Spectral Analysis of the Lyα Forest in a Cold Dark Matter Cosmology

Abstract

We simulate the Lyα forest in a standard cold dark matter universe using a two-level hierarchical grid code to evolve the dark and baryonic matter components self-consistently. We solve the time-dependent ionization equations for hydrogen and helium, adopting Haardt & Madau's recent estimate for the metagalactic UV radiation background. We compare our simulation results with the measured properties of the Lyα forest by constructing synthetic spectra and analyzing them using an automated procedure to identify, deblend, and fit Voigt line profiles to the absorption features. The H I column density and Doppler parameter distributions that we obtain agree closely with those measured by the Keck Observatory's High Resolution Eschelle Spectrograph (HIRES) and earlier high spectral resolution observations over the column density range 1012 cm-2 3.5). We also compare with measured values of the intergalactic He II opacity. Our results require an He II ionizing background lower than the Haardt & Madau estimate by a factor of 4, corresponding to a soft intrinsic QSO spectrum, with αQ ≈ 1.8-2.