Annu. Rev. Astron. Astrophys. 1999. 37: 487-531 Copyright © 1999 by Annual Reviews. All rights reserved

3.4. Narrow Absorption Line Results

In contrast to the BALs, intrinsic NALs might be the best abundance probes we have for QSO environments. Resolved measurements of NAL multiplets allow us to measure both the coverage fractions and true column densities (Section 3.2.2). The NALs also allow separate measurements of important lines that are often blended in BAL systems, such as NV 1239, 1243-Ly, OVI 1032, 1038-Ly, and many others. We therefore have potentially many more constraints on both the ionization and abundances.

Early NAL studies did not have the quality of data needed to derive column densities and abundances, but several groups noted a tendency for larger NV/CIV line strength ratios in z_a z_e systems compared with z_a << z_e (Weymann et al. 1981, Hartquist & Snijders 1982, Bergeron & Kunth 1983, Morris et al. 1986, Bergeron & Boissé 1986). This trend is probably not caused simply by higher ionization in z_a z_e absorbers, because recent studies show that z_a << z_e systems typically have strong OVI lines and therefore considerable high-ionization gas; NV appears to be weak relative to both CIV and OVI at z_a << z_e (Lu & Savage 1993, Bergeron et al. 1994, Burles & Tytler 1996, Kirkman & Tytler 1997, Savage et al. 1998). The lower NV/OVI and NV/CIV line ratios at z_a << z_e could be caused by an underabundance of nitrogen (compared with solar ratios) in metal-poor intervening gas (Bergeron et al. 1994, Hamann et al. 1997d, Kirkman & Tytler 1997). This would be the classic abundance pattern involving secondary nitrogen (Vila-Costas & Edmunds 1993). Relatively higher N abundances and thus stronger NV absorption lines should occur naturally in metal-rich environments near QSOs (see Section 2.6 above and Sections 6 and 7 below).

The first explicit estimates of z_a z_e metallicities were by Wampler et al. (1993), Möller et al. (1994), Petitjean et al. (1994), Savaglio et al. (1994) for QSOs at redshifts of ~ 2-4. These studies found that z_a z_e systems often have Z Z, which is at least an order of magnitude larger than the z_a << z_e systems measured in the same data. Several of the metal-rich z_a z_e systems have doublet ratios implying partial coverage and thus, very likely, an intrinsic origin (Wampler et al. 1993, Petitjean et al. 1994). The location of the other z_a z_e absorbers is not known, but Petitjean et al. (1994) noted a marked change from [C/H] -1 to [C/H] 0 at a blueshift of ~ 15,000 km s^-1 relative to the emission lines. If high abundances occur only in intrinsic systems, then these results suggest that most z_a z_e NALs are intrinsic (see also Möller et al. 1994).

More recent studies support these findings. Petitjean & Srianand (1999) measured Z Z and [N/C] > 0 in an intrinsic (partial-coverage) z_a z_e absorber. For z_a z_e systems of unknown origin, Savage et al. (1998) estimated roughly solar metallicities and Tripp et al. (1997) obtained [N/C] 0.1 and, very conservatively, [C/H] -0.8. (The lower limit on [C/H] for the latter system is -0.2 when more likely ionizing spectral shapes are used in the calculations.) Savaglio et al. (1997) revised the metallicities downward slightly from those in their 1994 paper to -1 < [C/H] < 0, based on better data. Those systems are of special interest because of their high redshift (z_a 4.1). Wampler et al. (1996) estimated Z ~ 2 Z (based on a tentative detection of OI 1303) for the only other z_a z_e systems studied so far at z > 4.

Hamann (1997), Hamann et al. (1995, 1997b, 1997e, 1999b) used the analysis outlined in Section 3.2 to determine metallicities or establish lower limits for several z_a z_e systems, including some mentioned above and some that are clearly intrinsic by the indicators in Section 3.1. The results generally confirm the previous estimates and show further that, even when there are no constraints on the ionization (for example, when only Ly and CIV lines are measured), the column densities can still require Z Z. A quick survey of those results suggests that bona fide intrinsic systems and most others with Z Z have [N/C] 0.0.

3.4.1. Uncertainties and Conclusions

Most of the NAL studies mentioned above would benefit from better data (higher signal-to-noise ratios and higher spectral resolutions) and more ionization constraints (wider wavelength coverage), but the frequent result for Z Z is convincing. Unlike the BALs, there are no obvious systematic effects that might lead to higher abundance estimates for z_a z_e systems compared with z_a << z_e. The possibility of ionization-dependent coverage fractions presents an uncertainty for those systems with partial coverage, but we do not expect that to cause systematic overestimates of the metallicities (Section 3.2.2). We conclude that many z_a z_e NALs and, more importantly, all of the confirmed intrinsic systems have Z 0.5 Z and usually Z Z. The upper limits on Z are uncertain. The largest estimate for a well-measured system is Z ~ 10 Z (Petitjean et al. 1994), but those data are also consistent with metallicities as low as solar because of ionization uncertainties (Hamann 1997). There are mixed and confusing reports in the literature regarding metal-to-metal abundance ratios, most notably N/C. In contrast to Franceschini & Gratton (1997), we find no tendency for subsolar N/C in z_a z_e systems. In fact, there is the general trend for stronger NV absorption at z_a z_e compared with z_a << z_e systems, and the most reliable abundance data suggest solar or higher N/C ratios whenever Z Z.

The only serious problem is in interpreting the abundance results for absorbers of unknown origin. High metallicities might correlate strongly with absorption near QSOs, but the metallicities cannot define the absorber's location. For example, Tripp et al. (1996) estimated Z Z and [N/C] 0 for a z_a z_e system in which the lack of excited-state absorption in CII^* 1336 (compared with the measured CII 1335) implies that the density is low, 7 cm^-3, and thus the distance from the QSO is large, 300 kpc. [The relationship between density and distance follows from the flux requirements for photoionization (Section 2.5.1.)] Super-solar metallicities at these large distances are surprising. At 300 kpc from the QSO, we might have expected very low intergalactic or halo-like abundances. The solution might be that the absorbing gas was enriched much nearer the QSO and then ejected (Tripp et al. 1996).

Unfortunately, the excited-state lines used for density and distance estimates are not generally available for z_a z_e systems (because they have low ionization energies, e.g. CII^* and SiII^*). Of the six z_a z_e absorbers known to be far ( 10) from QSOs based on these indicators, three of them clearly have z_a > z_e and are probably not intrinsic for that reason (Williams et al. 1975, Williams & Weymann 1976, Sargent et al. 1982, Morris et al. 1986, Barlow et al. 1997). Only one has a metallicity estimate - the system with Z Z at 300 kpc distance studied by Tripp et al. (1996).