The Ly alpha Forest from Gravitational Collapse in the Cold Dark Matter + Lambda Model

We use an Eulerian hydrodynamic cosmological simulation to model the Lyα forest in a spatially flat, COBE-normalized, cold dark matter model with {OMEGA} = 0.4. We find that the intergalactic, photoionized gas is predicted to collapse into sheetlike and filamentary structures which give rise to absorption lines having characteristics similar to the observed Lyα forest. A typical filament is ~500 h^-1^ kpc long with thickness ~50 h^-1^ kpc (in proper units), and baryonic mass ~10^10^ h^- 1^ M_sun_. In comparison our cell size is (2.5,9)h^-1^ kpc in the two simulations we perform, with true resolution perhaps a factor of 2.5 worse than this. The gas temperature is in the range 10^4^-10^5^ K, and it increases with time as structures with larger velocities collapse gravitationally. We show that the predicted distributions of column densities, b-parameters, and equivalent widths of the Lyα forest clouds agree reasonably with observations, and that their evolution is consistent with the observed evolution, if the ionizing background has an approximately constant intensity between z = 2 and z = 4. A new method of identifying lines as contiguous regions in the spectrum below a fixed flux threshold is suggested to analyze the absorption lines, given that the Lyα spectra arise from a continuous density field of neutral hydrogen rather than discrete clouds. We also predict the distribution of transmitted flux and its correlation along a spectrum and on parallel spectra, and the He II flux decrement as a function of redshift. We predict a correlation length of ~80 h^-1^ kpc perpendicular to the line of sight for features in the Lyα forest. In order to reproduce the observed number of lines and average flux transmission, the baryon content of the clouds may need to be significantly higher than in previous models because of the low densities and large volume-filling factors we predict. If the background intensity J_H I_ is at least that predicted from the observed quasars, {OMEGA}_b_ needs to be as high as ~0.25 h^-2^. The model also predicts that most of the baryons at z > 2 are in Lyα clouds, and that the rate at which the baryons move to more overdense regions is slow. A large fraction of the baryons which are not observed at present in galaxies might be intergalactic gas in the currently collapsing structures, with T~10^5^- 10^6^ K. All our results on the statistical properties of the simulated spectra are predictions that can be directly tested by applying the same methods to observed spectra. We are making the simulated spectra electronically available.