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S0 galaxies are generally viewed as “armless” disk galaxies. Once referred to as “lenticulars” because of their double convex lens-like appearance in the edge-on view, S0s first appeared as a galaxy type in Hubble’s 1936 book, The Realm of the Nebulae, where he presents his “tuning fork” depiction of his system (Figure 1.1). Although ellipticals, normal spirals and barred spirals were the main types recognized in Hubble’s original 1926 classification system, the possibility of S0 galaxies was raised even then. Something was puzzling about the original system in that Sa galaxies usually had fully developed arms while SBa galaxies did not. To account for this, Hubble hypothesized the existence of a class of disk-shaped galaxies lacking spiral arms. He placed these at the juncture of the “tuning fork”, where the most flattened-looking E galaxies link with the two parallel sequences of spiral galaxies. As Hubble increased his database of photographic images of “nebulae”, real examples of the hypothesized group of armless disk galaxies were found. Hubble was working on a revision to his system but died before publishing it. Sandage (1961) used Hubble’s notes to make the revision: S0s were interpreted in terms of two sequences of three “normal” stages: S0₁, S0₂ and S0₃, and three barred stages SB0₁, SB0₂ and SB0₃ (Figure 1.2). The subdivision of non-barred S0s was based on the development of structure such as the appearance of dust lanes, while the subdivision of barred S0s was based on the development of the bar from something very subtle to a more prominent feature.

Figure 1.17. Outer resonant features. The feature type is in the upper right of each frame

The tuning fork arrangement of galaxy types with S0s included began to be questioned when it was realized that S0s had lower luminosities than Sa or SBa galaxies and thus do not really link smoothly to those parts of the Hubble sequence. This led to the “parallel-sequence” interpretation of S0s, whereby S0s form a sequence parallel to the spirals (not to be confused with the normal/barred spiral prongs of the tuning fork). This interpretation is based on a nurture view that S0s are former spiral galaxies that have been stripped of their interstellar gas and dust through environmental interactions (van den Bergh 1976). On a sequence between spirals and S0s, van den Bergh placed what he called “anemic spirals” or galaxies in an intermediate state of stripping. Real examples of galaxies in such an intermediate state have been found in the Virgo Cluster (Koopmann and Kenney 2004) and the Coma Cluster (Yagi et al. (2010).

Figure 1.19 shows several S0 galaxies classified in the CVRHS system. The CVRHS system treats S0s differently from the tuning fork and parallel sequence ideas. Like spirals, S0s are divided into stages, families and varieties (Figure 1.3). The stage for S0s has three subdivisions: S0⁻, S0° and S0⁺, a sequence of increasing structure (but not necessarily decreasing bulge to total luminosity ratio, which determines stages along the spiral sequences). Intermediate CVRHS stages from average multiphase classifications (as in Buta et al. 2015 and Buta 2019) are S0^−/° and S0^°/+.

Figure 1.18. Twelve examples of barred galaxies having a “barlens”, which refers to the roundish section inside the bar. Several of these show light deficits around the bar, especially NGC 5335 (Buta 2017b; see also Gadotti and de Souza 2003b; Kim et al. 2016)

Lenses are common in early S0 galaxies. For example, typical S0⁻ and S0^−/° galaxies have mostly featureless extended envelopes that often have a subtle outer edge. Such features are classified as outer lenses (L). Faint traces of inner lenses (l) are also seen in such galaxies. As noted in section 1.4, the origin of lenses is uncertain. Some or most inner lenses are thought to be due to the dissolution of bars (Kormendy 1979), but the existence of outer and nuclear lenses suggests a possible link to the evolution of rings.

Figure 1.19. Stage sequences for both barred and non-barred S0 galaxies