Metal Abundances and Ionization in QSO Intrinsic Absorbers

This paper provides a general reference for deriving the ionizations and metal abundances for absorption-line clouds in photoionization equilibrium with a QSO spectrum. The results include a quantitative assessment of the theoretical uncertainties and firm lower limits on the metallicities when no ionization constraints are available. The calculations applied to the best available column densities for QSO intrinsic absorbers support previous studies; there is strong evidence for super-solar metallicities in broad absorption line (BAL) and at least some “associated” (za≈ ze) absorption regions. Even in cases where the level of ionization is unknown (for example, when only H I and C IV lines are measured), firm lower limits on the metal abundances (C/H) imply Z ∼> Z⊙ for typical BALs and some za ≈ ze absorbers. These results support the independent evidence for high metallicities in QSO broad emission-line regions. I argue that high gas-phase abundances, up to ∼10 Z⊙, can be expected in QSOs due to the normal enrichment by stars in the cores of young, massive (∼>10 11 M⊙) galaxies. The measured column densities in intrinsic absorbers also indicate that the gas is usually optically thin in the H I Lyman continuum out to energies above at least the C IV and N V ionization edges (i.e. ∼>100 eV). The column density ratios often require a range of ionization states in the same absorber. Neighboring za ≈ ze systems (having similar redshifts and often blended spectroscopically) also usually require different ionizations or metal abundances. If the abundances are similar in neighboring systems, their ionization parameters must sometimes differ by more than an order of magnitude, requiring a significant range of densities and/or distances from the ionizing QSO. These result, and the overall levels of ionization, are not consistent with single-zone models of the UV line and X-ray continuum (“warm”) absorbers. Subject headings: Galaxies: abundances — Galaxy evolution — Quasars: absorption lines — Quasars: general