What causes the Ly-alpha forest, clouds or large-scale velocity fields ?

Abstract

We show that in stochastic large scale velocity fields superposed on the general Hubble flow, the formation of the GP-depression in QSO spectra is intimately related to the formation of the absorption-line structure usually called `Ly-alpha forest'. Therefore the HI-density in the diffuse IGM might be substaintially underestimated if one determines the GP-effect from the apparent continuum in high resolution spectra of QSOs. Our tentative calculations imply a current baryon density Omega_b h^2_{100} = 0.015 which agrees well with the baryon density determination from the deuterium abaundance measurement by Levshakov, Kegel and Takahara (the same volume).