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

We have performed a multi-species hydrodynamical simulation of the formation and evolution of Ly clouds in a at CDM dominated universe with an external ux of ionizing radiation. We solve the fully coupled non-equilibrium rate equations for the following species: H, H + , H , H 2 , H + 2 , 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 nd excellent agreement for the following neutral hydrogen data: the distribution of column densities is t well by a power law with exponent = 1:55 with a possible de ciency of lines above column density 10 15 cm 2 ; the integrated distribution matches observed data over a broad range of column densities 10 13 to 10 17 cm 2 ; a Gaussian statistical t 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 10 14 cm 2 follows a power law with exponent = 2:22. Analogous calculations are presented for HeII absorption lines and we nd the ratio of Doppler parameters b HeII =b HI = 0:87. Our data also suggests that Ly clouds may belong to two morphologically di erent groups: small column density clouds which tend to reside in sheets or lamental structures and are very elongated and/or attened, and the large column density clouds which are typically found at the intersections of these caustic structures and are quasi-spherical.