Читать книгу Galaxies - Группа авторов - Страница 18

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 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.

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.