Ionization corrections in a multi-phase interstellar medium: Lessons
  from a z~2 sub-DLA
  We present a high resolution (FWHM=2.7 km/s), high S/N echelle spectrum for
the z = 2.26 QSO J2123-0050 and determine elemental abundances for the z = 2.06
sub-DLA 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 H
and HD. The plethora of detected transitions in various ionization stages is
indicative of a complex multi-phase structure 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
applied, 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 ionization-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 ionization corrections are
applied the ISM of this galaxy appears to be similar to some of the sightlines
through the Milky Way.
