular”. Eija Laurikainen provides here an historical and modern interpretation of this class of galaxies. S0s are often considered part of the vast class of the so-called early-type galaxies (ETGs) which also includes ellipticals. In Section 3.4 the classification of ETGs is considered by Pierre-Alain Duc, member of the ATLAS3D team. This research group is trying to organize ETGs into more physical classes, according to their bimodal, fast or slow, kinematic figures of rotation. Dwarfs, including irregular galaxies, were neglected by Hubble in The Realm of the Nebulae and have been considered only in later more complex classification schemes. Carme Gallart and Debra Elmegreen dealt with dwarfs in Section 3.5. “A good classification can drive the physics, but the physics must not be used to drive the classification. Otherwise the process becomes circular.” (A. Sandage, [107]). In Section 3.6 Didier Fraix-Bunet discusses the problem of galaxy classification and the use of multi-parametric approaches and genetic algorithms to provide a classification scheme independent from the HS. These techniques might potentially identify the evolutionary paths followed by galaxies in their evolution.