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.