Astron. Astrophys. 355, 848-862 (2000)

2. Observations and data reduction

Multi-object spectroscopic observations of Abell 521 were carried out at the ESO 3.6m telescope in December 1995 and at the CHFT in March 1997. At ESO, we used the Faint Object Spectrograph and Camera with the grism O300, yielding a dispersion of 230 Å/mm, and the TEK512 CCD chip (m 512 x 512 pixels); at CFHT we used the Multi Object Spectrograph facilities (Le Fèvre et al. 1994) with the grating O300, resulting in a dispersion of 240 Å/mm, and the STIS2 CCD (m 2048 x 2048 pixels). These combinations of gratings and detectors result in dispersions of 6.3 Å/pixel and 5 Å/pixel, respectively. Typically, two exposures, of 2700 s each, were taken for fields across the cluster. Wavelength calibration was done using arc lamps before each exposure (Helium-Argon, and Helium-Neon lamps). Data reduction was carried out with IRAF ¹, using the MULTIRED package (Le Fèvre et al. 1995). Radial velocities were determined using the cross-correlation technique (Tonry & Davis 1981) implemented in the RVSAO package (developed at the Smithsonian Astrophysical Observatory), with radial velocities standards obtained from observations of late-type stars and previously well-studied galaxies.

We have obtained 65 spectra of objects in the region of Abell 521. Star contamination was very low (only 3 of the 65 targets turned out to be stars). From these data we have retained 49 spectra of galaxies with signal-to-noise ratios sufficiently high to derive a measurement of the radial velocity with good confidence (with the parameter R parameter of Tonry & Davis greater than 3). This results in a completeness of our velocity sample of 30% of the galaxies brighter than . The finding chart for the objects with measured velocities is shown in Fig. 1. Spectra of the brightest BCG components were obtained with EFOSC at ESO using the long-slit mode, and 3 different exposures ( sec and 1800 sec). Imaging of Abell 521 was obtained in 1994 at the CFHT with the MOS in B and R (using the LORAL3 CCD) and in 1997 in the V and I bands (using the STIS2 CCD); the definition and analysis of the galaxy catalog from the V-band image are presented in Arnaud et al. 2000, and the complete photometric catalog will be provided in Slezak et al. 2000. Also, a J-band image of the cluster core was obtained in November 1996 at the CFHT with OSIS and 1800 sec of total exposure time (see Pelló et al. 1999). Velocity measurements from our spectroscopy are listed in Table 1. The columns are as follows: column (1): Identification number of each target galaxy in the cluster as shown in the finding chart; Columns (2) and (3): Right ascension and Declination (J2000.0) of the target galaxy; Column (4): Best estimate of the radial velocity resulting from the cross-correlation technique; Column (5): estimated error; Column (6): a listing of detected emission lines. Galaxies numbered 1-21 were observed at CFHT, while those from 22-49 were obtained at ESO.

Fig. 1. Finding chart for galaxies with successful velocity measurements in Abell 521. The galaxies are labeled as in Table 1.

Table 1. Heliocentric redshifts in the 10´x10´ frame centered on Abell 521.

© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
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