5.1. Ly Forest
The Ly forest evolves away dramatically from high to low redshift, as is strikingly clear from the spectra of z ~ 3 and z ~ 1 quasars in Figure 9. The evolution of the Ly lines with Wr(Ly) > 0.3 Å can be characterized by a double power law with ~ 2 for 1.8 < z < 4.5 and ~ 0.2 for z <1.8. Help in understanding the physical picture has come from sophisticated N-body/hydrodynamic simulations that incorporate the gas physics and consider cosmological expansion of the simulation box. The dynamical evolution of the HI gas can be described as outflow from the centers of voids to their surrounding shells, and flows along these sheets toward their intersections where the densest structures form. This picture is consistent with observational determinations of the ``sizes'' of Ly structures. It is difficult to obtain direct measurements of sizes except in some special cases to use ``double lines of sight'', close quasar pairs, either physical or apparent due to gravitational lensing. If the spectra of the two quasars both have a Ly absorption line at the same wavelength that implies a ``structure'' which covers both lines of sight. From these studies, it is found that ``structures'' are at least hundreds of kpc in extent.
Figure 9. Illustration of structure evolution of intergalactic gas from high to low redshift. The upper spectrum of a z = 3.6 quasar is a Keck/HIRES observation, while the lower spectrum is a FOS/HST observations of a z = 1.3 quasar. Higher redshift quasars show a much thicker forest of Ly lines. Slices through N-body/hydrodynamic simulation results at the two epochs z = 3 and z = 1 are shown in the right-hand panels. Three contour levels are shown: 1011 cm-2 (dotted lines), 1012 cm-2 (solid lines) and 1013 cm-2 (thick solid lines). Evolution proceeds so that the voids become more empty so that even the low column density material is found in filamentary structures at low redshifts.
At redshifts z = 5 to z = 2 dN/dz for Ly forest absorption is quite large, but it is declining very rapidly over that range. This dramatic evolution in the number of forest clouds is mostly due to the expansion of the universe, with a modest contribution from structure growth. At z < 2, the extragalactic background radiation field is falling, and Ly structures are becoming more neutral. Therefore, the more numerous, smaller N(H) structures are observed at a larger N(HI) and this will counteract the effect of expansion, thus slowing the decline of the forest.
The high redshift Ly forest was once thought to be primordial material, but in fact it is observed to have a metallicity of 0.1% solar, even at z = 3. For N(HI) < 1014 cm-2, the expected N(C IV) would be below the detection thresholds of current observations, so truly pristine material still eludes us. Perhaps it does not exist. To spread metals all through the intergalactic medium may have required a ``pre-galactic'' population of stars at z > 10 that polluted all of intergalactic space.