6. DIM ICEBERGS

Supercomputer simulations that incorporate hydrodynamics have been able to reproduce many of the large scale features of the universe. Cold dark matter modulates the formation of a filamentary web of low column density gas (e.g. Ostriker & Cen 1996). Quasar absorption can be used to trace the distribution and metallicity of this gas over nearly ten orders of magnitude in column density. Figure 4 shows the different scales probed by hydrogen absorbers of different column densities. The highest column densities correspond to damped Lyman- lines due to galaxy disks or their progenitors at high redshift. More modest column densities trace metal-enriched gas in the extended halos of bright galaxies. At z ~ 2 most of the baryons in the universe are in diffuse structures of about 10¹⁴ atoms cm^-2 (Hernquist et al. 1996; Miralda-Escude et al. 1996). Most of this gas has probably been heated to a high temperature by the present epoch (Cen & Ostriker 1998). Galaxies are the whitecaps that float on this churning sea of diffuse baryons and dark matter.

Figure 4. The column density and characteristic size of hydrogen quasar absorbers over ten decades in column density. The dot-dashed line is the level at which individual absorbers blend into a continuously fluctuating Gunn-Peterson effect. The dashed line is the mass density in luminous material measured locally.