Copyright © 2005 by Annual Reviews. All rights reserved
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| Annu. Rev. Astron. Astrophys. 2005. 43:
861-918 Copyright © 2005 by Annual Reviews. All rights reserved |
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)
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)
10-2 at
N(H I) 4 ×
1020cm-2. Because of the similarity with the
N(H I) 2 ×
1020cm-2 threshold, one might expect to find
f(H2) > 10-2 in a significant fraction
of damped
Ly
systems. Furthermore,
if damped
Ly 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
Ly 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
Ly 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
Ly 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
Ly 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
Ly 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
nH2, f(H2) is predicted
to decrease with decreasing dust-to-gas ratio, and since I
J
,
where J 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
Ly systems helps explain
the low values of f(H2). Indeed
Ledoux, Bergeron &
Petitjean (2002)
find a statistically significant positive correlation between
f(H2) and
.
What is the value of J in damped
Ly systems?
Levshakov et al. (2002)
inferred J 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,
J.
Levshakov et al. (2003)
found that J was comparable to the ambient interstellar radiation
intensity in the Galaxy, i.e.,
J
10-19 ergs
cm-2 s-1 Hz-1 sr-1 for this
system. Previous authors reached similar conclusions for other damped
Ly 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 h
10 eV and z
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
Ly 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
Ly 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
Ly systems.