THE LY-ALPHA FOREST FROM GRAVITATIONAL COLLAPSE IN THE COLD DARK-MATTER PLUS LAMBDA-MODEL

We use an Eulerian hydrodynamic cosmological simulation to model the L y alpha forest in a spatially flat, COBE-normalized, cold dark matter model with Omega = 0.4. We find that the intergalactic, photoionized g as is predicted to collapse into sheetlike and filamentary structures which give rise to absorption lines having characteristics similar to the observed Ly alpha forest. A typical filament is similar to 500 h(- 1) kpe long with thickness similar to 50 h(-1) kpc (in proper units), and baryonic mass similar to 10(10) h(-1) M(.). In comparison our cell size is (2.5, 9) h(-1) kpc in the two simulations we perform, with tr ue resolution perhaps a factor of 2.5 worse than this. The gas tempera ture is in the range 10(4)-10(5) K, and it increases with time as stru ctures with larger velocities collapse gravitationally. We show that t he predicted distributions of column densities, b-parameters, and equi valent widths of the Ly alpha forest clouds agree reasonably with obse rvations, and that their evolution is consistent with the observed evo lution, if the ionizing background has an approximately constant inten sity between z = 2 and z = 4. A new method of identifying lines as con tiguous regions in the spectrum below a fixed flux threshold is sugges ted to analyze the absorption lines, given that the Ly alpha spectra a rise from a continuous density held of neutral hydrogen rather than di screte clouds. We also predict the distribution of transmitted flux an d 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 similar to 80 h(-1) kpc perpendicular to the line of sight for features in the Ly alpha forest. In order to reproduce the observe d number of lines and average flux transmission, the baryon content of the clouds may need to be significantly higher than in previous model s because of the low densities and large volume-filling factors we pre dict. If the background intensity J(Hl) is at least that predicted fro m the observed quasars, Omega(b) needs to be as high as similar to 0.2 5 h(-2). The model also predicts that most of the baryons at z > 2 are in Ly alpha clouds, and that the rate at which the baryons move to mo re overdense regions is slow. A large fraction of the baryons which ar e not observed at present in galaxies might be intergalactic gas in th e currently collapsing structures, with T similar to 10(5)-10(6) K. Al l our results on the statistical properties of the simulated spectra a re predictions that can be directly tested by applying the same method s to observed spectra. We are making the simulated spectra electronica lly available.