The Low-Column Density Lyα Forest

Abstract

We develop an analytical method to compute the column density distribution of the Lyα forest in the low-column density limit. We compute the column density distributions for six different cosmological models and found that the standard, COBE-normalized CDM model cannot fit the observations of the Lyα forest at z = 3. The amplitude of the fluctuations in that model has to be lowered by a factor of almost 3 to match observations. However, the currently viable cosmological models like the slightly tilted COBE-normalized CDM+Λ model, the CHDM model with Ω_ν = 0.2, and the low-amplitude standard CDM model are all in agreement with observations for the value of the cosmological baryon density at or higher than the old standard big bang nucleosynthesis value of Ω_bh² = 0.0125 for the currently favored value of the ionizing radiation intensity. With the low value for the baryon density inferred by Rugers & Hogan, the models can only marginally match observations.