Abundances at high redshift can now be measured in a variety of ways involving emission and absorption lines, thanks to large telescopes and sensitive light detectors. Once the high redshifts of quasars were recognized, the potential to use these remarkable objects as probes of the early universe was clear. Derivation of abundances from the emission lines of AGN is difficult, however. The width of the broad lines impedes measurement of weak lines, and the high electron density in the broad line region (BLR) and the large optical depths of some of the lines makes analysis difficult. This is true in particular for the measurement of the electron temperature, necessary to calculate the line emissivity. Shields (1976) noted that relative abundances of C, N, and O could be derived with less sensitivity to the uncertainty in the electron temperature. The N/O and N/C ratio might in turn be an indicator of the overall metallicity, given the secondary nature of nitrogen production. He found high N/C in two QSOs and suggested a parallel to high nitrogen abundances in nearby normal galactic nuclei and AGN. Hamann and Ferland (1993) studied QSO abundances as a function of redshift by bringing together chemical evolution models and photoionization models. They concluded that most luminous, high redshift QSOs (z 2 to 4) have abundances higher than solar. Hamann & Ferland also noted that iron abundances in QSOs at high redshift might constrain cosmological models.
Abundances are also measured in absorbing gas clouds on the line of sight to a QSO. This includes the high column density "damped Ly" systems (DLAs) and the Ly forest. An example of the study of heavy element abundances in DLAs is the work of Pettini et al. (1999). They find [Zn/H] -1.2 in the redshift range z = 1 to 3, with rather little systematic dependence on redshift. The relative abundances of the heavy elements are consistent with a mild degree of depletion onto grains, and [Si/Zn] is essentially solar. (The ratio Si/Zn is a useful surrogate for O/Fe.) Thus, these absorbers do not appear to share the [O/Fe] enhancement of metal poor stars in the Galactic halo. Evidently, even at redshifts of 2 or so, past star formation in the DLA systems had proceeded gradually enough that iron production kept pace with oxygen and other SN II products.
Ly forest clouds often show C IV 1548, 1550 absorption lines when observed with sufficient resolution. Songaila & Cowie (1996) measured O VI as well and found that ionization ratios and line widths pointed to photoionization rather than collisional ionization. Mean abundances ratios appear to be [C/H] -1.5 and [O/C] [Si/C] +0.5, with a spread of order a factor 3 in C/H (Songaila & Cowie 1996; Davé et al. 1998; Ellison et al. 2000). These authors note that the high O/C resembles old halo stars; but as discussed above, O/C is also high in metal poor H II regions.
Abundances in DLAs resemble those measured in GEHRS in the more metal poor dwarf irregulars and in the outermost disks of spirals, that is, [O/H] -1.5. Silk, Wyse, & Shields (1987) suggested that this widespread minimum may result from reaccretion of enriched gas lost from an early population of dwarf galaxies. The more metal poor stars in the Galactic halo might then represent the cannibalized remains of the primordial dwarf galaxies. The very metal poor clouds of the Ly forest must then largely have escaped the reaccretion process, at least at the epoch at which they are observed.
Observations of abundances in ionized gas in galaxies at high redshifts are also becoming available. Kobulnicky & Zaritsky (1999) observed a sample of emission-line galaxies at redshifts z = 0.1 to 0.5 and found them to be only marginally more metal poor than the metallicity-luminosity relation observed for low redshift galaxies. In contrast, Kobulnicky & Koo (2000) obtained near infrared observations of two Ly emitting galaxies at z = 2.3 and 2.9, finding 12 + log O/H = 8.2 to 8.8. From these and other data, they find that emission-line galaxies at these redshifts have abundances substantially below the present day metallicity-luminosity relation. They note that these low abundances resemble those of metal rich globular clusters and raise the possibility that these objects may resemble the formation of galaxies like the Milky Way at z 3. The emission-line galaxies at high redshift have abundances higher than the DLAs, indicating that they represent different environments.