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1. Observations and data reduction

1. Description of the observations

The observations were performed with the ESO 3.6 m telescope equiped with MEFOS during 6 nights on November 5-11, 1994 and 2 nights on November 24-26, 1995. A description of the instrument can be found in Durret et al. (1998). The grating used with the Boller & Chivens spectrograph had 300 grooves/mm, giving a dispersion of 224 Å/mm in the wavelength region 3820-6100 Å. The detector was CCD #32, with 5122 pixels of $27 \times 27\ \mu$m.

The catalogue of galaxy positions used in this survey was obtained with the MAMA measuring machine and is presented in a companion paper (Slezak et al. 1999). This catalogue gives very accurate positions, but magnitudes in the photographic $b_{\rm J}$ band are known with an accuracy of 0.1 magnitude at best. CCD photometry of the central regions of the cluster in the V and R bands was later performed to recalibrate these $b_{\rm J}$ magnitudes and to obtain R magnitudes for the entire photometric sample. We observed spectroscopically a total of 15 fields, with exposure times of $2\times 30$ minutes, and we obtained 473 spectra in total (plus the same number of sky spectra). The limiting magnitude for the galaxies observed spectroscopically was $b_{\rm J}$ = 19, corresponding to a recalibrated magnitude $R\sim$18.7 (see Sect. 3).

{\psfig{,height=7cm} }\protect\end{figure} Figure 1: Spatial distribution of the 466 galaxies with redshifts in the direction of Abell 496 (map limited to $\pm $6000 arcsec from the cluster center)

Out of the 473 spectra obtained, we measured 410 reliable redshifts (the other ones were discarded due to insufficient signal to noise ratio). Since Abell 496 is at a relatively low galactic latitude, the effects of stellar contamination of our photographic plate catalogue are large. Our star-galaxy separation software was imperfect since out of the 410 objects for which redshifts were measured only 305 were galaxies. Such a high degree of contamination of the catalogue is at least partly due to the fact that we preferred to have a galaxy catalogue as complete as possible, in order not to "miss'' galaxies. Our catalogue includes these 305 galaxy spectra, plus those previously published by Proust et al. (1987), Quintana & Ramírez (1990) and Malumuth et al. (1992), and some galaxies from the CfA redshift catalogue (Huchra et al. 1992), reaching a total of 466 galaxy redshifts after eliminating objects observed twice.

The positions of the objects for which we gathered reliable spectra (either from our observations or from the literature) are shown in Fig. 1. These positions are relative to the cluster center taken to be the position of the maximum X-ray emission (Pislar 1998): $\alpha_{2000.0} = 04^{\rm h}33^{{\rm m}}37.9^{\rm s}, \delta_{2000.0} = -13^\circ
15'47''$. This center is within 7 arcsec of the position of the cD galaxy, a distance which is smaller than the ROSAT PSPC pixel size, and we will therefore consider hereafter that both positions coincide.

2. Data reduction

The spectra were reduced using the IRAF software. The frames were bias corrected in the usual way. Velocities were measured by cross-correlating the observed spectra with different templates: a spectrum of M31 (kindly provided by J. Perea) at a velocity of -300 km s-1, and stellar spectra of the standard stars HD 24331 and HD 48381, which were each observed every night during our 1994 run. The cross-correlation technique is that described by Tonry & Davis (1979) and implemented in the XCSAO task of the RVSAO package in IRAF (Kurtz et al. 1991). The errors on the velocities derived from absorption lines are given automatically by this task.

The positions of emission lines, when present, were measured by fitting each line with a Gaussian.

All the redshifts were measured by the same person (F.D.) in a homogeneous way. Redshifts of insufficient quality were discarded (i.e. those with a Tonry & Davis parameter smaller than 2.0, except for three galaxies with respective Tonry & Davis parameters of 1.5, 1.6 and 1.9 where the absorption lines seemed to be well enough defined for these redshifts to be kept in the final catalogue).

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