Galaxies. Группа авторов

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between these three parameters of elliptical galaxies define what has come to be known as the “fundamental plane” (Kormendy and Djorgovski 1989). The plane tells us that larger E galaxies tend to have lower average surface brightness than smaller E galaxies, and that more luminous E galaxies have higher central velocity dispersion than do lower luminosity E galaxies.

      The spiral structure of galaxies was discovered more than 170 years ago. The subtle patterns were first detected in 1845 with the world’s largest telescope at the time, the “Leviathan of Parsonstown” located in central Ireland. William Parsons, the Third Earl of Rosse, visually saw the spiral arms of the “Whirlpool Galaxy” M51 with his newly built 72-inch speculum metal reflector. In the parlance of 19th Century astronomy, M51 was called a “nebula”, not a galaxy, although the general view at the time was that most or all nebulae were distant systems of stars like the Milky Way (“Island Universe” hypothesis). Parsons built the Leviathan partly to test this idea. The discovery of spiral structure added mystique to the nebulae, and led to alternative ideas as to what the nebulae actually were. It would be nearly a century after Parsons’ discovery that any serious understanding of the nature of spiral structure would be achieved (section 1.11).

      Spiral galaxies are generally two-component systems consisting of a bulge and a disk. Although at one time bulges were thought to be generally less flattened components than disks, it is now clear that bulges include a mix of dissimilar structures, such as spheroidal “classical” bulges, highly flattened “pseudobulges” (Kormendy and Kennicutt 2004) and “boxy/peanut” bulges, the latter thought to be due to edge-on views of bars (e.g. Lutticke and Dettmar 1999).

      The sequence for non-barred galaxies in Figure 1.9 shows the rough correlation between central concentration and stage. Bulges are most prominent at stages Sbc and earlier, and are least prominent at stages Sc and later. The sequence shows well how arms are smooth at stage Sa and knotty, well-resolved, more open features at stage Sc. Intermediate stages are as well defined as regular stages: Sab galaxies often resemble Sa galaxies but with a greater degree of resolution into star-forming regions; Sbc galaxies typically have the bulge of an Sb galaxy in a disk with Sc arms; Scd is recognized as an Sc galaxy with only a trace of central concentration; and Sdm galaxies are typically bulge-less asymmetric systems with an offset bar and one spiral arm longer than the other. Similar underline stages (e.g. Sab. Scd) are used throughout the CVRHS sequence (de Vaucouleurs 1963).

Schematic illustration of a sequence of stages for non-barred galaxies in the VRHS/CVRHS system. Photos depict a sequence of stages for barred galaxies in the VRHS/CVRHS system.
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