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

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1. Introduction

Recently, the kinematics of the different types of galaxies in clusters, as well as their spatial distribution, has received some new attention, e.g. from Colless & Dunn (1996), Carlberg et al. (1996) & Mohr et al. (1996). Generally speaking, the late-type galaxies are found to avoid the central regions of their clusters, while their line-of-sight velocity dispersion with respect to the average velocity of the cluster, [FORMULA], appears to be higher than that of the early-type galaxies.

The ESO Nearby Abell Cluster Survey (ENACS, Katgert et al. 1996, 1998) has yielded redshifts for more than 5600 galaxies in the directions of about 100 rich clusters of galaxies, mostly in a cone around the South Galactic Pole. Using the ENACS, Biviano et al. (1997, hereafter Paper III) compared the spatial distribution and kinematics of the galaxies with emission lines (hereafter: ELG) with those of the galaxies without emission lines (hereafter: non-ELG). From a subsample of 545 galaxies for which a morphological type is known, Biviano et al. concluded that the ELG are almost exclusively late-type galaxies, i.e., spirals (and irregulars), while the non-ELG are a mix of early- and late-type galaxies. They found that [FORMULA] of the ELG is, on average, 20% larger than [FORMULA] of the non-ELG, while the spatial distribution of the ELG is significantly less peaked towards the cluster centre than it is for the non-ELG. These two facts, in combination with details of the kinematics, were interpreted as evidence for a picture in which the ELG are mostly on fairly radial, `first approach' orbits towards the central regions of their clusters, and thus not in full equilibrium with the population of non-ELG.

Biviano et al. estimated that the large majority of the ELG are spirals; however, the ELG appear to represent only about one-third of the total spiral population. Therefore, it was not clear whether the conclusion about the difference in kinematics of ELG's applies only to spirals with emission lines, or whether it applies to all spirals. If the spirals among the non-ELG would have identical kinematics and spatial distribution as the spirals with emission lines, the real differences between early- and late-type galaxies would be larger than the apparent differences. That would bring the result of Biviano et al. more in line with that of Colless & Dunn (1996) who found that the velocity dispersion of the late-type galaxies in the main concentration of the Coma cluster is very close to [FORMULA] times that of the early-type galaxies.

On the other hand, it is conceivable that only the spirals with emission lines are on `first approach' orbits, which would be consistent with the presence of sufficient amounts of line-emitting gas. The spirals without emission lines might then have traversed the central regions of their clusters and have lost most of their line-emitting gas in the process.

Recently, Ramírez & de Souza (1998) concluded that the orbits of elliptical galaxies in clusters are close to radial, while spirals have more circular shaped, or isotropic, orbits. Their conclusion is based on an analysis of the distribution of line-of-sight velocities.

In order to be able to investigate the kinematics of the spirals without emission lines, as well as to elucidate the cause for the apparent disagreement between the results of Ramírez & de Souza and that of Biviano et al., we need morphological types for the (non-ELG) galaxies in the ENACS. The obvious way to get these is through imaging. With well over 4000 galaxies without emission lines, that represents a major observational effort, on which we have embarked, but which will take some time to finish.

In this paper, we adopt a different approach, by using the ENACS spectra. The morphological types are estimated from the spectra with a Principal Component Analysis, in combination with an Artificial Neural Network. The network is `trained' with a subset of the ENACS galaxies for which a morphological type is available from imaging (Dressler, 1980, hereafter D80) and it is `tested' with the remaining galaxies with morphology from Dressler. With the morphological types estimated from the spectra, we investigate the kinematics and spatial distribution of, in particular, the late-type galaxies with and without emission lines.

The paper is organized as follows: in Sect. 2 the ENACS data is summarized. In Sect. 3 we describe the algorithm that we used to estimate the morphological type of a galaxy from its spectrum, by applying a PCA and an ANN. In Sect. 4 we discuss the results of the combined PCA/ANN and present the success rates achieved in assigning morphological types. In Sect. 5 we present an analysis of the spatial and kinematical differences between the (subsets of) early- and late-type galaxies. In Sect. 6 we summarize the results.

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© European Southern Observatory (ESO) 1999

Online publication: December 4, 1998
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