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 means of the Lyα forest in QSO spectra. Recent observations of the size and velocity of Lyα forest clouds have indicated that the Lyα absorption is probably not given by collapsed objects but, rather, 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 Lyα forest as the absorption of such precollapsed regions under a linear approximation regime. The simulated Lyα forests in the standard cold dark matter (SCDM) model, the cold plus hot dark matter (CHDM) model, and the low-density flat cold dark matter (LCDM) model have been confronted with observational features including (1) the number density of Lyα lines and its dependencies on redshift and equivalent width, (2) the distribution of equivalent widths and its redshift dependence, (3) clustering, and (4) the Gunn-Peterson effect. We find that the "standard" CHDM model, i.e., 60% cold dark matter, 30% hot dark matter, and 10% baryons, does not pass the Lyα forest test, probably because it produces structures too late and favors to forming structures on large scales instead of small-scale objects such as Lyα clouds. Within a reasonable range of Jv, the UV background radiation at high redshift, and δth, the threshold of the onset of gravitational collapse of the baryonic matter, the LCDM model is consistent with observational data in all four aspects mentioned above. The SCDM model can also fit with observations, but it requires a smaller Jv and a higher δth This suggests that whether or not a significant part of the Lyα forest lines is located in the halos of collapsed objects would be crucial to the success of the SCDM model.