Lyman-alpha clouds as a relic of primordial density fluctuations

We develop a model in which Lyα clouds are probes of primordial density fluctuations. We argue that they are pressure-driven relics whose expansion is triggered by photoionization at z ~ 4. This model finds its natural setting in the class of cold dark matter models with OMEGA = 1 and biased galaxy formation in which density peaks of subgalactic mass go nonlinear at z ~ 5. The more massive clouds are confined by dark matter and form dwarf galaxies, while the abundance and dimensions of the smaller clouds are comparable to those of the Lyα systems. A simple model of the photoionization history of the universe enables us to calculate the redshift evolution of the neutral hydrogen fraction in these clouds and simultaneously satisfy the Gunn-Peterson constraint on the ionization of the IGM. We present hydrodynamic computations of cloud histories in OMEGA = 1 CDM-dominated and baryon-dominated universes. By combining these results with the statistical properties of peaks in the Gaussian primordial density field, we determine the H I column density distribution as a function of NH I_ and redshift, and estimate the correlation function of the clouds. The observed cloud parameters come out naturally in the CDM model and suggest that Lyα clouds are the missing link between primordial density fluctuations and the formation of galaxies.