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 th rough the observation of damped Lyman alpha systems. However, the dete rmination of chemical abundances from the ionic column densities obtai ned 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 s ystems with hydrogen column densities N(HI) < 10(20) 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 d ensities up to 10(21) cm(-2).