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 [$N({\rm H~I}) > 3\ten{14}\cm2$] Lyman forest clouds toward $z \sim 3$ 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 \ion{C}{4} can be found in 75% of clouds with $N({\rm H~I}) > 3\ten{14}\cm2$ and more than 90% of those with $N({\rm H~I}) > 1.6\ten{15}\cm2$. Clouds with $N({\rm H~I}) > 10^{15}\cm2$ show a narrow range of ionization ratios, spanning less than an order of magnitude in \ion{C}{4}/\ion{H}{1}, \ion{C}{2}/\ion{C}{4}, \ion{Si}{4}/\ion{C}{4} and \ion{N}{5}/\ion{C}{4}, 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. \ion{Si}{4}/\ion{C}{4} 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 \ion{C}{4} systems for $\Delta v < 250\kms$, which we argue provides an upper limit to the clustering of \ion{H}{1} clouds. If the clouds are associated with galaxies, this requires a rapid evolution in galaxy clustering between $z = 3$ and $z = 0$.