The large number of illustrations in this article attests to the richness of the diversity of galaxy morphology. It is, of course, not possible to illustrate all aspects of morphology that might be worth discussing, but most interesting is how far physical galaxy morphology has come in the 35 years since Allan Sandage wrote his review of galaxy morphology in Volume IX of Stars and Stellar Systems. Galaxy morphology is no longer the purely descriptive subject it once used to be.
Internal perturbations such as bars are apparently capable of generating a great deal of the interesting structure we see in disk galaxies, and more theoretical and observational studies should elucidate this further. The impact of bars on morphology seems well understood, as summarized in detail by KK04 in their monumental review article on pseudobulges. Bars redistribute angular momentum and reorganize gas clouds to flow into resonance regions and fuel star formation. The gathering of gas into resonance regions can drive the formation of rings, and indeed can also build up the central mass concentration to the point of bar destruction. Even failing this, the pile-up of gas into the nuclear region can lead to the formation of a pseudobulge. The richness of barred galaxy morphology attests to the strong role secular evolution plays in structuring galaxies.
Progress in understanding the role of mergers and interactions on galaxy morphology has also proceeded at a rapid pace. Great success in numerical simulations and the theory of interacting galaxies has made it possible to link a specific type of interaction to a specific morphology (e. g., collisional ring galaxies). The complex structure of early-type galaxies, with boxy and disky isophote shapes, shells and ripples, and other features, shows that interactions and mergers play an important role in molding galaxies (see the excellent review by Schweizer 1998). With the advent of the Hubble Space Telescope, this role has been elucidated even more clearly because the merger rate was higher in the past.
In spite of the theoretical progress, it is interesting that classical morphology has not lost its relevance or usefulness even after more than 80 years since Hubble published his famous 1926 paper. No matter how much progress in understanding the physical basis for morphology is made, there is still a need for the ordering and insights provided by classical Hubble-Sandage-de Vaucouleurs galaxy classifications. Morphology went through a low phase in the 1980s and 90s when it was perceived that galaxy classification placed too much emphasis on unimportant details and was too descriptive to be useful. It was thought that the Hubble classification had gone as far as it can go, and that another approach needed to be tried to build a more physical picture of galaxies. At that time, there was a sense that the focus should be more on the component "building blocks" of galaxies, or what might be called galactic subsystems (e.g., Djorgovski 1992). Quantification of morphology became more possible as advanced instrumentation allowed more detailed physical measurements to be made. In the end, as morphology became better understood, it also became clear what a type such as "(R)SB(r)ab" might really mean, which enhanced the value of classification (KK04). In addition, numerical simulations became sophisticated enough to make predictions about morphology (e.g., the R1 and R2 subclasses of outer rings and pseudorings). These types of things, as well as the movement of morphology from the photographic domain to the digital imaging domain, the broadening of the wavelength coverage available to morphological studies from the optical to the ultraviolet and infrared domains, the Sloan Digital Sky Survey, and the accessiblity of high redshift galaxies to unprecedentedly detailed morphological study, all played a role in bringing galaxy morphology to the forefront of extragalactic research.
Even so, the writing of this article has shown that many important galaxies and classes of galaxies have not been studied well enough to have much modern data available. For example, in spite of the considerable interest in collisional ring galaxies the past 20 years or so, Struck (2010) was forced to lament that ring galaxies "are underobserved." The same can be said for resonance ring galaxies, giant low surface brightness galaxies, dwarf spirals, Magellanic barred spirals, counter-winding spirals, and other classes of interacting galaxies. The most that can be said about this is that further studies will likely be made, especially if instrumentation facilitates the objects in question. Rotation and dynamics are far short of photometry for most classes of galaxies, but would add a great deal of insight if obtainable.
At the other extreme, early-type (E and S0 galaxies) continue to be the focus of major photometric, kinematic, and theoretical research projects. Important clues to the formation and evolution of such galaxies are contained in their intrinsic shapes (oblate, prolate, triaxial), in the ages, metallicities, and radial mass-to-light ratios of their stellar populations, in their three-dimensional orbital structure, and in the kinematic peculiarities often found in such systems (de Zeeuw et al. 2002). Among the most recent studies are the massive photometric analysis of early-types in the Virgo Cluster by Kormendy et al. (2009), and the ATLAS3D project described by Cappellari et al. (2011). ATLAS3D is the largest kinematic database of high-quality two-dimensional velocity field information ever obtained for early-type galaxies, including 260 such galaxies in a well-defined and complete sample. This survey is simply the latest part of the long-term effort by many researchers, beginning in the 1980s, to understand early-type galaxies in terms of quantitative parameters that can be tied to theoretical models. Early-types have been a persistent enigma in morphological studies, and considerable evidence suggests that the E, S0 sequence as defined by Hubble, Sandage, and de Vaucouleurs hides a great deal of important physics associated with these objects. The ATLAS3D project was designed to exploit the R parameter described by Emsellem et al. (2007; see section 5.1), which separates early-types into fast and slow rotators and discriminates galaxies along the red color sequence (section 15.1).
These advances for early-type galaxies do not mean that quantitative analyses of later-type galaxies are lacking. As codes for two-dimensional photometric decomposition become ever more sophisticated (e. g., Peng et al. 2010; Laurikainen et al. 2010 and references therein), parameters that characterize the bulges, disks, bars, lenses, rings, and spiral patterns are being derived for large numbers of galaxies (especially barred galaxies) that were not reliably decomposable with earlier one-dimensional approaches.
For the future, it is to be hoped that the Sloan Digital Sky Survey will be extended to cover the whole sky, and provide access to high quality morphological studies of several million more galaxies, some of which might have new and exotic structures. The James Webb Space Telescope should be able to carry the HST's torch to greater depths and resolutions of high redshift galaxies, to further enhance our understanding of galaxy evolution.
This article is dedicated to Allan Sandage (1926-2010), one of the 20th century's greatest astronomers, who helped set the stage for galaxy morphology to be one of the most active fields of modern extragalactic research. It was Dr. Sandage's efforts that firmly cemented Hubble's ideas on morphology into astronomy. The author is grateful to Dr. Sandage for the inspiration he provided for this article and for his standard of excellence in astronomy.
The author also gratefully acknowledges the helpful comments and suggestions from the following people that considerably improved this article: Martin Bureau, Adriana Durbala, Debra Elmegreen, William Keel, Jeffrey Kenney, Johan H. Knapen, Rebecca Koopmann, John Kormendy, Eija Laurikainen, Barry Madore, Karen L. Masters, Patrick M. Treuthardt, Sidney van den Bergh, and Xiaolei Zhang. The author also thanks Debra Elmegreen for providing the images of high redshift galaxies shown in Figure 46, Masfumi Yagi for the illustrations in Figure 29, and John Kormendy, Jason Surace, Donald P. Schneider, and Rogier Windhorst for the use of published illustrations from specific papers. This article uses images from many sources too numerous to acknowledge here, but mainly drawn from the dVA, the NASA/IPAC Extragalactic Database, the Ohio State University Bright Spiral Galaxy Survey (OSUBSGS), the Sloan Digital Sky Survey, and several published papers by other authors. The NASA/IPAC Extragalactic Database (NED) is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Funding for the OSUBSGS was provided by grants from the NSF (grants AST 92-17716 and AST 96-17006), with additional funding from the Ohio State University. Funding for the creation and distribution of the SDSS Archive has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, NASA, NSF, the U.S. Department of Energy, the Japanese Monbukagakusho, and Max Planck Society. Observations with the NASA/ESA Hubble Space Telescope were obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under contract NAS 5-26555. The Spitzer Space Telescope is operated by the Jet Propulsion Laboratory, California Institute of Technology, under NASA contract 1407. The Two Micron All-Sky Survey is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. GALEX is a NASA mission operated by the Jet Propulsion Laboratory. GALEX data is from the Multimission Archive at the Space Telescope Science Institute (MAST). Support for MAST for non-HST data is provided by the NASA Office of Space Science via grant NNX09AF08G and by other grants and contracts. This article has also made use of THINGS, "The HI Nearby Galaxy Survey" (Walter et al. 2008), and BIMA-SONG, the Berkeley-Illinois-Maryland Survey of Nearby Galaxies (Helfer et al. 2003).