Constraints from the Lyα forest power spectrum

We use published measurements of the transmission power spectrum of the Ly alpha forest to constrain several parameters that describe cosmology and thermal properties of the intergalactic medium (IGM). A six-parameter grid is constructed using particle-mesh dark matter simulations together with scaling relations to make predictions for the gas properties. We fit for all parameters simultaneously and identify several degeneracies. We find that the temperature of the IGM can be well determined from the falloff of the power spectrum at small scales. We find a temperature of around 2 x 10(4) K, dependent on the slope of the gas equation of state. We see no evidence for evolution in the IGM temperature. We place constraints on the amplitude of the dark matter fluctuations. However, contrary to previous results, the slope of the dark matter power spectrum is poorly constrained. This is because of uncertainty in the effective Jeans smoothing scale, which depends on the temperature as well as the thermal history of the gas.