Cosmological hydrodynamic simulations of the Lyman-alpha forest
Several three dimensional, multi-species, cosmological hydrodynamic simulations of the formation and evolution of Ly$\alpha$ forest absorbers were performed in three CDM-like cosmological models: the standard CDM model (SCDM), the flat CDM model with a nonzero cosmological constant ($\Lambda$CDM) and the open CDM model (OCDM). Synthetic spectra along random lines-of-sight were generated and analyzed by using a powerful spectrum analysis method designed to mimic observational data reduction. The SCDM model was studied in great detail and found to be in excellent agreement with observations in: the slope and shape of the column density distribution, the value and distribution of the Doppler parameter and the equivalent width, the evolution of line density, effective opacity and flux decrement, etc. A number of sensitive tests were devised to compare and contrast different cosmological models, and it is found that both the SCDM and the $\Lambda$CDM models agree with observations quite well, while the OCDM model fails to match most of the observational data satisfactorily. It is discovered that the low column density (log $N\sb{HI}\ \simeq$ 13.3 and log $N\sb{HeII}\ \simeq$ 14.5) H sc I and He sc II Ly$\alpha$ absorbers which are optically-thin at the line center ($\tau\sb0 <$ 1) actually originate from density fluctuations in the underdense regions--cosmic minivoids, which have lower temperature than is allowed by thermal equilibrium, expand faster than the cosmic expansion, and contain a significant percentage of baryons previously thought to be in the diffuse form. A new picture of IGM is established with a continuous spectrum of clumpiness: from the traditional high column density Ly$\alpha$ clouds, to intermediate column density Ly$\alpha$ sheets and filaments, to a new class of absorbers--Ly$\alpha$ voids--the under-brushes in the Ly$\alpha$ forest.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign