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Astron. Astrophys. 333, 841-863 (1998)

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2. Observations and data reduction

2.1. The sample

Since no confirmed DLAS had been discovered by the HST at the time when our study was undertaken, we had to rely on the presence of 21 cm absorption or a high [FORMULA] ratio to select our targets (see Paper I for details). Three quasars in our sample satisfy the first criterion: 3C 196, PKS 1229-021 and 3C 286. It is to be noted that the presence of 21 cm absorption does not necessarily guarantee that the Ly [FORMULA] line is damped because part of the radio flux originates from extended components such as jets or lobes (3C 196 and PKS 1229-021) and the H I distribution in the absorber may exhibit spatial structure at scales comparable to the extent of these features. The other three objects, EX 0302-223, PKS 0454+039 and Q 1209+107, have a system with a high [FORMULA] ratio.

We have retrieved from the HST database the G270H spectrum of EX 0302-223 which has been observed by another team (program 6224, dataset Y2SH0103T). The main characteristics of the QSOs and absorption systems of interest, as well as the log of the HST observations, are given in Table 1 (the absolute magnitudes of the candidate absorbers listed in Paper I are affected by an error; corrected values are given in Table 1).


[TABLE]

Table 1. QSO/Damped Ly [FORMULA] system characteristics and observation log


2.2. Data reduction

The only modifications introduced to the HST pipe-line reduced data involves the wavelength scale and zero point of the intensity scale. When two distinct exposures were obtained for a given object and grism, the two spectra were averaged (with weights according to the exposure time). To define the absolute wavelength scale, we apply uniform shifts determined using strong Galactic lines from singly ionized species, the latter being assumed at rest. In some spectra, no such line is seen with a good enough signal-to-noise ratio; we then rely on strong unblended lines from already known absorption systems with well determined optical redshifts. Further, when a common transition is detected both in the G190H and G270H spectra, this feature is used to constrain the relative shift of these two spectra. The shifts applied to the original wavelengths for PKS 0454+039, PKS 1229-021 and 3C 286 are 1.4, 2.3 and 1.2 Å respectively for the G190H spectra and 1.5, 2.3 and 1.8 Å for the G270H data (the G270H spectrum of EX 0302-223 has been shifted by -0.6 Å). For the G160L data, only the strongest absorption features from known systems are useful; -5.5 Å and 6 Å have been added to the original wavelengths for Q 1209+107 and 3C 196 respectively.

Regarding the intensity scale, we used the profiles of damped Ly [FORMULA] lines or of Lyman edges to determine the true zero level. Generally, damped Ly [FORMULA] lines seen in the high resolution data do not go exactly to zero, even when they are clearly saturated at the line core. Although the observed offsets are quite small, they would have a significant effect on the fitting procedures, so we subtracted them. These offsets are generally positive and are presumably due to scattered light in the instrument. Their effect on [FORMULA] measurements is however always negligible as compared to [FORMULA].

2.3. Data analysis and results

Figs. 1 to 6 present the spectra obtained for EX 0302-223 (G270H), PKS 0454+039 (G190H and G270H), 3C 196 and Q 1209+107 (G160L), PKS 1229-021 and 3C 286 (G190H and G270H). The full width at half-maximum (FWHM) of an unresolved line is 1.5, 2.0 and 6.3 Å in the G190H, G270H and G160L data respectively. All spectra are given in flux units of [FORMULA] erg cm-2 s-1 Å-1. The detection, measurement and identification of all absorption lines have been performed interactively. The uncertainty on observed equivalent widths is derived (in the same manner as Young et al. 1979) from the noise level measured in selected portions of the spectra which look free from any absorption line. For weak unresolved features, [FORMULA] lies in the range 0.06 - 0.10 Å over most of all the G190H and G270H ranges (0.07 - 0.08 Å being by far the most common values). Locally, [FORMULA] can be smaller (e.g. on emission lines where values as low as [FORMULA]  Å are reached) or larger (e.g. when blending occurs). The central wavelengths of the absorption lines are obtained by fitting a Gaussian to the observed profile (after normalizing the adjacent continuum if necessary, e.g. for absorption lines located on an emission line). For the identification of features significant at the [FORMULA] level, we first search for absorption from already known metal systems using the list of strong lines given by Bahcall et al. (1993; their Table 1). The presence of weaker transitions that might be expected on the basis of the results obtained in this first step is then examined, especially from the DLAS, for which we accept lines at a lower significance level. To this aim, we consider the extensive line list given by Verner et al. (1994). The identification is performed on the basis of criteria involving redshift agreement, line width as compared to the instrumental profile and relative strength, when several transitions from the same species are detected. When two or more lines strongly overlap, wavelengths and equivalent widths are measured after deblending. This was performed using the context FIT/LYMAN developed by A. Fontana within MIDAS, the ESO data analysis software package; this routine was also used to fit line profiles and extract gaseous column densities for damped Ly [FORMULA] lines or Lyman edges. Among the lines left unidentified, we have searched for the presence of metal doublets and, in the Ly [FORMULA] forest, for lines from the Lyman series possibly associated with the strongest candidate Ly [FORMULA] features.

This results in the line lists given in Tables 2 to 5. For EX 0302-223, only lines from metal systems are given in Table 2. Although the S/N ratio is quite good, some ambiguities remain due to 1) the large number of absorption features expected from the DLAS and 2) the presence in EX 0302-223, PKS 0454+039 and PKS 1229-021 of several other metal systems. When two or more transitions might contribute to a given feature, they are indicated in the Tables. Le Brun & Bergeron (1997) have performed an identification of Ly [FORMULA] absorbers in the field of 3C 286 and give for this object a more extensive list of candidate Ly [FORMULA] lines (down to the [FORMULA] significance level).


[TABLE]

Table 2. Absorption lines detected in the spectrum of EX0302-223


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© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998

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