Ionization properties and elemental abundances in damped Lyα systems

We analyze extant data of Al+2, Al+, and other low ions with the aim of studying the ionization properties of damped Ly alpha systems (DLAs) from the analysis of the ratio R(Al+2/Al+) equivalent to N(Al+2)/N(Al+). We find the good correlations log N(Al+)-log N(Si+) and log N(Al+)-log N(Fe+) that we use to indirectly estimate N(Al+) from N(Si+) and/or N(Fe+) measurements. In this way, we determine the ratio R(Al+2/Al+) for a sample of 20 DLAs. Contrary to common belief, the ratio can attain relatively high values (up to 0.6), suggesting that the gas of the intermediate ionization state plays an important role in DLAs. On the other hand, the lack of any trend between abundance ratios, such as Si/H and Si/Fe and R(Al+2/Al+) indicates that abundances are not severely influenced by ionization effects. We find a log R(Al+2/Al+)-log N(H-0) anticorrelation that we use in conjunction with idealized photoionization equilibrium calculations to constrain the ionization properties and to predict ionization corrections in DLAs. We consider two possible origins for the species of the low- and intermediate-ionization state: (1) neutral regions devoid of Al+2 and/or (2) partially ionized, Al+2-bearing regions. The log R(Al+2/Al+)-log N(H-0) anticorrelation can be naturally explained in terms of a two-region model with a soft, stellar-type ionizing radiation field. We present abundance ionization corrections for 14 elements of astrophysical interest derived with different types of ionizing spectra. For most of these elements, the corrections are generally below measurements errors, which is contrary to the predictions of recent models presented in the literature. We briefly discuss the potential effects of inaccuracies in the Al recombination rates used in the photoionization calculations.