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Astron. Astrophys. 341, 709-724 (1999)

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3. Observed abundances in DLA systems

We have compiled from the literature all available data on element abundances in DLA systems for comparison with results from our chemically consistent galaxy evolution models. Publications on abundance observations in DLA systems span more than one decade in time and the data experienced considerable improvement in quality and quantity in recent years. The total resulting compilation of element abundances is very inhomogeneous. We confine our comparison to abundances of Al, Ni, S, Fe, Si, Mn, Cr, and Zn, because for those eight elements enough observations are available. For all other elements included in our models the number of published abundance measurements in DLA systems is too small for a reasonable comparison. For our final compilation we take the following criteria into account.

  • Many DLA systems (characterized by the redshift [FORMULA] of the absorption lines and the background QSO) are reported by more than one author. In these cases the most reliable data will be used.

  • Different methods to determine column densities from observed absorption lines are in use: curve of growth analysis, profile fitting methods (e.g. VPFIT etc.), apparent optical depth method (Savage & Sembach 1991).

  • The quality of data depends on instrumental capabilities. Resolution and signal to noise ratio of the spectrographs have been improved considerably in recent years using 4m class telescopes and the 10m Keck Telescope.

  • All methods (mentioned above) used to derive column densities from observed absorption lines need oscillator strength f. Different f values are published in the literature and have been used by different observers. In column "reff" of Table 3 (cf. also footnote [FORMULA]) we list the references for oscillator strengths used for the DLA observations. In our final compilation we prepare a homogeneous database by referring all observed abundances to the oscillator strengths given by Morton (1991) since those are most widely used.

  • Different solar element abundances are used by different authors to calculate abundances in the usual form given in Eq. (5):


    where X denotes the number density [FORMULA] or column density [FORMULA] of element X measured from the absorption line and H denotes the hydrogen density, respectively. The symbol [FORMULA] denotes solar values which are used as reference values (cf. Savage & Sembach, 1991). In column "[FORMULA]" of Table 3 (cf. also footnote a) we list the references to solar abundances used to calculate [X/H] by different authors. Our final compilation is homogeneously normalized to solar abundances published by Anders & Grevesse (1989) since they are most widely used.

Taking all these points into account we have compiled abundances in DLA systems for those eight elements which have the largest number of abundance determinations: Al, Ni, S, Fe, Si, Mn, Cr and Zn. According to the method used for abundance determination and the estimation of reliability of the data by the authors themselves we divide our sample into two classes of reliability: reliable and less reliable .

In Table 3 we list 29 papers, each reports at least one abundance for any of our eight elements. The quantity of data in each paper is given in the columns labeled with respective elements. The number of reliable data for any element is given by the second number separated by a comma. For instance, Boissé et al. (1998) report Fe abundances for three DLA systems, one of these is reliable and two are less reliable measurements. Obviously the main share of data comes from about five papers published during the past five years (their running number is printed heavy).

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© European Southern Observatory (ESO) 1999

Online publication: December 16, 1998