Dependencies of QSO Lyman-alpha absorption line statistics on cosmological parameters

Abstract

We have performed high-resolution hydrodynamic simulations of the Lyman-alpha forest in a variety of popular cold dark matter dominated cosmologies, including a low density and a vacuum-dominated model. The fluctuation amplitude of these models is chosen to match the observed abundance of galaxy clusters at low redshift. We assume that the intergalactic medium is photoionized and photoheated by a uniform UV-background with the required amplitude to give the observed mean hydrogen absorption. We produce simulated spectra, analyze them by fitting Voigt profiles and compare line statistics with those obtained from high resolution observations. All models give column density distributions in good agreement with observations. However, the distribution of line widths (the b-parameter distribution) reflects differences in the temperature of the intergalactic medium between the models, with colder models producing more narrow lines. All the models with a low baryon density, Omega_b h^2=0.0125, are too cold to produce a b-parameter distribution in agreement with observations. Models with a higher baryon density, Omega_b h^2=0.025, are hotter and provide better fits. Peculiar velocities contribute significantly to the line widths in models with low matter density, and this improves the agreement with observations further. We briefly discuss alternative mechanisms for reconciling the simulations with the observed b-parameter distributions.