Statistical characteristics of the observed Ly α forest and the shape of initial power spectrum

Abstract

Properties of approximately 4500 observed Ly α absorbers are investigated using the model of formation and evolution of dark matter (DM) structure elements based on the modified Zel’dovich theory. This model is generally consistent with simulations of absorber formation, describes the large-scale structure (LSS) observed in the galaxy distribution at small redshifts reasonably well and emphasizes the generic similarity of the LSS and absorbers. The simple physical model of absorbers asserts that they are composed of DM and gaseous matter. It allows us to estimate the column density and overdensity of DM and gaseous components and the entropy of the gas trapped within the DM potential wells. The parameters of the DM component are found to be consistent with theoretical expectations for the Gaussian initial perturbations with the warm dark matter-like power spectrum. The basic physical factors responsible for the evolution of the absorbers are discussed. The analysis of redshift distribution of absorbers confirms the self-consistency of the adopted physical model, Gaussianity of the initial perturbations and allows one to estimate the shape of the initial power spectrum at small scales that, in turn, restricts the mass of the dominant fraction of DM particles to M DM 1.5‐5 keV. Our results indicate possible redshift variations of intensity of the ultraviolet background by approximately a factor of 2‐3 at redshifts z ∼ 2‐3. One of the most promising methods for studying the processes responsible for the formation and evolution of the structure of the Universe is the analysis of properties of absorbers observed in the spectra of the furthest quasars. The great potential of such investigations has been discussed by Oort (1981, 1984) just after Sargent et al. (1980) established the intergalactic nature of the Ly α forest. The available Keck and VLT high-resolution observations of the forest provide a reasonable data base and allow one to apply statistical methods for such investigations. The essential progress achieved recently through numerous highresolution simulations of absorber formation and evolution confirms that this process is closely connected with the initial power spectrum of perturbations. These results allow us to consider the properties of absorbers in the context of the non-linear theory of gravitational instability (Zel’dovich 1970; Shandarin & Zel’dovich 1989) and to apply the statistical description of structure formation and evolu