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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