Statistical characteristics of observed Ly-$α$ forest and the shape of linear power spectrum

Abstract

Properties of $\sim$ 6 000 Ly-$α$ absorbers observed in 19 high resolution spectra of QSOs are investigated using the model of formation and evolution of DM structure elements based on the Zel'dovich theory. This model asserts that absorbers are formed in the course of both linear and nonlinear adiabatic or shock compression of dark matter (DM) and gaseous matter. It allows us to link the column density and overdensity of DM and gaseous components with the observed column density of neutral hydrogen, redshifts and Doppler parameters of absorbers and demonstrates that at high redshifts we observe a self similar period of structure evolution with the Gaussian initial perturbations. We show that the colder absorbers are associated with rapidly expanded regions of a galactic scale which represent large amplitude negative density perturbations. We extend and improve the method of measuring the power spectrum of initial perturbations proposed in Demiański & Doroshkevich (2003b). Our method links the observed separations and the DM column density of absorbers with the correlation function of the initial velocity field. We recover the cold dark matter (CDM) like power spectrum at scales 10> D > 0.15Mpc/h with a precision of ~15%. However at scales $\sim 3 - 150 h^{-1}$kpc the measured and CDM--like spectra are different. This result suggests a possible complex inflation with generation of excess power at small scales.