The typical neutral hydrogen column density is N(HI)
3.0 x
1018 cm-2 and
the Doppler parameter is in the range 5 < b < 25 km
s-11. Excited fine structure
lines of CII* and SiII* are usually not seen, implying an electron
density in the
absorbing gas ne
10 cm-3. There is a
wide range of ionization since N(CII)
1015
cm-2 and N(CIV)
1015 cm-2. From this
we can deduce that the gas is probably
photoionized by a flat-spectrum source (for example, the meta-galactic
QSO flux), that the ionization parameter is
= n(H) /
n(
) = 6.5 x
103 and that a significant
fraction of the gas is neutral. The heavy-elements are somewhat underabundant;
Z
0.1
Z
. Only crude
limits can
be placed on the sizes of the absorbing clouds. The
line depths show that the clouds cover the QSO emission region which is
estimated to
have a size D
1019 cm. The Jeans length in the gas
is estimated to be 2.4 x 1021 cm.
The two lines of sight to the gravitationally lensed QSO, Q0957+551
(zem = 1.40),
show the same C IV absorption redshifts at zem = 1.12,
but the detailed absorption profiles are different
(Boksenberg and
Sargent 1983),
implying individual cloud sizes of
D < 2.7 x 1022 cm. On the other hand, this observation
together with the observation
(Shaver and
Robertson 1983a;
Boksenberg and
Sargent 1983)
of common absorption
redshifts in the QSO pair UM680 / 681 whose separation is 1 arcmin or 300
kpc, implies
that the absorption occurs in relatively small clouds which are embedded
in much
larger structures. The heavy-element redshift systems often exhibit
velocity structure
scales extending from 20 km s-1 (the lower limit that has
been resolved) up to 600
km s-1. The value 150 km s-1 is typical. The
number density of typical strong
CIV redshift systems is dN / dz
1.5 at z = 2. As we shall
see, there is no strong
cosmological evolution in the density of absorbers.