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Astron. Astrophys. 331, 493-505 (1998)

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2. The data sample

The current work is based on a subsample of 29 clusters from the catalogue of Abell et al. (1989, ACO hereafter) with a redshift less than 0.1, for which at least 10 ENACS galaxy redshifts are available in the whole area. The typical redshift for these clusters is 0.07. We have selected those clusters for which galaxy [FORMULA] magnitudes and positions were available to us from the Cosmos catalogue, and we have limited the sample to clusters with very regular contours of projected galaxy density. In this way, we expect to reduce the scatter in the FP, due to the noise in the structural parameter measurements of clusters. In Fig. 1 we show surface density plots for two clusters (A0119 and A3111) which are representative for the total sample. For the other 27 clusters, contour plots will be given in Paper VII.

[FIGURE] Fig. 1. Adaptive-Kernel maps of projected galaxy density for A0119 and A3111, from the Cosmos catalogue. Coordinates are in arcsec with respect to the geometric cluster center.

We used the Cosmos data to calculate the integrated luminosity of the cluster galaxies in a given selected area, as well as to determine the characteristic scale of the cluster galaxy distribution. Note that the clusters A2734, A2764, A2799, A2800, A2911, A2923 and A3122 are in the EDSGC area around the Southern Galactic Pole, while the other 22 clusters are outside the EDSGC area but in the general Cosmos area. We assumed the entire Cosmos catalogue to have the same magnitude limit as the EDSGC subset, which is nearly complete at [FORMULA] = 20.

The ENACS dataset was used to obtain an estimate of the degree of background (or foreground) field galaxy contamination in the cluster area. The ENACS data also allow us to determine the cluster velocity dispersions. However, we cannot calculate the integrated luminosity from those data. Spectroscopy was attempted for galaxy samples with well-defined completeness limits in magnitude. However, as the spectroscopy has not yielded redshifts for all galaxies that were observed, the galaxies with redshifts do not define a truly magnitude-limited sample. In Paper V we illustrate this by comparing the differential [FORMULA] magnitude distribution of the Cosmos galaxies to the differential [FORMULA] magnitude distribution of the ENACS galaxies, for each cluster separately. Since the two catalogues may cover different regions of each cluster (typically, the ENACS samples are restricted to the inner regions and rarely extend beyond 1.5 h-1 Mpc ), we have selected only the ENACS and Cosmos galaxies in the intersection of both surveys.

Absolute magnitudes are derived from apparent magnitudes using the cluster distance estimated from the mean cluster velocities, using a Hubble constant [FORMULA] = 100 km s [FORMULA]  Mpc-1 and a deceleration parameter [FORMULA]. We scaled the [FORMULA] magnitudes to the [FORMULA] system by adopting a constant galaxy colour, [FORMULA] - [FORMULA] (see Paper V). The Cosmos and ENACS magnitude distributions are quite similar down to a given limiting magnitude which corresponds to the (a priori unknown) magnitude completeness limit of the ENACS samples, which varies somewhat between clusters. If the Cosmos catalogue is complete to [FORMULA] = 20, the ENACS catalogue is complete down to an absolute [FORMULA] magnitude of about -20.

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© European Southern Observatory (ESO) 1998

Online publication: February 16, 1998