Owing to the non-linear collapse of cosmic structures, the IGM is well known to be highly inhomogeneous. The discrete gaseous systems detected in absorption in the spectra of high-redshift quasars blueward of the Ly emission line are assigned different names based on the appearance of their absorption features (see Figure 1).
Figure 1. High resolution ( / = 37,0000) spectrum of the ze = 3.50 quasar Q1159+123, taken with the Keck High Resolution Spectrograph (exposure time 8 h). The data are taken from Songaila (1998). The Ly forest is clearly seen in absorption blueward of the atomic hydrogen Ly emission line from the quasar (the broad peak at 5470 Å), and is produced by resonant Ly scattering in gas clouds along the line-of-sight between us and the quasar. The spectrum shows a Lyman-limit system just shortward of 4150 Å, i.e. close to the quasar emission redshift. Most of the features between the Ly and C IV emission (the other broad peak just below 7000 Å) are C IV intergalactic absorption lines.
The term ``Ly forest'' is used to denote the plethora of narrow absorption lines whose measured equivalent widths imply H I column densities ranging from 1016 cm-2 down to 1012 cm-2. These systems, observed to evolve rapidly with redshift between 2 < z < 4, have traditionally been interpreted as intergalactic gas clouds associated with the era of baryonic infall and galaxy formation, photoionized (to less than a neutral atom in 104) and photoheated (to temperatures close to 20,000 K) by a ultraviolet background close to the one inferred from the integrated contribution from quasars. Recent spectra at high resolution and high signal-to-noise obtained with the Keck telescope have shown that most Ly forest clouds at z ~ 3 down to the detection limit of the data have undergone some chemical enrichment, as evidenced by weak, but measurable C IV lines. The typical inferred metallicities range from 0.3% to 1% of solar values, subject to uncertainties of photoionization models. Clearly, these metals were produced in stars that formed in a denser environmemt; the metal-enriched gas was then expelled from the regions of star formation into the IGM.
An intervening absorber at redshift z having a neutral hydrogen column density exceeding 2 x 1017 cm-2 is optically thick to photons having energy greater than 13.6 eV, and produces a discontinuity at the hydrogen Lyman limit, i.e. at an observed wavelength of 912 (1 + z)Å. These scarcer ``Lyman-limit systems'' (LLS) are associated with the extended gaseous haloes of bright galaxies near the line-of-sight, and have metallicities which appear to be similar to that of Ly forest clouds.
In ``damped Ly systems' the H I column is so large (NHI 1020 cm-2, comparable to the interstellar surface density of spiral galaxies today) that the radiation damping wings of the Ly line profile become detectable. While relatively rare, damped systems account for most of the neutral hydrogen seen at high redshifts. The typical metallicities are about 10% of solar, and do not evolve significantly over a redshift interval 0.5 < z < 4 during which most of today's stars were actually formed.
Except at the highest column densities, discrete absorbers are inferred to be strongly photoionized. From quasar absorption studies we also know that neutral hydrogen accounts for only a small fraction, ~ 10%, of the nucleosynthetic baryons at early epochs.