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brightness as a function of galactocentric distance, with no inflections or other apparent structure. Type E⁺ galaxies are considered as “late” ellipticals showing some traces of structure. The structure can be in the form of a subtle lens, a feature that when more apparent is seen to have a shallow brightness gradient interior to a sharp edge (Sandage 1961; Kormendy 1979). In the Third Reference Catalogue of Bright Galaxies (RC3; de Vaucouleurs et al. 1991), the E⁺ classification was also applied to brightest cluster members having a shallow brightness gradient (or an apparent extended envelope) in the outer regions. In the Morgan (1958) classification system, such objects are classified as type cD. Examples of E and E⁺ galaxies are shown in Figures 1.5 and 1.6. Other aspects of E galaxy morphology include the following:

Disky versus boxy isophotes: The shapes of E galaxy isophotes carry important clues to their history. Kormendy and Bender (1996) noted how Hubble’s main E galaxy classification criterion, the apparent flattening (as in, e.g. E3), had little physical significance, being determined mainly by how a galaxy is viewed. Very few ellipticals are recognized as being more flattened than E4. NGC 4623 in Figure 1.5 is as close as any real galaxy comes to being a genuine E7. Nevertheless, it shows a faint trace of a thin disk (section 1.5).

Figure 1.5. Examples of elliptical galaxies ranging from round in apparent shape (E0) to fairly elongated (E6). Sandage and Bedke (1994) considered NGC 4623 (lower right frame) to be a possible example of a genuine E7 galaxy. Very few ellipticals are more flattened than E4

Capaccioli (1987) noted two families of elliptical galaxies at a time when any trace of a disk component in a galaxy classified as type E was taken to mean that the galaxy was a misclassified S0 galaxy. Kormendy and Bender (1996) proposed a reclassification of ellipticals into disky and boxy categories based on the sign of the cos 4θ relative Fourier deviation from perfectly elliptical isophotes. If the relative amplitude of this term is positive, the isophotes are said to be cuspy (or “disky”), but if negative, the isophotes are boxy. Cuspy isophotes are interpreted as indicating the presence of a subtle disk component, likely to be made of accreted material. Boxy isophotes are also thought to be indicative of interactive history. If an E galaxy is disky, it is classified as type E(d), while if an E galaxy is boxy, the type is E(b). The distinction of these subcategories can depend on how we are viewing the structures. For example, if the subtle disk in an E galaxy is oriented more nearly face-on rather than edge-on, then the isophotes will not necessarily show a cuspy shape. Both boxiness and diskiness are favored to be seen in the edge-on view. Several examples are shown in Figure 1.7. The CVRHS adopts the E(b) and E(d) terminology of Kormendy and Bender (1996) when these distinctions are obvious in a visual inspection of images.

Photos depict E plus galaxies are generally elliptical galaxies with very subtle traces of structure, usually in the form of a lens, a feature with a shallow brightness gradient interior to a sharp edge.

Figure 1.6. E⁺ galaxies are generally elliptical galaxies with very subtle traces of structure, usually in the form of a lens, a feature with a shallow brightness gradient interior to a sharp edge. Lenses are more prominent in S0 galaxies. The bottom row shows three brightest cluster members that are best interpreted as Morgan cD (supergiant) galaxies but which have also been considered E⁺ galaxies by de Vaucouleurs

Isophote twisting: Isophote twisting, where the ellipticity and position angle of the major axis of the isophotes of an E galaxy change systematically with increasing radius, is thought to be the principal evidence favoring triaxial intrinsic shapes (e.g. Fasano and Bonoli 1989). In a triaxial galaxy, there are three principal axes, each having a different radial scale-length. This highlights how the observed ellipticity of an E galaxy cannot be interpreted as easily as that of a disk-shaped galaxy. There is also the possibility, as noted by Nieto (1988), that some isophote twisting in E galaxies is due to an incipient bar structure in a very “early” SB0 galaxy. Tsatsi et al. (2017) detected evidence of prolate rotation in eight early-type galaxies, again supporting the idea of triaxial shapes.

Schematic illustration of examples of dusky and boxy elliptical galaxies.

Figure 1.7. Examples of disky and boxy elliptical galaxies. Generally, the disky/boxy nature of E galaxy isophotes can only be determined with detailed isophotal ellipse fits. The examples illustrated here are more obvious cases that can be distinguished in color displays of the images

Dust and gas: The presence of interstellar dust and gas in E galaxies is a likely indicator of interaction/merger history (Schweizer 1987). The orientations of planar dust lanes may be tied to the likely triaxial structure of ellipticals. This follows from the presence of minor axis dust lanes, where the dust appears along the short apparent axis of the isophotes (Bertola and Galletta 1978). In disk-shaped galaxies, the planar dust lane always appears parallel to the apparent major axis. In general, minor, major and intermediate axis dust lanes are found in E and S0 galaxies. Examples are shown in Figure 1.8. In the CVRHS system, dust-lane Es are specified as E (dust-lane).

Figure 1.8 also includes the peculiar example of NGC 4459, where a small dusty ring, here recognized as a nuclear ring, is found. Although the presence of such a feature would normally warrant a classification of SA(r)0⁺ (Buta et al. 2007), the ring is clearly a small disk embedded in a much larger elliptical galaxy.

Schematic illustration of twelve examples of dust-lane early-type galaxies, including ellipticals and S0s.

Figure 1.8. Twelve examples of dust-lane (d.l.) early-type galaxies, including ellipticals and S0s. Major axis (e.g. NGC 442, 4370 and 6314) and minor axis (e.g. NGC 810) cases are included

Luminosity profiles: de Vaucouleurs (1948) first recognized the well-known “ law” for ellipticals, and for a while it was generally thought that this characterized the luminosity distributions of all E galaxies. When the Sérsic (1968) approach was used (e.g. Huang et al. 2013 and references therein), it became clear that n = 4 only characterized the more luminous ellipticals (see review by Graham 2013). Lower luminosity ellipticals have lower values of n. Other aspects of luminosity profiles include the distinction between “core Es”, which have a flat central profile, and “power-law Es”, which have a central cusp (Kormendy and Bender 1996).

The fundamental plane of E galaxies: Elliptical galaxies are characterized by a well-defined interconnectedness of their physical parameters. In the three-dimensional space defined by the parameters Re, the effective radius that transmits half the total luminosity; Ie, the average surface brightness within this radius; and σo, the central velocity dispersion,

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