Metal enrichment and Ionization Balance in the Lyman Alpha Forest at Z = 3

The recent discovery of carbon in close to half of the low neutral hydrogen column density [N(H I)> 3 x 10^14^ cm^-2^] Lyman forest clouds toward z ~ 3 quasars has challenged the widely held view of this forest as a chemically pristine population uniformly distributed in the intergalactic medium, but has not eliminated the possibility that a primordial population might be present as well. Using extremely high signal-to-noise observations of a sample of quasars we now show that C IV can be found in 75% of clouds with N(H I)>3 x 10^14^ cm^-2^ and more than 90% of those with N(H I) > 1.6X 10^15^ cm^-2^. Clouds with N(H I) > 10^15^ cm^-2^ show a narrow range of ionization ratios, spanning less than an order of magnitude in C IV/H I, C II/C IV, Si IV/C IV and N V/C IV, and their line widths require that they be photoionized rather than collisionally ionized. This in turn implies that the systems have a spread of less than an order of magnitude in both volume density and metallicity. Carbon is seen to have a typical abundance of very approximately 10^-2^ of solar and Si/C about three times solar, so that the chemical abundances of these clouds are very similar to those of Galactic halo stars. Si IV/C IV decreases rapidly with redshift from high values (>0.1) at z > 3.1, a circumstance which we interpret as a change in the ionizing spectrum as the intergalactic medium becomes optically thin to He^+^ ionizing photons. Weak clustering is seen in the C IV systems for {DELTA}ν < 250 km s^-1^, which we argue provides an upper limit to the clustering of H I clouds. If the clouds are associated with galaxies, this requires a rapid evolution in galaxy clustering between z = 3 and z = 0.