Elliptical Galaxy - Sizes and Shapes

Sizes and Shapes

Elliptical galaxies vary greatly in both size and mass, from as little as a tenth of a kiloparsec to over 100 kiloparsecs, and from 107 to nearly 1013 solar masses. This range is much broader for this galaxy type than for any other. The smallest, the Dwarf elliptical galaxies, may be no larger than a typical globular cluster, but contain a considerable amount of dark matter not present in clusters. Most of these small galaxies may not be related to other ellipticals.

The Hubble classification of elliptical galaxies contains an integer that describes how elongated the galaxy image is. The classification is determined by the ratio of the major (a) to the minor (b) axes of the galaxy's isophotes:

Thus for a spherical galaxy with a equal to b, the number is 0, and the Hubble type is E0. The limit is about E7, which is believed to be due to a bending instability that causes flatter galaxies to puff up. The most common shape is close to E3. Hubble recognized that his shape classification depends both on the intrinsic shape of the galaxy, as well as the angle with which the galaxy is observed. Hence, some galaxies with Hubble type E0 are actually elongated.

There are two physical types of ellipticals; the "boxy" giant ellipticals, whose shapes result from random motion which is greater in some directions than in others (anisotropic random motion), and the "disky" normal and low luminosity ellipticals, which have nearly isotropic random velocities but are flattened due to rotation.

Dwarf elliptical galaxies have properties that are intermediate between those of regular elliptical galaxies and globular clusters. Dwarf spheroidal galaxies appear to be a distinct class: their properties are more similar to those of irregulars and late spiral-type galaxies.

At the large end of the elliptical spectrum, there is further division, beyond Hubble classification. Beyond gE giant ellipticals, lies D-galaxies and cD-galaxies. These are similar to their smaller brethren, but more diffuse, with larger haloes. Some even appear more akin to lenticular galaxies.