A homogeneous sample of sub-damped Lyman α systems -: I.: Construction of the sample and chemical abundance measurements

In this first paper of a series, we report on the use of quasar spectra obtained with the high-resolution Ultraviolet-Visual Echelle Spectrograph (UVES) and available through the European Southern Observatory (ESO) Very Large Telescope (VLT) archive to build the first homogeneous and unbiased sample of sub-damped Lyman alpha (sub-DLA) systems, absorbers with HI column densities > 10(19) cm(-2) but lower than the classical definition of damped Lyman alpha systems (2 x 10(20) cm(-2)). A systematic investigation of the properties of these systems and a comparison with those of the DLAs (Paper II of this series) is expected to provide new clues on the association of high column density absorbers with galaxies and on the overall evolution of the neutral hydrogen gas mass and metal content in the Universe. In the spectra of 22 quasars which were found suitable for a sub-DLA search, we identified 12 sub-DLAs and one borderline case between the DLA and sub-DLA systems in the redshift interval z = 1.8-4.3. We measured the column densities of HI and of up to 16 additional ions of low, intermediate and high ionization: OI, CII, CIV, SiII, SiIV, NI, SII, MgI, MgII, AlII, AlIII, FeII, Fe III, NiII, ZnII and Cr II. We further investigated the significance of the ionization corrections in the determination of the chemical abundances from the low-ionization ions in the sub-DLA HI column density range. Using the predictions of different ion ratios as a function of the ionization parameter computed using the CLOUDY software package, we have estimated that with the exception of one case, the ionization corrections to the abundances of nine systems for which we were able to constrain the ionization parameter are lower than 0.2 dex for all of the elements except Al II and Zn II down to HI column densities of log N( H I) = 19.3 cm(-2). Finally, we present the first sub-DLA chemical abundance data base, which contains the abundance measurements of 11 different elements (O, C, Si, N, S, Mg, Al, Fe, Ni, Zn and Cr). We took advantage of the lower HI column densities in sub-DLAs to measure, in particular, the O and C abundances using lines that are normally saturated in DLAs.