The Lyman-alpha Forest in the Cold Dark Matter Model

Abstract

Cosmological simulations with gas provide a detailed description of the intergalactic medium, making possible predictions of neutral hydrogen absorption in the spectra of background QSOs. We present results from a high-resolution calculation of an $Ω=1$ cold dark matter model. Our simulation reproduces many of the observed properties of the \lya forest surprisingly well. The distribution of HI column densities agrees with existing data to within a factor of $\sim$ two over most of the range from $10^{14}\cm^{-2}$ to $10^{22}\cm^{-2}$; i.e., from unsaturated \lya forest lines to damped \lya systems. The equivalent width distribution matches the observed exponential form with a characteristic width $W_* \approx 0.3$ angstroms. The distribution of b-parameters appears consistent with that derived from QSO spectra. Most of the low column density absorption arises in large, flattened structures of moderate or even relatively low overdensity, so there is no sharp distinction between the \lya forest and the ``Gunn-Peterson'' absorption produced by the smooth intergalactic medium. Our results demonstrate that a \lya forest like that observed develops naturally in a hierarchical clustering scenario with a photoionizing background. Comparison between simulations and high-resolution QSO spectra should open a new regime for testing theories of cosmic structure formation.