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

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3. Characteristics of the system

The absorption lines detected in the quasar spectrum are presented in Table 1. Besides the strong MgII doublet, several lines of FeII are the only other conspicuous features at our spectral resolution and [FORMULA] ratio. Upper limits on the strength of the MgI 2852 Å (0.6 Å) and CaII 3934 Å (0.3 Å) lines have also been estimated.


[TABLE]

Table 1. Absorption lines detected
Notes:
[FORMULA] All [FORMULA] values measured at positions calculated using the mean redshift of 0.80787 adopted for the MgII doublet
[FORMULA] in unknown blend or anomalous noise trough


The strengths of the Mg II and Fe II lines strongly suggest the system is a DLAS. Photoionization models (Bergeron & Stasiska 1986), show that the presence of Fe II lines is a good indicator of high neutral hydrogen column densities, and that whenever the Fe II /Mg II ratio (measured by [FORMULA]) is greater than about unity, the absorber is very likely a strong DLAS ([FORMULA] cm-2), where a Fe II /Mg II ratio greater than about 0.6 indicates a neutral hydrogen column density above [FORMULA] cm-2 (Bergeron, private communication). The criterion has been successfully used by Wampler et al. (1993), and by Le Brun et al. (1997) to study the lower redshift DLAS, for which the Ly[FORMULA] line was in the UV range, and thus unobserved. In the [FORMULA] absorber present in the spectrum of D089/898, we measure [FORMULA], a value comparable to the lower limit inferred by Bergeron & Stasiska (1986) for the DLAS.

We have also listed in Table 2 the rest equivalent widths and the Fe II /Mg II ratio of the already known intermediate redshift DLAS, together with the candidate DLAS recently detected at [FORMULA] in the spectrum of PKS 0118-272 by Vladilo et al. (1997). As can be seen, the system discovered in the spectrum of D089/898 has line rest equivalent widths and a Fe II /Mg II ratio within the range of the other systems. All the confirmed systems have [FORMULA] cm-2, and the candidate DLAS in the spectrum of PKS 0118-272 displays Ca II and Ti II lines, which are only detected in DLAS. Taken all together, these facts indicate that the system detected in our UV-selected QSO spectrum is very likely a DLAS.


[TABLE]

Table 2. Mg II and Fe II absorption lines in low redshift systems
Notes:
[FORMULA] Fe II /Mg II ratio estimated from the Fe II 2600 line


Because of the small size of the FOCA/NORRIS QSO sample, it is legitimate to address the questions of the probability to find such DLAS candidates, and of favorable effects possibly at play in our selection process. As a comparison, the HST/FOS Key Project did discover only one system among 35 quasars. But half of them have redshifts below 0.7 (of which 6 have [FORMULA]), and thus the HST redshift path is very short. In contrast, the actual FOCA sample of QSOs around A2111 contains only 8 objects but with redshifts in the range [FORMULA]. Given that i) the NORRIS spectroscopic observations allow to observe Mg II absorption lines with [FORMULA], yielding a cumulative redshift path in the FOCA sample of 5.45; and ii) the average number of DLAS at [FORMULA] being [FORMULA] (Lanzetta et al. 1995), one expects to detect [FORMULA] DLAS, a value compatible with our detection of one system. Indeed, it must be emphasized that compared to the HST/FOS Key Project, we are probing higher redshifts that give access to larger z paths in regions of higher DLAS density.

More generally, ground-based spectroscopic follow-up of VUV imaging surveys is an efficient way to discover DLAS candidates with measurable Ly[FORMULA] lines up to [FORMULA], since the detection in a VUV bandpass picks up with a high probability QSOs that are bright enough in the region of the Ly[FORMULA] absorption line, including the scarce high redshift ones that are unaffected by the intergalactic Lyman continuum opacity (Moller & Jakobsen 1990). In this context, the GALEX project (planned for launch in 2001 as a NASA Small Mission Explorer) will provide a VUV survey quite suited to such a program in its imaging mode. In addition, with a spectroscopic mode that can by itself discover [FORMULA] DLAS from their Ly[FORMULA] line directly measured in the UV data, GALEX is expected to increase the present sample of known DLAS by an order of magnitude.

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© European Southern Observatory (ESO) 1998

Online publication: November 9, 1998
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