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

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2. Data

In the present analysis we use the galaxy spectra obtained with the OPTOPUS instrument at the ESO 3.6m telescope at La Silla, Chile in the context of the ENACS. For a detailed description of the characteristics of the survey, we refer to Katgert et al. (1996, 1998). A brief summary of those aspects of the observations that are relevant to the analysis in this paper may be useful, however.

The observations were done between September 1989 and October 1993. The observed galaxies all lie in the direction of rich Abell clusters. The redshifts of these clusters are mostly [FORMULA] and most clusters lie around the South Galactic Pole in the solid angle defined by [FORMULA] and [FORMULA]. The galaxies were selected either on film copies of the SERC IIIa-J survey or on glass copies of the first Palomar Sky Survey. Areas of between 1 and 4 square degrees centered on the target clusters were scanned with the Leiden Astroscan plate-measuring machine. The magnitude limits are between 16.5 and 17.5 in the R-band (see Katgert et al. 1998 for details). These limits correspond to absolute magnitudes of -19.8 and -18.8 at the median redshift [FORMULA] of the survey, assuming a Hubble parameter of 100 km s-1 Mpc-1.

The OPTOPUS system used fibres with a diameter of 2.3 arcseconds, which corresponds to a linear scale of 2.1 [FORMULA] kpc at a redshift of [FORMULA]. In 6 of the 9 observing runs the same spectrograph setup was used. In general, the wavelength range was from [FORMULA] Å to [FORMULA] Å, but it varies slightly between runs. The spectral resolution is almost always 130 Å/mm, or about 5 Å, except in the run of September 1989, which has a lower resolution. Due to different pixel sizes of the CCD detectors used, the spectra were sampled at either 1.9 or 3.5 Å/pixel.

In the wavelength range covered by the observations, and for the redshifts of the clusters observed, the principal emission lines that were observable are [OII] (3727 Å), H[FORMULA] (4860 Å) and the [OIII] doublet (4959 and 5007 Å). Possible emission lines were identified independently by two persons: in the 2-D frames and in the 1-D extracted spectra (see Paper III for details). In the spectra of about 1200 galaxies one or more emission lines were detected. For 554 1 galaxies in the 10 ENACS clusters in common with the sample of Dressler (1980) the morphology is available. Of the 71 ELG which have a morphological classification in D80, 61 are spirals or irregulars (86%), 8 are S0's (11%) and 2 are ellipticals (3%). On the other hand, of the 181 spirals that the D80 set has in common with ENACS, 61 show emission lines. So, while 6 out of 7 ELG are spirals/irregulars, only 1/3 of the spirals are ELG. A small fraction (about 7% according to Biviano et al. 1997) of the ELG are active galactic nuclei, as is evident from the line widths.

The spectra that were obtained in September 1992 often exhibit peculiarities, such as deviant pixels at beginning and end of the spectrum. These probably were introduced in the reduction process and have not influenced the redshift determination. We have not reduced these spectra again for the present analysis, but we have excluded them from the analysis, as they could produce below-average classification results (see Sect. 4.2).

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© European Southern Observatory (ESO) 1999

Online publication: December 4, 1998