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.
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.
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.
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 “
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, an important characteristic emerges. The correlations between these three parameters of elliptical galaxies define what has come to be known as the “fundamental plane” (Kormendy and Djorgovski 1989). The plane tells us that larger E galaxies tend to have lower average surface brightness than smaller E galaxies, and that more luminous E galaxies have higher central velocity dispersion than do lower luminosity E galaxies.
1.4. Spiral galaxies
The spiral structure of galaxies was discovered more than 170 years ago. The subtle patterns were first detected in 1845 with the world’s largest telescope at the time, the “Leviathan of Parsonstown” located in central Ireland. William Parsons, the Third Earl of Rosse, visually saw the spiral arms of the “Whirlpool Galaxy” M51 with his newly built 72-inch speculum metal reflector. In the parlance of 19th Century astronomy, M51 was called a “nebula”, not a galaxy, although the general view at the time was that most or all nebulae were distant systems of stars like the Milky Way (“Island Universe” hypothesis). Parsons built the Leviathan partly to test this idea. The discovery of spiral structure added mystique to the nebulae, and led to alternative ideas as to what the nebulae actually were. It would be nearly a century after Parsons’ discovery that any serious understanding of the nature of spiral structure
