It is difficult to determine the original galaxy forms of the systems discussed in this chapter. Compared to other galaxy types, they exhibit a much larger range of possible forms.

THE MORPHOLOGY OF IRREGULAR GALAXIES AND INTERACTING SYSTEMS

In this chapter, irregular galaxies and interacting galaxy systems are considered together. Based on their morphology, which in both cases is characterized by asymmetry and irregularities, the two cases are usually distinguished in astronomy. The examples discussed here, however, demonstrate that one often cannot sharply separate irregular and interacting galaxies. In fact, one often has to take into account combinations of events and interdependencies in the astrophysical interpretation of the visible forms.

About three to four per cent of galaxies cannot be classified as elliptical or spiral since they are lacking the basic structures in their appearance. For example, often a plane of symmetry or clearly defined centre is lacking, which sometimes leads to the observation of collections of large-scale star-forming regions, loose H II regions and individual dust filaments. Their masses are in the range 108–1010 solar masses and at a few thousand to 30 000 light years, their diameters are very small. The term “irregular galaxy” should not be confused with “peculiar”. The latter is used as a qualifier to the standard galaxy types, for example to indicate that a spiral structure exists but is perturbed, but still allows the original classification to be recognized. In the case of irregular galaxies, however, there is no sharp dividing line between perturbation and typical structure, especially since the irregular forms can also be interpreted with respect to the diverseness of their histories.

Interactions with other galaxies are mainly responsible for the appearance of irregular galaxies. There is a broad spectrum of possible interactions. These range from weak perturbations in the gravitational field of two approaching galaxies to stronger tidal interactions which can cause a flow of material to a collision of galaxies which can even end in a merger and produce a new galaxy.