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Astron. Astrophys. 340, 381-383 (1998)

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2. Presentation of the data

The new DLAS candidate has been identified thanks to the FOCA balloon-borne survey at 200 nm (see Milliard, Donas & Laget, 1991). This 40 cm telescope has been routinely flown over years and has revealed a large set of UV sources with monochromatic magnitudes [FORMULA]. Most of them ([FORMULA]) are extragalactic, as recently confirmed by a spectroscopic follow-up (Milliard et al. 1998), performed in the optical domain toward a high galactic latitude ([FORMULA]) field of 0.6 square degree including the extragalactic cluster Abell 2111 (A2111).

Optical spectroscopy of the FOCA's UV-detected candidates around A2111 has been carried out with the NORRIS multifiber spectrograph (Hamilton et al., 1993) mounted at the Cassegrain f/16 focus of the Hale 5m telescope at Mount Palomar. A detailed description of the observations from 1995 through 1997, their calibration and methods of reduction will be given in a forthcoming paper (Milliard et al. 1998), but for the sake of clarity of the present paper let us just mention that the wavelength domain was [FORMULA] Å with a resolution varying from [FORMULA] Å FWHM over the full CCD field, and normally two exposures of 50-60 min were taken on the sky, each one being bracketed by 2 FeAr arc exposures for wavelength calibration, while flat-fielding was achieved using dome-flats.

Special attention was payed to the sky background subtraction, so critical in the infrared, and a relative flux calibration was provided by the very hot sdO standard BD[FORMULA], while the continuous and selective atmospheric absorptions have been approximately corrected for, with satisfactory results in terms of spectral flux distribution over the full wavelength domain in most cases, and in particular for D089/898 as can be checked on Fig. 1.

[FIGURE] Fig. 1. Optical spectrum of QSO D089/898. The absolute flux calibration is based on the POSS2 B[FORMULA] magnitude value. The absorption lines listed in Table 1 are labelled.

Of the 113 field extragalactic objects spectroscopically observed around A2111, 8 are QSOs (previously unidentified) with redshifts in the range [FORMULA]. One of them is FOCA's id. number D089/898, a [FORMULA] magnitude object that shows a strong, unresolved emission of the MgII 2796-2803 Å doublet at [FORMULA] (together with several weaker emission lines, see Fig. 1), but also partially resolved absorptions by the MgII doublet at a lower redshift of [FORMULA], and with rest equivalent widths [FORMULA] Å.

Notice that the only deep optical image presently available for this field is a B-band POSS2 image. With a limiting magnitude of 22, it is not deep enough neither for identifying the absorbing galaxy nor for constraining the brigthness.

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© European Southern Observatory (ESO) 1998

Online publication: November 9, 1998
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