FOREST
Tytler (1987a,
1988)
has recently derived the distribution of the number
density of
Ly- absorption lines as a
function of column density and has found that the observed
distribution may be represented as a single power law over a wide range
in column
density from N(HI) = 1012 to 1021 cm-2.
The low end of this range is dominated by lines of the
Ly- forest, while the high
end is dominated by Ly- lines
associated with redshifts containing heavy-element
lines. Tytler points out that the single power law makes it hard to
believe that the
lines are produced by two distinct kinds of objects. Moreover, the
absence of a break
in the distribution at a column density of N(HI)
1017
cm-2 makes it hard to
believe that the objects which produce the lines are highly ionized as
is commonly
believed. Instead, Tytler proposes that the clouds are neutral at all
column densities;
for this to be true they must be small, dense objects in order not to be
ionized by
the meta-galactic QSO flux. In fact, Tytler advocates a typical size of
about 3 pc.
This is hard to reconcile with the observations of common lines in the
spectra of
gravitationally lensed QSOs. Moreover, the claim of one population of
objects is in
stark disagreement with other observed regularities - in particular, the
differences in
composition, cosmological evolution, and clustering which were
summarized above.
In addition, some authors have questioned Tytler's claim that a simple
power-law distribution actually applies
(Bechtold 1987;
Wolfe 1988);
the distribution of raw equivalent widths derived by
Sargent et al. (1980)
certainly showed different slopes
for the Ly- forest and
heavy-element Ly-
lines. Therefore, it seems unwise at this
stage to pursue the consequences of Tytler's radical suggestion until
the facts are more clearly established.
A second development, which observationally is on firmer ground, concerns the
behavior of the Ly- forest in
the vicinity of the Ly-
emission line.
Carswell et al. (1984)
pointed out that, contrary to a previous analysis by
Sargent et al. (1980),
there is statistical evidence for a deficiency of absorption lines in the blue
wing of the Ly-
emission line. They interpreted this as being due to the effect of the
QSO in effecting
the ionization of the Ly-
clouds in its immediate vicinity. In a related development,
Murdoch et al. (1986)
pointed out that whereas in QSO spectra
overall there is a marked tendency for the density of
Ly- forest lines to increase with
redshift, in a
given QSO spectrum there is a flat or even declining tendency with
increasing redshift (see also
Hunstead 1988).
This result, named the inverse effect by Murdoch et al.
was shown to be due to a statistical deficiency of
Ly- absorption
lines close to the
emission line, as Carswell et al. had found.
Tytler (1987b)
extended this result, which
now seems to be firmly established. Bajtlik, Duncan and Ostriker
(1987,
1988) have
quantified Carswell et al.'s suggestion and have shown that
Tytler's data may be fitted
very well if it is assumed that close to the QSO the local radiation
field dominates over the meta-galactic QSO flux.
The discovery of the inverse effect, or the proximity effect as it is
called by
Bajtlik et al. (1988),
is very important because it allows a direct
determination to be made
of the total meta-galactic ionizing flux at high redshifts, where the
contribution of
lower luminosity sources such as star-forming galaxies and Seyfert
nuclei cannot be
observed. Interestingly, Bajtlik et al. found a good fit to the
observations with a
meta-galactic flux at z = 2.5 of
I
=
1021 ergs s-1 cm-2 ster-1, a
value which
has been estimated as the total contribution of QSOs alone. Moreover,
the success of
Bajtlik et al.'s calculation probably enhances one's confidence
that the Ly- clouds are
indeed highly ionized by the meta-galactic UV flux as
Sargent et al. (1980)
proposed.