(RGs; upper frames in Figure 1.24) are one type of cataclysmic ring; these are thought to result from a head-on collision between a disk galaxy and a companion (Appleton and Struck-Marcell 1996). Polar ring galaxies (PRGs; lower frames in Figure 1.24) are a second type; these are thought to originate from the disruption of a small satellite galaxy along a polar orbit around a more massive disk galaxy (Schweizer et al. 1983). None of the examples of this type of feature shown in Figure 1.24 are certain cases. The possible PR feature in NGC 2918 is very faint but favorably oriented nearly edge-on (Whitmore et al. 1990). The possible case in NGC 4247 may be nearly face-on and less clearcut.
Figure 1.24. Possible examples of cataclymic ring galaxies. Top row: collisional ring galaxies. Bottom row: polar ring or inclined ring galaxies
1.10. General properties along the CVRHS sequence
1.10.1. Morphological systematics
The classification of a large sample of galaxies in the CVRHS system allows investigation of the systematics of different morphological types and classes, i.e. how aspects of morphology might vary with stage and family. Figure 1.25 first shows the distribution of stages for three galaxy samples: (a) a sample of 1240 galaxies from the Spitzer Survey of Stellar Structure in Galaxies (S4G; Buta et al. 2015); (b) a sample of 441 galaxies from the Analysis of the Interstellar Medium in Isolated Galaxies (AMIGA; Buta et al. 2019; Verdes-Montenegro et al. 2005); and (c) a sample of 2,628 galaxies from the Extraction of the Forms of Galaxies from Images survey (French acronym: EFIGI; Buta 2019; Baillard et al. 2011). Each plot is restricted to galaxies inclined at ≤60°. The three graphs show how the distribution of stages depends sensitively on the selection criteria for a given sample. The S4G sample is distance-limited and used 21 cm radial velocities to judge distances; this accounts for the significant emphasis of this sample on extreme late-type (Sd-Sm) spirals and Magellanic irregulars (Im). The EFIGI sample was selected largely on the basis of angular diameter and emphasizes mainly Sb-Sc spirals. The AMIGA sample of isolated spirals is similar in showing an emphasis on Sb-Sc spirals.
Figure 1.25. Distribution of CVRHS stages from (a) Buta (2019; EFIGI), (b) Buta et al. (2019; AMIGA) and (c) Buta et al. (2015; S4G). The histograms are each restricted to relatively face-on galaxies (i ≤ 60°)
Figure 1.26 shows the bar, inner variety and outer variety fractions ( fbar, fIV, and fOV, respectively) as a function of stage for subsets of EFIGI galaxies inclined ≤66°. The numbers are N = (a) 2,440, (b) 749, (c) 1,309, (d) 260 and (e) 2,642 galaxies. Figure 1.26(a) shows that the bar fraction has a minimum near stage Sc, with the highest bar fractions for extreme late-type spirals; Figure 1.26(b) shows that inner rings and well-defined inner pseudorings have the highest fraction around stage Sab; Figure 1.26(c) shows that (s) variety is uncommon for types Sab and earlier, but jumps to ≥80% for stages Sc and later; Figure 1.26(d) shows that lenses, ring-lenses and pseudoring-lenses are found mainly among stages Sb and earlier; and Figure 1.26(e) shows that outer features (rings, pseudorings, lenses and ring lenses) are most common between stages S0+ and Sb. Although outer features are rare for types later than Sb, Figure 1.26 shows a secondary peak among extreme late-type spirals.
Figure 1.26. Systematics of CVRHS classifications from Buta (2019; EFIGI). The samples used for these graphs were restricted to inclinations ≤66°
1.10.2. Astrophysical systematics
The CVRHS sequence of galaxy types from ellipticals to irregulars has astrophysical significance. This is because some measured properties of galaxies correlate with position along the sequence. For example, the integrated color index,
Other galactic properties that vary along the VRHS sequence include the average surface brightness and HI mass-to-blue luminosity ratio. The average surface brightness ranges from ≈12.4 mag arcmin−2 for stages E to S0+ to 14.9 mag arcmin−2 for stage Im, a factor of 10 drop (Buta et al. 1994). This correlation is directly related to one of Hubble’s classification criteria for spirals: that Sa galaxies have more prominent central concentrations than do Sc galaxies. The latest stages (Sd-Im) have virtually no central concentration and the lowest average surface brightnesses. Similarly, there is a relatively smooth variation in HI mass-to-blue light ratio across the spiral sequence, ranging from 0.08 at stage S0/a to 0.5 at stage Im, a factor of 6.5 change (Buta et al. 1994).
1.11. Other approaches to galaxy classification
The CVRHS system is only one approach to galaxy classification, but it has several advantages: (1) a high focus on features that are likely intimately connected to dynamics and evolution, such as bars, rings and spirals; (2) correlation with star formation history; and (3) the broadest perspective on galaxy morphology without being too unwieldy. However, in the era of large imaging surveys like
