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Ly-alpha and metal line systems are clearly distinguished in terms of N(HI). Since HI is the only species detected in the Ly-alpha systems, and it is ubiquitous in metal line systems, N(HI) is the only column density that can be used for a comparative study of the different narrow lined systems. As we saw above, this is unfortunate since it would be much more useful to classify systems in terms of their total column densities, or better still, their masses.

The first determination of the distribution of N(HI) in the Ly-alpha systems by Carswell et al. (1984) revealed a roughly power law distribution. This has now been extended to include a hopefully representative small sample of all narrow lined systems (Tytler 1987a).

The most impressive feature of the column density distribution is the fact that the narrow lined systems cover an enormous range of logN(HI), from below 12.8 for the weakest Ly-alpha systems, up to 21.9 for metal systems (Smith et al. 1986). This 9 order of magnitude spread is comparable to the difference in luminosity between the white dwarf stars and the supergiants. Here then is the clearest evidence for diversity amongst the absorption systems, a diversity which encourages one to search for subclasses of systems.

It then came as a surprise to find that to first order the column density distribution could be represented by a single power law. It appears that the majority of systems with logN(HI) leq 17 are Ly-alpha systems, while those with larger N(HI) are predominantly metal line systems. If the power law fit to the Ly-alpha systems is extrapolated up to the larger column densities of the metal line systems, one obtains a good estimate of the frequency of occurrence of the latter. These preliminary results suggest that there is some commonality between the Ly-alpha and metal line systems, as has been noted by Bergeron and Boissé (1984).

New data now show that the power law slope flattens at both low column densities below logN(HI) appeq 14.35 (Carswell et al. 1987), and also above logN(HI) appeq 20 (Smith et al. 1986). In addition, the distribution remains very poorly constrained at other N(HI). However a number of interesting questions are suggested. Why are there so few Ly-alpha systems with logN(HI) geq 17? Why are metal line systems with logN(HI) leq 17 extremely rare, and could possible connections between the Ly-alpha and metal line systems account for these observations?

The simplest response is that the Ly-alpha are really metal line systems which have too low a total column density to show metal lines. For all reasonable levels of ionization, metal lines would be observed if total column densities exceed 1018.3 for abundances greater than 0.01 solar (Chaffee et al. 1986). For a typical logN(HI) = 14, this limits ionization to n(H+) / n(H) leq 104.3. Those Ly-alpha systems which have larger N(HI) must have lower ionization or lower metal abundances.

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