Abundances at high redshift: ionization correction factors

In the last few years, improvements in high-resolution observations of quasi-stellar objects (QSOs) are enabling the use of absorption line systems as probes of the metallicity at early epochs, in particular through the observation of damped Lyman α systems. However, the determination of chemical abundances from the ionic column densities obtained from the observed absorption lines depends on unobserved ions. In this paper, it is shown that the ionization correction factors for OI, NI, CII, MgII and SiII, relative to HI, may be significant only for systems with hydrogen column densities N(HI) < 1020 cm-2 for a soft radiation field. For a harder radiation field, the correction factors for the singly ionized ions are significant for systems with column densities up to 1021 cm-2.