Spectral Analysis of the Lyman Alpha Forest in a Cold Dark Matter Cosmology
Abstract
We simulate the Lya forest in a standard CDM universe using a 2-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 the Haardt & Madau (1996) estimate for the metagalactic UV radiation background. We compare our simulation results with the measured properties of the Lya forest by constructing synthetic spectra which we analyze in a manner similar to the method used by observers. The HI column density and Doppler parameter distributions we obtain agree closely with those measured by the Keck HIRES and earlier high spectral resolution observations over the column density range 10^{12} < NHI < 10^{16}. In particular, we find a power-law column density distribution persists to the lowest values reported from the HIRES measurements, in agreement with the incompleteness-corrected observations. We find evolution in the cloud number density and opacity comparable to the observed evolution. Our Doppler parameters are somewhat smaller and more narrowly distributed than found in similar SPH simulations. It is unclear at present whether this is due to differences in the line-finding and fitting procedures, or reflects a genuine difference in the numerical techniques.Keywords
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