The relation between QSO absorption systems and high redshift galaxies

The relation between high redshift galaxies and QSO absorption systems is discussed in the context of hierarchical galaxy formation. It is demonstrated that imprints of the reionization history of the universe are detectable in the $b$-parameter distribution of the \Lya forest at redshifts two to four, favoring models in which hydrogen and helium are reionized simultaneously at or before redshift five by a quasar-like spectrum of UV photons. Hydrodynamical simulations including star formation, feedback due to supernovae and chemical enrichment are also presented. Energy feedback in form of kinetic energy can give rise to an efficient transport of metals out to distances of a few hundred kpc. The observational signature of this metal transport mechanism compared to simple homogeneous enrichment models is discussed. Finally, it is shown that present day $L^*$ galaxies typically have several progenitors at $z\approx3$ spread over a few hundred kpc. These progenitors are closely associated with damped \Lya and Ly-limit systems, and have velocity dispersions, luminosities and colors comparable to U-dropout galaxies at $z\approx 3$.