Comparison of the Magellanic Clouds by other irregular barred spirals

Abstract

I. The main structural characteristics of the irregular barred spirals of the magellanic type are illustrated in Figure 1. They were first detected 10 years ago on small-scale photographs of the Large Cloud taken at Mount Stromlo (de Vaucouleurs 1954, 1955) and were later found to be uniformly present in many other “late-type” barred spirals designated SB(s)d and SB(s)m in the revised classification system (de Vaucouleurs 1959). These stages form a continuous transition between the regular barred spirals of the “S-shaped” sequences, such as NGC 1365: SB(s)b, NGC 1300: SB(s)bc, NGC 7479: SB(s)c, and the completely irregular system IBm showing only an axial bar and little or no traces of whorls, such as NGC 4449 and 4214. Examples of the transition stages are NGC 7741: SB(s)cd, NGC 1313: SB(s)d, NGC 4027: SB(s)dm, NGC 4618: SB(rs)m, and the Large Magellanic Cloud: SB(s)m (Fig. 2). The Small Cloud, because of its interaction with the Large Cloud and its unfavourable inclination, is more difficult to classify precisely; it may be designated as SB(s)mp or IB(s)mp to indicate the presence of residual (or incipient) spiral structure (de Vaucouleurs 1957a).