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)>3x10(14) cm(-2)] Lyman forest clouds toward z similar to 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)>3x10(14) cm(-2) and more than 90% of those with N(H I)>1.6x10(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 v <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. (C) 1996 American Astronomical Society.