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Astron. Astrophys. 341, 371-384 (1999)

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6. Summary and conclusions

We studied the spatial and kinematical properties of early- and late-type galaxies in a subset of the rich Abell clusters observed in the ENACS. We compared these properties for galaxies with emission lines (ELG) and without (non-ELG).

As for only about 10% of the galaxies in the ENACS the morphological type was known from imaging, we applied a Principal Component Analysis (PCA) in combination with an Artificial Neural Network (ANN), to try and classify all galaxies in the ENACS on the basis of their spectrum. The PCA is an important first step in the classification as it compresses essentially all significant information of a spectrum into a limited number of Principal Components (PCs).

These PCs, which are linear combinations of the original spectral fluxes, were subsequently used in an ANN. The ANN was trained to classify spectra using a subset of 150 galaxies for which the morphological type is known (D80). Another 120 galaxies for which the morphology is available from D80, the so-called test set, were used to determine the success rate of the classification algorithm.

Classifying galaxies into three classes (E, S0 and S+I), the ANN yielded the correct galaxy morphology of D80 for [FORMULA]% of the galaxies in the test set. The success rate increased to [FORMULA]% when the galaxies were classified into only two classes, early- (E+S0) and late-type (S+I) galaxies. Furthermore, [FORMULA]% of the galaxies with emission lines in their spectrum (ELG) was classified as late-type. This fraction is larger than the [FORMULA]% that one expects from the individual success rates for each morphological type separately. Apparently, the success rate for the ELG is larger than for the entire set of galaxies.

We discussed several factors that may produce misclassifications. First, one does not always know the true galaxy type. Using galaxies which Dressler classified twice, we estimate that at least between 5 and 10% of the classifications based on imaging are incorrect. Secondly, even within one morphological type the spectra may be substantially different. Thirdly, S0 galaxies may be hard to separate from ellipticals from their spectrum alone, and we find that a rather large number of E's is classified as S0 and vice versa, whereas the number of misclassifications between S0 and S+I is much smaller. Finally, spiral galaxies with a large bulge may have a spectrum that leads to a bona-fide early-type classification with PCA and ANN.

We investigated how galaxies of different type are distributed in the plane defined by the two most significant PCA components. There appears to be a distinction between E, S0 and S+I galaxies in this plane, although it is not very large. On the contrary, the ELG and non-ELG have clearly different distributions, which shows that the PCs contain significant information about the morphological type of a galaxy.

Finally, we extended the analysis of Biviano et al. (in Paper III), who studied the differences in the spatial and kinematical properties of ELG and non-ELG, to galaxies of different morphology. We conclude that the presence of emission lines, rather than the galaxy morphology, is the basic property that is correlated with the spatial and kinematical properties of a galaxy. Thus, the correlation between morphology on the one hand and spatial and kinematical properties on the other hand seems to result mainly, if not exclusively, from the presence of emission lines.

The line-of-sight velocity dispersion with respect to the average cluster velocity is larger for the ELG than it is for the non-ELG, and. A similar, but smaller, difference is found between late- and early-type galaxies. This supports the idea that the ELG are on fairly radial, and possibly `first approach' orbits towards their cluster centres, while their line-emitting gas has not yet been stripped. In addition, the late-type galaxies without emission lines (i.e. with little gas) appear to have spatial and kinematical properties that more resemble those of the early-type galaxies than those of the late-type galaxies with emission lines. Apparently, if a late-type galaxy has passed through the cluster centre, most of its gas will have been stripped and the galaxy will not show emission lines anymore.

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© European Southern Observatory (ESO) 1999

Online publication: December 4, 1998