Galaxies. Группа авторов

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Galaxies - Группа авторов


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are the most common types of galaxies. Encompassing a wide range of luminosities from dwarfs to giants, Magellanic spirals and irregulars are generally HI rich systems with considerable active star formation. Several examples are shown in Figure 1.20.

      The Magellanic Clouds are considered to be examples of SB(s)m galaxies, i.e. extreme late-type galaxies having clear bars but very subtle spiral structure showing a characteristic asymmetry with a single main arm and weaker features (de Vaucouleurs and Freeman 1972). This asymmetry is recognizable even in the nearly edge-on view (Buta et al. 2015). The transition stage from Sd to Sm, or Sdm, is also well defined in showing two arms with one much longer than the other. As for Scd and Sd galaxies, Sdm, Sm and Im galaxies are often barred, except for low luminosity dwarfs that may lack the mass required to show such a feature. Non-barred giant late-type galaxies like NGC 5474 show a similar asymmetry to SBm galaxies and can be classified as type SA(s)m.

Photos depict magellanic irregular 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 image law. In some cases, there is subtle structure similar to what is seen in conventional S0 galaxies, and the system is referred to as a dwarf S0, or dS0, galaxy. In either case, the object may be nucleated, and if so, the classification is dE,N or dS0,N. In contrast, dwarf spirals, or dS types, are extremely rare.

      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.

      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.

Photos depict edge-on galaxies from stage Sa to stage Im.

      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.

Photos depict embedded disks and thick or thin disks.


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