Annu. Rev. Astron. Astrophys. 2005. 43: 861-918
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Molecular gas is ubiquitous throughout the Galaxy ISM. In a survey for Lyman and Werner band absorption, the most efficient tracers of H2 molecules in diffuse interstellar gas, Savage et al. (1977) detected H2 in 90 out of 103 sightlines toward background galactic stars. More specifically, Savage et al. (1977) showed that the molecular fraction, f(H2) ident 2N(H2) / [2N(H2) + N(H I)], undergoes a steep transition from f(H2) < 10-4 at N(H I) < 4 × 1020cm-2 to f(H2) geq 10-2 at N(H I) geq 4 × 1020cm-2. Because of the similarity with the N(H I) geq 2 × 1020cm-2 threshold, one might expect to find f(H2) > 10-2 in a significant fraction of damped Lyalpha systems. Furthermore, if damped Lyalpha systems are the neutral gas reservoirs for star formation at high redshifts, and since stars form out of molecular clouds, molecules should be present.

However, the H2 content of damped Lyalpha systems is much lower than that in the Galaxy. In their compilation of accurate searches for H2, Ledoux, Petitjean & Srianand (2003) report the detection of H2 in only 5 out of 23 cases qualifying as confirmed damped Lyalpha systems, which brings to mind the H2 content of the LMC and SMC. In particular, Tumlinson et al. (2002) found H2 in only 50% of the sightlines through the LMC. Moreover, the LMC resembles the damped Lyalpha systems in that the typical upper limits are f(H2) < 10-5. In addition, the mean value of f(H2) for the positive detections is about 10-2 for the damped Lyalpha systems, the LMC and the SMC, which is about a factor of 10 lower than in the Galaxy.

Why is the H2 content in damped Lyalpha systems so low? The answer is partially related to low dust content. In the Galaxy ISM, H2 forms on the surfaces of dust grains and is destroyed by photodissociation due to FUV radiation. In that case f(H2) = 2R nH / I (Jura 1974), where R is the formation rate constant and I is the photodissociation rate. Because R propto kappa nH2, f(H2) is predicted to decrease with decreasing dust-to-gas ratio, and since I propto Jnu, where Jnu is the mean intensity of FUV radiation, f(H2) should decrease with increasing radiation intensity. The low molecular fractions of the LMC and SMC are plausibly attributed to low dust content and high radiation intensities. Similarly, the low dust content of damped Lyalpha systems helps explain the low values of f(H2). Indeed Ledoux, Bergeron & Petitjean (2002) find a statistically significant positive correlation between f(H2) and kappa.

What is the value of Jnu in damped Lyalpha systems? Levshakov et al. (2002) inferred Jnu for DLA0347-38 at z = 3.025 by showing that FUV pumping was responsible for populating several excited rotational levels in the ground electronic state. By combining the excitation equations with the formation equations it is possible to deduce I, independent of the functional form of R, and from that, Jnu. Levshakov et al. (2003) found that Jnu was comparable to the ambient interstellar radiation intensity in the Galaxy, i.e., Jnu approx 10-19 ergs cm-2 s-1 Hz-1 sr-1 for this system. Previous authors reached similar conclusions for other damped Lyalpha systems (see Black, Chaffee & Foltz 1987; Ge & Bechtold 1997; Petitjean, Srianand & Ledoux 2000). This is an important result because intensities of this magnitude are significantly higher than the background radiation intensity predicted at hnu approx 10 eV and z approx 3 (Haardt & Madau 2003). Therefore, a local source of radiation is required to maintain the right balance between H2 formation and destruction.

While the molecular content of the diffuse gas detected in damped Lyalpha systems is low, dense molecular clouds with high dust content could be present. Such objects would be missed owing to obscuration of the background QSOs or due to a low covering factor. While future surveys for radio-selected damped Lyalpha systems may eventually rule out such scenarios, they are consistent with the current data. In fact, dense molecular clouds may be required as the sites of the star formation, which has been inferred for damped Lyalpha systems.

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