Probing the universe with the Lyman-alpha forest: II. The column density distribution

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  • 27 June 1997
Abstract
I apply the well controlled Hydro-PM approximation of Gnedin & Hui to model the column density distribution of the Lyman-alpha forest for 25 different flat cosmological scenarios, including variants of the standard CDM, tilted CDM, CDM with a cosmological constant, and CHDM models. I show that within the accuracy of the HPM approximation the slope of the column density distribution reflects the degree of nonlinearity of the cosmic gas distribution and is a function of the rms linear density fluctuation at the characteristic filtering scale only. The amplitude of the column density distribution, expressed as the value for the ionizing intensity, is derived as a function of the cosmological parameters (to about 30% accuracy). The observational data are currently consistent with the value for the ionizing intensity both being constant in the redshift interval z~2-4 and peaking by about 30% at z~3. The majority (if not all) of flat cosmological models have difficulty in simultaneously achieving sufficiently low power spectrum amplitude at the cluster scale to fit the observed cluster abundances, and sufficiently high amplitude at the Lyman-alpha forest scale to reproduce the observed shallow column density distribution. While the discrepancy is not statistically significant at the moment, if confirmed in the future, it may indicate that there exist another population of objects contributing to the unsaturated Lyman-alpha absorption along lines of sight towards distant quasars.

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