The Lyman Alpha Forest from Gravitational Collapse in the CDM+Lambda Model

Abstract

We model the $\lya$ forest in a spatially flat, CDM model with $Ω=0.4$, with an Eulerian hydrodynamic simulation, and find that the intergalactic, photoionized gas collapses into sheet-like and filamentary structures which produce absorption lines like those observed in the $\lya$ forest. A typical filament is $\sim 1 h^{-1} \mpc$ long with thickness $\sim 50-100 h^{-1} \kpc$ (in proper units), and baryonic mass $\sim 10^{10} h^{-1} \msun$. The gas temperature increases with time as structures with larger velocities collapse gravitationally. The predicted distributions of column densities and b-parameters of the absorption lines agree with the observed ones, and their evolution is consistent with our model if the ionizing background has an approximately constant intensity between $z=2$ and $z=4$. We also predict the distribution of transmitted flux and its correlation along a spectrum and on parallel spectra, and the \heii flux decrement as a function of redshift. We predict a correlation length of $\sim 80 h^{-1}\kpc$ perpendicular to the line of sight for features in the Lyman alpha 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 we predict. If the background intensity $\jhi$ is at least that predicted from the observed quasars, $Ω_b$ needs to be $\sim 0.025 h^{-2}$, higher than expected by light element nucleosynthesis; the model also predicts that most of the baryons at $z>2$ are in $\lya$ clouds, and that the rate at which the baryons move to more overdense regions is slow.