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.