Annu. Rev. Astron. Astrophys. 1998. 36:
267-316 Copyright © 1998 by Annual Reviews. All rights reserved |
Theoretical modelling of absorption systems can be traced back to Spitzer's (1956) prediction (expanded by Bahcall & Spitzer 1969) that normal galaxies have large gaseous halos giving rise to heavy element UV absorption lines. Bahcall & Salpeter (1965) considered groups of galaxies, Arons (1972) suggested forming low mass protogalaxies as the probable sites of Ly absorption. The interpretation by Sargent et al 1980 of their observations of the Ly forest alerted researchers to differences between metal and Ly forest absorption systems, with the evidence pointing away from galaxies, to distinct astronomical objects, intergalactic gas clouds.
4.1. Ly
Clouds confined by the Pressure of an Intercloud Medium
If the Ly absorbers
correspond to overdense clumps of gas,
their persistence throughout of the history of the universe must be
either due to only a slow change in their properties, or to
replenishment of the clouds
on a shorter time scale. An apparent lack of rapid
evolution in the properties of the forest (later shown to be a
statistical fluke), and the short electron and
proton relaxation time scales and mean free paths appeared to justify
treating the clouds as "self-contained entities in equilibrium"
(Sargent et al 1980).
A two phase intergalactic medium was postulated, with the hot, tenuous
intercloud medium (ICM) in pressure equilibrium with the cooler and denser
Ly clouds. The standard
version of the pressure confinement model
(Sargent et al 1980;
Ostriker & Ikeuchi
1983;
Ikeuchi & Ostriker
1986)
considers spherical, and, since gravity is ignored,
homogeneous clouds. This model is self-consistent, but there are no very
compelling physical reasons for
preferring pressure to gravitational confinement or to no confinement
at all, and the possibility of self-gravitating clouds as an
alternative was discussed soon
(Melott 1980;
Black 1981).
Nevertheless, the pressure confinement model for
Ly
clouds is appealing for several reasons: It combines the concept of a
multiphase
structure of the intergalactic medium, familiar from the interstellar
medium, with the idea of separate entities, "clouds", in analogy to,
but different from galaxies. A hot intercloud medium may have
been a possible source of the X-ray background. In addition the explosion
scenario (see below) provided a theory of cloud formation. Finally, the
model
made testable predictions, a rare but risky undertaking for astrophysical
theories, which paved the way to its eventual demise.
The basic properties of pressure confined clouds, as worked out in detail
by Sargent et al (1980),
Ostriker & Ikeuchi
(1983),
and Ikeuchi & Ostriker
(1986)
can be summarized as follows:
The Ly clouds are supposed
to be in photoionization equilibrium
with an ionizing UV background. The gas is heated by photoionization,
and cools via thermal bremsstrahlung, Compton cooling, and the usual
recombination and collisional excitation processes. The cloud
evolution consists of several phases, depending on the relative lengths
of cooling and expansion time scales. The ICM is expanding
adiabatically by the cosmic expansion at all times because the high
degree of ionization does not allow for efficient photoionization
heating. The denser clouds embedded in the hot ICM start out in
isothermal expansion with a temperature fixed by thermal ionization
equilibrium (Tc ~ 3 × 104K),
until the density
nc
PICM / Tc
(1 +
z)5 has dropped sufficiently that photoheating cannot
compensate for the work of expansion any longer, and the clouds begin to
cool and to expand less rapidly. The sound speed drops even faster so
ultimately pressure equilibrium with the ICM ceases and the clouds
enter free expansion. The available range of cloud masses is
constraint by the requirements that the clouds must be small enough not
to be Jeans-unstable, but large enough not to be evaporated rapidly when
heated by thermal conduction from the ambient ICM
(Sargent et al 1980,
Ostriker & Ikeuchi
1983).
Clouds formed at z ~ 6 would survive
down to accessible redshifts (~ 4) only if their masses range
between 10 5 < Mc < 1010
M.
THE ORIGIN OF LYMAN ALPHA CLOUDS FROM COSMIC SHOCKS
The explosion scenario of structure formation
(Schwarz et al 1975;
Ostriker & Cowie 1981)
provided a non-gravitational origin
for the pressure confined Ly
clouds. Large scale explosions
from galaxies (e.g., from starbursts) and QSOs may have driven shock waves
into the intergalactic medium. These events may also have provided the
energy for collisional re-ionization and heating of the ICM, since
photoionization cannot produce temperatures high enough to maintain
pressure confinement
(Ikeuchi & Ostriker
1986).
This way a two-phase
medium of hot "cavities" enclosed by a system of cooler shells could
have arisen
(Ozernoy &
Chernomordik 1976).
Ly absorption is
caused by the fragmenting shells
(Chernomordik & Ozernoy
1983;
Vishniac et al 1985).
Among the observational consequences may be pairs of
Ly lines that occur wherever the
line of sight intersects an expanding, spherical shell. It has been
argued that such pairs have been seen
(Chernomordik 1988,
and refs.
therein); this may be the case in individual systems, but it is
difficult to prove in a statistical sense because the two-point correlation
does not show the expected signal at the relevant velocity scale,
~ 100 kms-1
(Rauch et al. 1992).
A potential problem for this model is
implied by the observed lack of a correlation with galaxies. If
Ly
clouds are shells expelled by galaxies, the absorbers should be
clustered in a manner similar to that of galaxies
(Vishniac & Bust
1987).
Neither the auto-correlation function among absorbers nor the
cross-correlation with galaxies show a signal of the requisite strength
(Barcons & Webb 1990).
A similar pattern of shell formation ensues if
QSOs, in the initial event of reionization, surround themselves with
Strömgren spheres
(Arons & McCray 1969,
Shapiro & Giroux
1987),
which can lead to shocked shells of gas at the boundary of the HII
regions. The shells fragment as in the explosion scenario and may be
visible as Ly absorbers
(Madau & Meiksin
1991).
THE ELUSIVE INTERCLOUD MEDIUM The need for a
confining intercloud medium has led to a number of searches for a
residual absorption trough between the absorption lines, caused by the
HI in the intercloud space. These measurements came to be referred to
in the literature as the Gunn-Peterson (GP) test, though
Gunn & Peterson's
(1965)
original result of a 40% average absorption was a detection of the
unresolved
Ly forest as a whole. Values for
the residual (i.e., intercloud, or diffuse, as opposed to line) GP
effect require (a) a precise knowledge of the unabsorbed QSO continuum
level, and (b) either subtracting the contribution from
Ly
"lines", or the use of line free regions (whatever that may be) for
the estimate.
Steidel & Sargent
(1987b)
measured the total flux
decrement of a sample of 8 QSOs against continua extrapolated from the
regions redward of Ly
emission. After subtracting a model population of discrete
Ly lines they obtained a
residual
GP < 0.02 ±
0.03 (< z > ~ 2.67), i.e., a null result.
Giallongo et al (1992),
and Giallongo et al (1994)
have compared apparently line free regions with an
extrapolated continuum, and they found
GP < 0.013 ±
0.026(< z > ~ 3) and (
GP < 0.02 ±
0.03(< z > ~ 4.3),
respectively. Given the line crowding at high redshift, the small error
bars of the z > 4 work betray a certain degree of optimism.
The clean-cut decomposition into line and continuum absorption makes
theoretical
sense for pressure confined clouds, but observationally we can never be
sure whether there is a flat continuum trough from diffuse gas, or
whether there are many weak lines blended together.
Jenkins & Ostriker
(1991),
and Webb et al (1992),
acknowledging this
problem, attempted to model the pixel intensity distribution with
continuum absorption and variable contributions from discrete lines.
Their results show that
GP can be produced
both ways,
by blending of weak lines below the detection threshold, or by a constant
pedestal of absorption.
In any case the weakness or non-detection of a residual GP trough puts
an upper
limit on the density of the ICM. A lower limit on the ICM pressure
(
nICM TICM) can be derived from the
absorption line
width (which gives an upper limit on the radius of the expanding cloud
(Ostriker & Ikeuchi
1983).
The condition that the cloud must be large
enough not to evaporate gives an upper limit on the pressure. Another
independent upper limit on the pressure of the ICM comes from the lack
of inverse Compton distortions in the spectrum of the cosmic microwave
background (CMB)
(Barcons et al 1991).
This result rules out the
intergalactic medium as the source of the hard X-ray background.
It also may spell trouble for the explosion model of galaxy formation,
which is the origin of the
two-phase IGM in the current picture) and of apparently non-existing
structure in the CMB. When all the limits are combined all the limits
only a relatively small corner of allowed (n, T) parameter
space remains for the intercloud medium.
PROBLEMS WITH THE COLUMN DENSITY DISTRIBUTION For
pressure confined clouds the large range of neutral hydrogen column
densities observed must correspond to a range in the parameter
combination
To reproduce only the low column density systems between
13 < log N(HI) < 16 the mass has to vary by 9 orders
of magnitude, or the radiation
field by 3 orders, or the pressure by a factor of 63. To ensure cloud
survival the mass range is limited to less than 4 dex (see above), and the
temperature is constant; therefore, we may need to invoke pressure
inhomogeneities
(Baron et al 1989).
However,
Webb & Barcons (1991),
looking for pressure related spatial correlations among the equivalent
widths of Ly forest lines
excluded pressure fluctuations
P / P > 14%
at the 2 level, and a
similar limit must hold for
the radiation field J. Extremely flattened clouds would help
somewhat in
that they would increase the column density range, allowing a wider
range of path lengths through the clouds
(Barcons & Fabian
1987),
but that may introduce other problems.
Williger & Babul
(1992)
taking these constraints
into account investigated pressure confined clouds with detailed
hydrodynamical simulations and found that the small mass range leads
not only to a failure in producing the column density range but also to
a faster drop in the number of clouds with redshift, than observed.
To summarize, the pure pressure confinement model is unlikely to explain
the Ly forest as a whole,
though it is clear that some LOS must
go through sites where gas is locally confined by external hydrostatic
or ram
pressure. Low redshift gaseous galactic halos, the likely hosts of the
dense Lyman limit absorbing clouds, may be such environments.
Formed by local instabilities the dense clouds may be in pressure
equilibrium with a hot gas phase at the virial temperature of the halo
(Mo &
Miralda-Escudé 1996,
and references therein).
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