![]() | Annu. Rev. Astron. Astrophys. 2005. 43:
861-918 Copyright © 2005 by Annual Reviews. All rights reserved |
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.