A SIMULATION OF LY-ALPHA ABSORPTION FORESTS IN LINEAR-APPROXIMATION OF COLD DARK-MATTER AND COLD PLUS HOT DARK-MATTER MODELS

Our goal in this paper is to test some popular dark matter models by m eans of the Ly alpha forest in QSO spectra. Recent observations of the size and velocity of Ly alpha forest clouds have indicated that the L y alpha absorption is probably not given by collapsed objects but, rat her, by precollapsed regions in the baryonic density field. Therefore, a linear approximation description would be able to provide valuable information. We developed a technique to simulate the Ly alpha forest as the absorption of such precollapsed regions under a linear approxim ation regime. The simulated Ly alpha forsts in the standard cold dark matter (SCDM) model, the cold plus hot dark matter (CHDM) model, and t he low-density flat cold dark matter (LCDM) model have been confronted with observational features including (1) the number density of Ly al pha lines and its dependencies on redshift and equivalent width, (2) t he distribution of equivalent widths and its redshift dependence, (3) clustering, and (4) the Gunn-Peterson effect. We find that the ''stand ard'' CHDM model, i.e., 60% cold dark matter, 30% hot dark matter, and 10% baryons, does not pass the Ly alpha forest test, probably because it produces structures too late and favors to forming structures on l arge scales instead of small-scale objects such as Ly alpha clouds. Wi thin a reasonable range of J(v), the UV background radiation at high r edshift, and delta(th), the threshold of the onset of gravitational co llapse of the baryonic matter, the LCDM model is consistent with obser vational data in all four aspects mentioned above. The SCDM model can also fit with observations, but it requires a smaller J(v) and a highe r delta(th). This suggests that whether or not a significant part of t he Ly alpha forest lines is located in the halos of collapsed objects would be crucial to the success of the SCDM model.