Since the time of their discovery it has been recognized that the
Ly- systems
should be photoionized by the intergalactic radiation field
(Arons 1972).
Although direct spectroscopic measurements of the level of ionization in the
Ly-
systems will
have to wait for far-ultraviolet observations with the HST, estimates
are already available for the metal line systems.
Bergeron and
Stasinska (1987)
have found that line
strengths observed in metal systems indicate a very narrow range for the
ionization parameter U =
n
/
ne. High ionization systems have
1.5 x 10-3
U
2 x 10-2, and
low ionization systems have 2.5 x 10-4
U
2.5 x 10-3.
Let us postulate that additional systems exist with the same range of gas
densities, but sufficiently small line of sight columns that they are,
like the Ly- systems,
optically thin in the Lyman continuum. Since these systems would experience the
same background ionizing radiation, and have the same range of values for the
ionization parameter, we deduce that they should have 1.9
log
n(H+) / n(H)
3.8.
This shows that most metal line systems do have ionizations such that
metal lines
would not be detected if their N(HI) were as low as the values found in
typical Ly-
systems. Note that this level of ionization is not low enough to hide
the metals in the
exceptional Ly-
systems with
logN(HI) = 17. These systems must have abundances
of under 0.003 solar if they have gas densities like the metal line systems.
The gas densities in Ly-
systems have also been deduced from limits on the sizes of the systems.
Sargent et al. (1982)
failed to find any
correlation between the Ly-
systems in the spectra of a close pair of QSOs, leading to a maximum
size of about 1 Mpc. However a strong correlation was found by
Foltz et al. (1984)
in two images of
the gravitationally lensed QSO 2345+007A,B. This observation leads to a minimum
size of about 5 kpc for Ly-
systems with both low and moderate equivalent widths.
The implications are most interesting for the low equivalent width
systems. At least 5
of 7 systems with Ly-
rest
frame equivalent widths in the range 0.12-0.23 Å were seen
in both lensed images. These systems will have 13.5
logN(HI)
14.6, maximum
total gas densities of n
10-3.5 cm-3, and total
column densities of logN
18.2
assuming that they are spherical. This alone is sufficient to limit
abundances to under 0.01 solar. The level of ionization is high with log
n(H+) / n(H)
4.7, about an
order of magnitude above the maximum level of ionization found for common metal
line systems. Were this same ionization to apply to systems with larger
N(HI), even lower abundances would apply.
Persuasive as these arguments are, they are not unique, as a number of
investigators
have pointed out. The Ly-
systems need not be pressure supported, in which
case they could have a variety of levels of ionization. It is also
possible that the Ly-
lines seen in the lensed QSO images arise from the passage of light
through highly
flattened rather than spherical clouds, in which case the level of
ionization would be much lower than assumed.
We are then left with two possible explanations for the scarcity of metal line
systems with the low N(HI) typical of the
Ly- systems. The first is
that, at these
low column densities, metals do not exist, or are much less abundant
than in the
observed metal line systems. The second is that such systems do occur, and they
have the same gas densities and low levels of ionization as the observed
metal line
systems. The metal lines would not be observed at the N(HI) of typical
Ly-
systems.
Further evidence that Ly-
systems may have a low level of ionization comes from
the observation of unexpectedly narrow
Ly-
lines in a few systems.
Chaffee et al. (1983)
reported the observation of a line which, if
Ly-
, must have a
temperature of
under 16,700°K to limit thermal line broadening. Other similar
systems have been
observed, although there is disagreement about their frequency of
occurrence. These
systems must either be of high density and hence low ionization, or they
could be cooled by metals or H2.
Barcons and Fabian (1987)
have stressed that an intergalactic medium which is
capable of producing the X-ray background in the energy range 3-300 keV is only
compatible with Ly- clouds if
the latter are of high density and low ionization. A
medium with a current density n = 10-6 cm-3 and T
= 3 x 108 °K would crush
Ly-
clouds of T
104 °K unless their densities were
n
1 - 10 cm-3.
Explanations for the lack of
Ly- systems with logN(HI) >> 17
are then that the
processes which make the clouds do not operate at these column
densities, that metals
form in Ly-
clouds whenever column densities are this large, or that
metals exist for
a wide range of column densities and are only observed when column
densities exceed this value.