Galaxies. Группа авторов

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Galaxies - Группа авторов


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lenses mainly, outer varieties refer to outer rings and lenses. While a typical inner ring or pseudoring in a barred galaxy has about the same size as the bar, the typical outer ring is about twice the size of the bar. Outer features are typically more diffuse than inner features, and of lower surface brightness. The main types of outer features include outer rings (R), outer pseudorings (R′), outer lenses (L), outer ring-lenses (RL) and outer pseudoring-lenses (R′L). An example of each of these, including a doubled outer ring case (RR), is shown in Figure 1.16. In all cases, the outer feature classification is positioned ahead of the family classification, as in (RL)SA(l)0° or (R′)SB(rs)ab.

      In spiral galaxies, outer features tend to be pseudorings or pseudoring lenses. The best-defined outer rings tend to be found in S0+ or S0/a cases. The galaxies NGC 2859 (Figure 1.16) and 3945 (Figure 1.14) are two of the best examples of outer rings. Both are classified by Buta (2019) as (R)SABa(rl,bl,nb)0+, meaning they are “late” S0s. A typical outer pseudoring is seen in NGC 5610, type (R′)SB(s)ab (Figure 1.10). Outer pseudorings are much more common than outer rings.

Photos depict examples of different types of outer features.

Photos depict outer resonant features.

      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).

Photos depict twelve examples of barred galaxies having a barlens, which refers to the roundish section inside the bar. Photos depict sage sequences for both barred and non-barred S0 galaxies.


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