THE LYMAN-ALPHA FOREST IN THE COLD DARK-MATTER MODEL

Cosmological simulations with gas provide a detailed description of th e intergalactic medium, which makes possible predictions of neutral hy drogen absorption in the spectra of background QSOs. We present result s from a high-resolution calculation of an Omega = 1 cold dark matter model. Our simulation reproduces many of the observed properties of th e Ly alpha forest surprisingly well. The distribution of H I column de nsities agrees with existing data to within a factor of similar to 2 o ver most of the range from 10(14) cm(-2) to 10(22) cm 2; i.e., from un saturated Ly alpha forest lines to damped Ly alpha systems. The equiva lent width distribution matches the observed exponential form with a c haracteristic width W-double dagger approximate to 0.3 Angstrom. The d istribution 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 Ly alpha forest and the ''Gunn-Peterson'' absorption produced by the smooth intergalactic medi um. Our results demonstrate that a Ly alpha forest like that observed develops naturally in a hierarchical clustering scenario with a photoi onizing background. Comparison between simulations and high-resolution QSO spectra should open a new regime for testing theories of cosmic s tructure formation.