Published online by Cambridge University Press: 05 September 2017
Elliptical galaxies cover a wide range in size. Thus some of the largest galaxies belong to the galaxy family discussed in this chapter.
THE CLASSIFICATION OF ELLIPTICAL GALAXIES
In the last few years, amateur astrophotography has become more professional. This allows interesting features to be seen even in the apparently uninteresting elliptical galaxies, making visible the results of current research. The high efficiency of sensitive CCDs, their large dynamic range and above all the possibilities of electronic image processing have played an enormous role in making these popular subjects for many photographers.
The classification of elliptical galaxies from E0 to E7 in the Hubble diagram describes their form, from the circular E0 to strongly flattened E7 types. The numbers from 0 to 7 following the E are computed according to the formula 10 × (a − b)/a, where a is the semimajor and b the semiminor axis of an ellipse. This allows one to quantify the ellipticity of galaxies. An ellipse also has two foci whose distance from the centre is known as the linear eccentricity, e. In the case of circular E0 types, the two foci coincide at the centre and the eccentricity is thus zero. With increasing distance of the foci and thus increasing flattening, the value of the linear eccentricity increases. The classification E0 to E7, however, has no physical background, since depending on the orientation the general triaxial form of a spindle-shaped elliptical galaxy could appear as a round E0 type. The almost circular E2 and E3 types are the most common in surveys.
Dwarf elliptical galaxies with low surface brightness are classified as dE. Some of the dwarf satellite galaxies of the Milky Way are classified as dwarf spheroidal, dSph, which are even dimmer than the dE. The Local Group containing the Milky Way also has irregular satellites. Relatively new are blue compact dwarfs, which are usually classified as irregular dwarf galaxies. At 15–20 per cent, their gas fraction is relatively large and explains their conspicuously large starformation rates.