Some Musings on Galaxy Classification

Abstract

The present paper presents a discussion of classification systems for galaxies, with special emphasis on possible modifications of the Hubble "tuning fork" diagram, and on galaxy types not included in Hubble's original scheme. Hubble's morphological types were defined in terms of standards observed at small look-back times that were mostly located in the field, or in poor clusters. It is pointed out that it is often difficult to shoehorn galaxies located in the cores of rich clusters, or objects viewed at large look-back times, into the Hubble classification system. The evolutionary relationships between E, S0 and dSph galaxies are presently still controversial and poorly understood. It is suggested that S0 galaxies may have arrived at their present morphology along various evolutionary tracks. Late-type barred spirals are found to be systematically less luminous than normal late-type galaxies. This suggests that the dichotomy between normal and barred spirals may reflect significant differences in their evolutionary histories. Such differences might be explored by searching for systematic differences between the [O/Fe] values in normal and barred spirals. Finally it is pointed out that the Large Magellanic Cloud may have been a low surface brightness galaxy for an ~8 Gyr period that ended 3-5 Gyr ago. This suggests that some galaxies can jump from one morphological classification type to another.