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Astron. Astrophys. 319, 747-756 (1997)

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2. Traditional morphologists' classification

Most of our present knowledge of galaxy morphology is based on the pioneering works of a few observers who classified thousands of galaxies. The observational material available for the classification (better material allows a more detailed scheme) and the aims of the classification work generally govern the choice of the classification scheme. The details of these schemes depend on the authors; nevertheless, some points are common to most authors.

  1. The morphological 'system is usually defined by a set of standards or prototypes' (Buta 1990), and the galaxies are classified according the resemblance to these standards (although it is not always the case; see e.g. Kormendy 1979, Kennicutt 1981 or Schombert 1986).
  2. The observational material used for classifying galaxies is very often photographic plates (or copies of them). Historical reasons and the large angular extent of plates with respect to CCDs made this choice obligatory for large samples of galaxies as well as for galaxies larger than a few arcmin.
  3. Almost all galaxies are on the same plate or on very similar plates exposed under similar observing conditions (seeing, sky level, etc.). The observational data are thus uniform, i.e. the quality of the observational material is the same for the whole sample.
  4. Structural components (disk, bar, etc.) are not measured by most morphologists (Hubble's definition of types does not require such a measurement), but only visually estimated. Up to now, it was not reasonable, in terms of computer time, to measure such structural components for large samples of galaxies: morphologists take 30 sec. to classify a galaxy (Naim et al. 1995), whereas, for example, the determination of the Hubble type by means of structural components (see Sect. 5) may reasonably take 30 min. per galaxy.
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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998