CLOUDS
There are considerable differences between the heavy-element and single
Ly-
redshift systems. The typical column density is N(HI)
1015
cm-2. There is a
power-law distribution of column densities of the form n(HI)
N-1.5. (This is the
area-weighted observed distribution which translates into a true
distribution n(HI)
N-3.5). The line widths are in the range 10 < b
< 40 km s-1; b
25 km s-1 is
typical. There are several limits on the sizes of the absorbing
clouds. The observation
of common Ly- absorption lines
in the spectra of the two brighter components of the
gravitationally lensed QSO, Q1115+080, implies that D > 0.4
kpc. An upper limit of D < 400 kpc is inferred from the absence
of common Ly- forest
lines in the spectra
of the QSOs UM 680 and UM 681 referred to above. In a critical measurement,
Foltz et al. (1984)
showed that most, but not all,
Ly- forest lines were observed in
both components of the pair Q2345+007 which is thought to be a
gravitational lens,
although the lensing galaxy has not been observed. Making a plausible
assumption
about the redshift of the lensing galaxy, Foltz et al. deduced
that the minimum size of
the Ly- clouds was 8 kpc; this
is likely to be close to the actual value if it is true that
some strong lines are not seen in both lines of sight. If the pair is
not a lens then the
value of D would be about 15 kpc. Statistical methods have been
used to set limits on
the abundances of any heavy-elements associated with the
Ly- clouds.
Norris et al. (1983)
found marginal evidence for absorption in the OVI
1034, 1038 doublet
to be statistically associated with
Ly- absorption lines in the
spectra of several
high-redshift QSOs. They derived an abundance ratio 0/H
10-2(O/H)
on the assumption (see below) that the
Ly- clouds are photo-ionized by
the meta-galactic flux of radiation from QSOs. Later,
Chaffee et al. (1985)
conducted an analysis of a
single high column density system in the QSO Q0014+81 and deduced that they had
detected SiIII 1206 line. The
problem is that this line lies in the
Ly- forest. More
detailed calculations of the expected ionization conditions in the
clouds indicated that
SiIII 1206 would be the
strongest observable heavy-element line in their spectral
range. The inferred abundance was (Si/H) = 0.001
(Si/H). A Lyman limit system
with no detectable lines of heavier elements was discovered in the QSO
Q2126-158 (zem = 3.30) by
Boksenberg and
Sargent (1983).
This system has zabs = 2.99 and the
HI column density is well determined to be N(HI) = 3 x 1017
cm-2. The limit on the
possible heavy-element abundance, as reinterpreted by Chaffee et
al. in the light of
their improved calculations, was Z = 0.001
Z. In summary, it is
still not established
in my opinion that any lines of heavier elements have been detected
associated with
the Ly- forest lines. This is
true for both the weaker lines when added together and
for single strong systems. In this respect the
Ly- forest redshifts are
clearly different from the heavy-element redshifts.
Only weak clustering has been detected in the 2-point correlation function of
the Ly- clouds
(Webb 1987, personal communication). An earlier study by
Sargent, Young and
Schneider (1982)
revealed no clustering either in the
correlation function of
a single QSO, Q1623+268 (zem = 2.55), or in the
cross-correlation between the Ly-
forest lines in this QSO and a second object, Q1623+269, separated from
Q1623+268
by 3 arcmin (or 1 Mpc) on
the plane of the sky. If the local, galaxian correlation
function is written as dP = N0[1 +
(r)]dV
where (r) =
(r/rc)-1.77 and where
rc = 5
Mpc, the simplest hierarchical clustering model predicts that
rc changes with redshift
according to the relation rc(z) =
rc(0)(1 + z)-5/3. In fact, the limit
obtained by
Sargent et al. was rc = 0.2 Mpc at z = 2.5 as
compared with rc = 1 Mpc predicted by
the simple theory. The marginal clustering detected by Webb is
consistent with the upper limit obtained earlier.
Rees and Carswell
(1987)
also pointed out that there is no evidence for voids in the
Ly- cloud redshift distribution.
The Ly- clouds exhibit strong
evolution in redshift
(Peterson 1978;
Young, Sargent and
Boksenberg 1982;
Murdoch et al. 1986).
According to the latest work, the
number density of the absorption lines evolves as dN / dz
(1 + z)2.3±0.1 and that for
lines with rest equivalent width above 0.36 Å, dN / dz
60 at z = 2.5.
Thus, the Ly- lines
differ qualitatively from the heavy-element redshifts in at
least three important respects: composition, clustering and evolution in
number density.