Astron. Astrophys. 346, L21-L24 (1999)

2. Data

2.1. Target selection, observation and data reduction

A 30mn equivalent I-band image centered on J2233-606 was obtained in the ESO Director's discretionary time with the EMMI-NTT red-imaging channel and made available to us for target selection. Photometric calibration has been done with standard stars from Landolt (1992) and the accuracy of the zero point is better than 0.1 magnitude. Source extraction and star-galaxy separation were performed with Sextractor (Bertin & Arnouts, 1996) resulting in a catalogue of 1159 objects with complete at the 90% level. The US Naval Observatory catalogue was used as reference for the astrometry. This gives an accuracy close to 1" for absolute positions and about three times better for relative positions within the field. Multi-slit spectroscopy has been carried out with EMMI where about thirty objects can be observed simultaneously. We used slits of 1.02 or 1.34" in width, leading to a spectral resolution of FWHM  10.6 or 13.9 Å. The spectral range lies within 3900-10000 Å but its actual length depends on the location of the object within the mask. Reduction was done with standard techniques using an updated version of Multired under the IRAF reduction package. The residuals of the wavelength calibration fits have an r.m.s. of less than 0.7 Å, but the positioning of the objects relative to the slit was accurate to 0.3", leading to wavelength uncertainties of order 3 Å. The accuracy in measuring the wavelengths of lines in the galaxy spectra is close to one tenth of the resolution element, i.e. 1. to 1.4 Å. Thus the redshift accuracy is mainly limited by the position of the galaxies within the slits, and our redshifts are accurate to about 0.001. Spectra were flux calibrated using standard stars from Stone & Baldwin (1984) with no attempt to correct here for aperture losses. Full details of observations of the complete sample, data reductions and measured parameters are in Dennefeld et al. (1999, in preparation).

2.2. Objects around J2233-606

Galaxies for which we have performed spectroscopy are shown by a filled circle in Fig. 1. Those galaxies lying within 1´ of J2233-606 are listed in Table 1 two of which are of particular interest: Q433 (J2233415-603255) is another QSO, and G486 (J2233378-603324) is a spiral galaxy at only 4.7" from J2233-606. Their spectra are displayed in Fig. 2. The redshift of Q433, , is determined from the broad Mg II 2799 and C III] 1909 emission lines. Also seen are the broad Fe II 2400, 2600 complex, and the as yet unidentified broad feature around 2100 Å (see for instance Francis et al. 1991). The observed (V-I) spectral index is 0.5; using the corresponding spectral energy distribution gives . G486 is at ([O II] , [O III] , CaK) and has a late-type spectrum consistent with its Sc morphology and the presence of numerous H II regions in the HST image. The observed (V-I) spectral index is 1.5, giving (i.e. ).

Fig. 1. Objects at around J2233-606 (diamond symbol) with the running number of our photometric catalogue. North is at the top and East to the left. Objects with a compactness of more than 0.95, i.e. likely to be stars, are shown with crosses. For the remaining objects, circles represent the 16 objects at of which 11 spectra have been obtained (filled circles). Squares represent fainter objects.

Fig. 2. Spectrum of Q433 (top) and G486 (bottom).

Table 1. Galaxies within a 1´ radius of J2233-606 with the running number of our photometric catalogue (ID), the coordinates (, ), the I-band apparent magnitude (I), the heliocentric redshift (z), the angular separation () to J2233-606, and the impact parameter ().

© European Southern Observatory (ESO) 1999

Online publication: May 21, 1999
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