Figure 1.20. Magellanic irregular galaxies
1.7. Dwarf elliptical, S0, and spheroidal galaxies
In the environment of the Virgo Cluster, the most common type of galaxy is not an irregular, but what Sandage and Binggeli (1984) called a dwarf elliptical (dE) type. Of nearly 2,000 catalogued galaxies in the Virgo Cluster, Sandage and Binggeli (1984) found that 80% were of this type. A dE galaxy is a low luminosity system with a relatively smooth luminosity distribution that resembles a conventional elliptical galaxy but has a low surface brightness and a luminosity distribution closer to an exponential than to the
The exact placement of the dE and dS0 systems within the Hubble tuning fork can be examined by studying the luminosity distributions and investigating scaling relations among elliptical galaxies of all luminosities. Based on such studies, Kormendy and Bender (2012) concluded that dE and dS0 galaxies are not merely the low luminosity extensions of classical E galaxies, but are environmentally modified former irregular and dwarf irregular galaxies. Kormendy and Bender (2012) suggested that all such galaxies be referred to as “Spheroidal” galaxies, and that their existence supports the idea of the van den Bergh (1976) parallel-sequence classification.
In the CVRHS system, the dE and dS0 classification symbols of Sandage and Binggeli (1984) are adopted in the form dE/dS0/Sph or dE/S0,N/Sph. Ann et al. (2015) show numerous examples.
1.8. Edge-on galaxies
Edge-on disk galaxies can be more difficult to classify than face-on galaxies, owing to the foreshortening of features such as spiral arms, bars and rings. Nevertheless, the degree of central concentration is often still detectable in spite of heavy planar extinction, and the degree of resolution in the disk can still be used, so that the stage is still discernible (Figure 1.21). Features like inner varieties have to be left out of the classification, leaving types such as Sbc sp, Sd spw, etc. The characteristic “sp” (meaning “spindle-shaped”) is only used to single-out edge-on or nearly edge-on disks, and does not imply prolate rotation. A galaxy classified as “spw” is one that shows evidence of disk warping in the edge-on view. Some extreme cases of disk warping are shown in Buta et al. (2015).
Embedded disks and thick disks: An embedded disk system is a case where a disk galaxy is embedded within a clear 3D component, usually an E galaxy of low flattening. In a disky elliptical galaxy, classified as type E(d) in the notation of Kormendy and Bender (1996), the presence of a faint disk is inferred from subtle distortions of isophotes. As noted in section 1.3, disky isophotes are detected through positive values of the cos(4θ) relative Fourier component of deviations from a perfectly elliptical shape. Embedded disk systems are similar to disky ellipticals, only the disk is much more obvious.
The 3D systems the disks are embedded in may be considered part of the bulge, and the disk may appear to underextend, fill or overextend the isophotes of that system (Buta et al. 2015). While disky E systems are best detected when the disk is close to edge-on, more prominent disks need not be so inclined. Figure 1.22 shows several examples. Embedded disks are recognized in CVRHS classification by a notation such as, for example, S0− sp/E(d)2-3 where the “S0− sp” part is the spindle-shaped edge-on disk component and the “E(d)2-3” refers to a low flattening E galaxy the disk is embedded within. Obvious embedded disks, where the background spheroidal system has a low flattening, are not common. In general, the flattening of the spheroidal part of an embedded disk system, and the character of the isophotes as boxy or disky, are estimated using a color display of the galaxy’s isophotes. In rare cases, the extended component is boxy rather than cuspy, as in the case of NGC 4638 where the CVRHS type is S0− sp/E(b)3.
Figure 1.21. Edge-on galaxies from stage Sa to stage Im. The “sp” stands for “spindle” and is used to characterize the edge-on orientation. If “spw”, it means the edge-on disk shows signs of warping
Most spiral and S0 galaxies do not have dominant classical bulges, but many have a two-component disk: the thin disk, which has a vertical scale height of a few hundred pc, and the thick disk, which shares the same plane with the thin disk but has a vertical scaleheight of about 1 kpc. Especially in spirals, the two disks can have very different stellar populations, with population I material occupying the thin disk and population II material occupying the thick disk. As for embedded disks, thin and thick disks are recognized in CVRHS classifications by combining a normal stage classification with an E(d) or E(b) classification as in, for example, S0− sp/E(d)7. In this case, the “S0− sp” part refers to the thin disk and determines the stage index T of the galaxy. The “E(d)7” part refers to the thick disk, whose apparent flattening and isophotal character (cuspy or boxy) are judged from a color display. Note that thick disks may have cuspy, boxy or perfectly elliptical isophotes, the latter requiring no (d) or (b) added to the classification.
Figure 1.22. Embedded disks and thick/thin disks. The main embedded disk cases shown are NGC 4660, NGC 5382, NGC 3675 and NGC 4474, and the thick/thin disk examples include NGC 4452, NGC 4527, NGC 4710 and NGC 4544
1.9. Morphology of interacting and merging galaxies
Interacting and merging galaxies make up about 2–4% of nearby galaxies (Knapen and James 2009) and are often unclassifiable in any standard Hubble-based morphological bins. Figure 1.23 shows several examples, all of which are classified as Pec (merger) in the CVRHS system. Distorted shapes, tidal tails, sharp edges (“shells” or “ripples”; e.g. Schweizer and Seitzer 1988), multiple nuclei, and in some cases enhanced star formation characterize these objects. Although major mergers of two spiral galaxies have been thought to generally lead to an elliptical galaxy as a remnant (e.g. Schweizer 1982), more recent numerical studies have shown that a disk galaxy can also be a remnant of such mergers (Athanassoula et al. 2016).
Figure 1.23. Examples of interacting and merging galaxies
The types of rings in normal galaxies that define inner, outer and nuclear varieties are likely features that result from internal dynamics (section 1.12). Figure 1.24 shows examples of the extremely rare types of ring-like features that likely result from a cataclysm, such as a galaxy interaction or
