When our structural morphological scheme is reduced to the 3 coarse Hubble types and is applied to a sample of galaxies in the Coma cluster, it agrees to within 15 or 20% with other detailed traditional morphological analyses. This agreement is comparable to the one obtained among traditional morphologists. This shows that our criteria for classifying galaxies lead to the same results as the standard ones, and that our method is acceptable for classifying galaxies.
Most disagreements occur for galaxies difficult to classify because they are faint (), because images of higher resolution are required, and/or because of a disagreement on the significance of structural parameters in borderline cases between S0 and S.
At least half of the disagreements arise in spiral galaxies whose spiral arms (or spiral pattern) have not been detected or taken into account by previous analyses, even if S0/a galaxies are spirals by definition (e.g. BO, RC3 p. 15). Therefore traditional morphologists underestimate the spiral fraction in nearby clusters by 7 to 10 %. Even though this does not seem to be a large quantity, it represents an error of 100 % in the relative number of spirals in clusters, because of their scarcity with respect to early-type galaxies. This helps to resolve apparent differences between the number of spirals in nearby and distant clusters, for which various explanations and theories have been proposed in the recent past. At any rate, this underscores the need for higher resolution images as one classifies fainter and/or smaller galaxies.
The method we adopt uses quantitative criteria (the presence of structural properties) to classify galaxies, our types are thus reproducible and stable to a higher degree than the ones of morphologists. We also use better observational material for galaxies difficult to classify. The first results of analyses based on this method, summarized in Sect. 6.4, are promising. For all these reasons, our classification method should be preferred for classifying galaxies in clusters, even if it is more time and telescope consuming than the others.
© European Southern Observatory (ESO) 1997
Online publication: July 3, 1998