The Lyman Alpha Forest from Gravitational Collapse in the Cdm+λ Model

We use an Eulerian hydrodynamic cosmological simulation to model the Lyα forest in a spatially flat, COBE normalized, cold dark matter model with Ω = 0.4, and find that the intergalactic, photoionized gas is predicted to collapse into sheet-like and filamentary structures which give rise to absorption lines having similar characteristics as the observed Lyα forest. A typical filament is ∼ 1hMpc long with thickness ∼ 50 − 100h kpc (in proper units), and baryonic mass ∼ 10h M⊙. In comparison our nominal numerical resolution is (2.5, 9)h kpc (in the two simulations we perform) with true resolution, perhaps a factor of 2.5 worse than this; the nominal mass resolution is (10, 10)M⊙ in gas for the two simulations. The gas temperature 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 with the observed ones, and that their evolution is consistent with our model, if the ionizing background has (as expected from our simulations) an approximately constant intensity between z = 2 and z = 4. The 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 ∼ 80h kpc perpendicular to the line of sight for features in the Lyman alpha forest. 1 Institute for Advanced Study, Princeton, NJ 08540 2 Peyton Hall, Princeton University, Princeton, NJ 08544 3, California Institute of Technology, Pasadena, CA 91125 4 Hubble Fellow