A Multispecies Model for Hydrogen and Helium Absorbers in Lyman-Alpha Forest Clouds

We have performed a three-dimensional multispecies hydrodynamical simulation of the formation and evolution of Lyα clouds in a flat cold dark matter (CDM) dominated universe with an external flux of ionizing radiation. We solve the fully coupled nonequilibrium rate equations for the following species: H, H+, H-, H2, H2+, He, He+, He++, and e-. The statistical properties of the distribution and evolution of both hydrogen and helium absorption lines are extracted and compared to observed data. We find excellent agreement for the following neutral hydrogen data: the distribution of column densities is fitted well by a power law with exponent β = 1.55, with a possible deficiency of lines above column density 1015 cm-2; the integrated distribution matches observed data over a broad range of column densities, 1013-1017 cm-2; a Gaussian statistical fit to the Doppler parameter distribution yields a median of 35.6 km s-1; the evolution of the number of clouds with column densities larger than 1014 cm-2 follows a power law with exponent γ = 2.22. Analogous calculations are presented for He II absorption lines, and we find the ratio of Doppler parameters bHe II/bH I = 0.87. Our data also suggest that Lyα clouds may belong to two morphologically different groups: small column density clouds which tend to reside in sheets or filamental structures and are very elongated and/or flattened, and the large column density clouds which are typically found at the intersections of these caustic structures and are quasi-spherical.