What do we really know about Spiral Galaxy Morphology

Beckman, J. E.; Irwin, P.; Pohlen, M.
Bibliographical reference

Lecture Notes and Essays in Astrophysics, vol. 2, p. 199-214.

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stronomers have long learned to classify spirals in terms of their morphological features, primarily their bulges, discs, and bars. In addition other properties of discs are used in classification, notably spiral arms, warps and truncations. In this paper we show how with modern techniques parts of the basis of these standard classifications are shown to be unsatisfactory, even misleading. In particular we will look carefully at bulges and at truncations, and explain how on purely morphological grounds we need to make basic changes in our concepts. We will put in question the traditional view of the spheroidal bulge and will also show that the majority of galaxy discs are not in fact truncated.