Ionization corrections in a multiphase interstellar medium: lessons from a zabs∼ 2 sub-DLA

We present a high resolution (FWHM= 2.7 km s −1 ), high S/N echelle spectrum for the zem = 2.26 QSO J2123−0050 and determine elemental abundances for the zabs = 2.06 subDLA in its line of sight. This high redshift sub-DLA has a complex kinematic structure and harbours detections of neutral (Si, Ci), singly (e.g. Cii, Sii) and multiply ionized (e.g. Civ, Siiv) species as well as molecular H2 and HD. The plethora of detected transitions in various ionization stages is indicative of a complex multi-phase st ructure present in this high redshift galaxy. We demonstrate that the ionization corrections in this sub-DLA are significant (up to �0.7 dex). For example, if no ionization correction is applie d, a super-solar metallicity is derived ([S/H] = +0.36), whereas a single phase ionization correction reduces this to [S/H] = −0.19. The theoretical impact of a multi-phase medium is investigated through Cloudy modelling and it is found that the abundances of Si, S and Fe are always over-estimated (by up to 0.15 dex in our experiments) if a single-phase is assumed. Therefore, although Cloudy models improve estimates of metal column densities, the simplification of a single phase medium leaves a systematic error in the result, so that even ionizat ion-corrected abundances may still be too high. Without ionization corrections the properties of this sub-DLA appear to require extreme scenarios of nucleosynthetic origins. After ioniz ation corrections are applied the ISM of this galaxy appears to be similar to some of the sightlines through the Milky Way.