|Annu. Rev. Astron. Astrophys. 2005. 43:
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The best evidence for ionized gas in damped Ly systems comes from the detection of C IV 1548.1, 1550.7 in every object for which accurate spectra have been obtained. Examples of the C IV velocity profiles are shown in Wolfe & Prochaska (2000a) who compare them with the corresponding low-ion profiles. The absence of a one-to-one alignment between the velocity components indicates that the gas producing C IV absorption is not the same gas that produces low-ion absorption. The difference is not surprising. While the X-ray background at z ~ 3 is about 30 times brighter than that at z = 0 (Haardt & Madau 2003), it is still not sufficiently bright to produce C3+ ions by photoionization in neutral gas with the Lyman limit optical depths, LL >> 103 (Wolfe et al. 2004). Note that collisional ionization is ruled out, as many of the C IV profiles exhibit components with v < 5 km s-1, indicating T < 3 × 104 K, which is lower than the T > 4 × 104 K threshold required to produce significant fractions of C3+. In that respect damped Ly systems differ from the Galaxy ISM in which (a) C3+ is collisionally ionized in gas with T ~ 105 K (Sembach & Savage 1992), and (b) the velocity components of the C IV and low ions are aligned. This is interpreted as evidence for corotation of hot halo gas with the neutral disk (Savage, Edgar & Diplas 1990). By contrast, the C3+ ions in damped Ly systems are more likely produced in gas that is photoionized, has a temperature T 104 K, and is kinematically distinct from the neutral gas.
Damped Ly systems also exhibit Si IV 1393.7, 1402.7 and Al III 1854.7, 1862.7 absorption. Because the Si IV and C IV velocity components are closely aligned, the Si3+ ions must be produced by photoionization in the same gas containing the C3+ ions (Wolfe & Prochaska 2000a). In the Galaxy ISM the Al III lines arise in a warm ionized medium (WIM), which is an extensive region of warm (T 104 K) photoionized hydrogen that pervades the disk of the Galaxy (Reynolds 2004). Therefore, it is surprising that in damped Ly systems the Al III velocity components are aligned with the low ions and misaligned with the C IV and Si IV components that arise in gas resembling a WIM. This implies that the Al2+ and low ions both arise either in neutral gas or in photoionized gas, which is kinematically distinct from the C3+-bearing gas. Photoionization equilibrium calculations indicate that in most damped Ly systems both Al2+ and the low ions arise in gas that is mainly neutral (Prochaska et al. 2002b, Vladilo et al. 2000, Wolfe et al. 2004). The soft X-ray background at z ~ 3 is sufficiently bright to produce Al2+ by photoionization and to leave hydrogen mainly neutral and carbon singly ionized (see Wolfe et al. 2004). On the other hand, in about 10% of damped Ly systems the regions giving rise to low ion and Al III absorption contain hydrogen that is significantly ionized. These cases can be recognized by the large Fe2+ / Fe+ ratios and low Ar0 / Si+ ratios (Prochaska et al. 2002b).
These studies raise some interesting questions. First, photoionization by X-ray background radiation produces ions Xj for which the ionization potential of the preceding ionization state IP(Xj-1) > 1 Ryd. In many damped Ly systems photoionization by locally generated radiation with h < 1 Ryd produces low ions for which IP(Xj-1) < 1 Ryd (see Section 8). The question is why doesn't local radiation with h 1 Ryd photoionize sufficient neutral gas to generate a co-rotating WIM like that in the Galaxy? The answer may be related to the higher neutral-gas content of damped Ly systems, which could reduce the escape fraction of ionizing radiation from H II regions in damped Ly systems. Second, do damped Ly systems contain hot (T > 105 K) collisionally ionized gas? Hot gas is a byproduct of feedback processes such as galactic winds or shock heating by supernova remnants (Ferrara & Salvaterra 2004). Because of the evidence for type II supernovae (see Section 3.2.2) and the possible existence of winds (see Section 2.5.2), hot gas could be present in damped Ly systems. The most efficient technique for finding hot gas is through the detection of the O VI 1031.9, 1037.6 doublet (Sembach et al. 2003). But no surveys for O VI in damped Ly systems have been carried out.