Ionization corrections in a multi-phase interstellar medium: Lessons from a z~2 sub-DLA

TL;DR

This study analyzes a high-redshift sub-DLA galaxy's complex multi-phase interstellar medium, demonstrating significant ionization corrections are necessary for accurate elemental abundance measurements, which alter previous interpretations of its chemical properties.

Contribution

It provides the first detailed analysis of ionization corrections in a high-redshift, multi-phase sub-DLA, highlighting their importance for accurate abundance determinations.

Findings

Ionization corrections can be as large as 0.7 dex.

Single-phase models overestimate metal abundances by up to 0.15 dex.

Corrected abundances suggest the galaxy's ISM is similar to the Milky Way.

Abstract

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.