This chapter concerns the smallest of all galaxies. Examined more closely, they reveal, apart from their small dimensions, further properties which distinguish them strongly from their larger siblings.

THE MORPHOLOGY OF DWARF GALAXIES

The classification of irregular galaxies often contains a separate group of especially small, dim galaxies. These are dwarf galaxies. Dwarf galaxies are differentiated according to their appearance: blue compact dwarfs (BCDs), dwarf spheroidals (dSph), dwarf ellipticals (dE), and dwarf irregulars (dIrr), which also include tidal dwarfs. The terms are not used uniformly in the literature, which is due to the fact that because of the difficulty of detecting them, studies of these galaxies began in earnest only in the last two decades. A further type is ultra-compact dwarf galaxies (UCDs), which were discovered in nearby galaxy clusters in 1999. The UCDs are more compact than other dwarf galaxies and their appearance is similar to that of bright globular clusters.

The term “d” for dwarfs was introduced to extend the “ regular” galaxies in the Hubble sequence to dwarf morphologies. Thus, a “d” was placed before the “E” or “Irr” to denote an elliptical or irregular galaxy as a dwarf. The BCDs have the highest surface brightness among dwarfs. Their prominent blue colour is due to a high star-formation rate at the centre. Much dimmer are dwarf spheroidal galaxies. These have less or even almost no concentrated gas and, along with the UCDs, are the smallest type of dwarf galaxy.

The classification of dwarf galaxies also includes so-called tidal dwarf galaxies which can form from the material of tidal tails which are common in the interaction of large galaxies. These tails extend from a few ten thousand to a few hundred thousand light years into space. As the interacting galaxies orbit each other, twisted tracks of gas, dust, and stars form which surround the pair. A local gravitational potential far enough away from the centre of the galaxy can collect and concentrate material, which can lead to a tidal dwarf galaxy forming in the tidal tail.