Probing the universe with the Lyman-alpha forest: III. The mean opacity of the IGM

I apply the well controlled Hydro-PM approximation of Gnedin & Hui to study the consistency between the column density distribution of the Lyman-alpha forest and the mean opacity of the IGM for 25 different flat cosmological scenarios, including variants of the standard CDM, tilted CDM, CDM with a cosmological constant, and CHDMmodels. I show that observational data of Press, Rybicki, and Schneider are not compatible with the hypothesis that the Lyman-alpha forest arises from fluctuations in the low density IGM, whereas the data of Zuo and Lu and Lu et al. are in a full agreement with the level of the ionizing intensity required to reproduce the observed column density distribution of the Lyman-alpha forest. Thus, either the data of Press, Rybicki, and Schneider are contaminated, or the hypothesis that the Lyman-alpha forest arises from density fluctuations in the IGM is invalid.