A principal goal of extragalactic studies has been to understand what drives the morphology of galaxies. It is important to determine the dynamical and evolutionary mechanisms that underlie the bewildering array of forms that define the various galaxy classification schemes used today (e.g., Sandage & Bedke 1994; Buta et al. 2007), because this will allow us to establish the relationships, if any, between different galaxy types. Physical interpretations of galaxy morphology have revolved around two different domains: (1) formative evolution, where rapid, violent processes, such as hierarchical clustering and merging, led to formation of major galactic components, such as bulges, disks, haloes, and presumably, the Hubble sequence (e.g., White & Rees 1978; Firmani & Avila-Reese 2003); and (2) secular evolution, where disk material is slowly rearranged through the collective interaction of instabilities, such as bars, ovals, spirals, and triaxial dark matter haloes (Kormendy & Kennicutt 2004, hereafter KK04). KK04 argued that the Universe is in a state of transition, where secular evolution is becoming the dominant process of morphological change.
Galaxy morphology is a classical subject with a rich history in astronomy. Associated with many famous names, such as the Herschels, Lord Rosse, Curtis, Shapley, Hubble, Lundmark, Reynolds, Sandage, Morgan, and de Vaucouleurs, morphology and classification are the first step in the study of galaxies as fundamental units of matter in space. As elegantly noted by Peng et al. (2002), clues to galaxy formation and evolution `are hidden in the fine details of galaxy structure.'
With this series of lectures, I not only have the great privilege of laying out the fine details of galaxy morphology in the tradition of these earlier observers, but also I must do so in the context of secular evolution. This is very challenging, not just because the subject is so broad, but because we are only beginning to understand how secular processes operate in galaxies. Fortunately, my colleagues at this School bring a considerable expertise on this subject, and I feel like I can cover galaxy morphology at the level of detail that I think is needed.
Galaxy morphology may be a classical subject in astronomy, but it has a surprising freshness that has defied predictions of its impending irrelevance to true understanding of galaxies. Classical galaxy morphology and classification have survived into the modern era for several reasons: the Hubble Space Telescope (HST), the Sloan Digital Sky Survey (SDSS), and the improved understanding of the meaning of different morphological types through extensive theoretical and observational studies. There can be little doubt that morphology holds the key to recognising the processes of galactic evolution, and that it will continue to provide insight as the relationships between different types of galaxies become better established.
My goals with this series of lectures on galaxy morphology are to: (1) provide a historical overview of galaxy morphology and classification; (2) illustrate phenomenology and highlight notation; (3) introduce non-optical galaxy classification; (4) describe interpretative galaxy classification; (5) describe environmental impacts on galaxy morphology; (6) describe the important quantitative tools used for modern morphological studies and the use of large surveys to explore morphology on an unprecedented scale; and finally (7) highlight the importance and relevance of morphology to the evidence for secular evolution in galaxies.
In describing morphology, I will draw heavily on two recent sources: the de Vaucouleurs Atlas of Galaxies (dVA, Buta et al. 2007) and the major review on galaxy morphology by Buta (2013, hereafter B13) for the new series: Planets, Stars, and Stellar Systems.