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The influence of Omega(b) on high-redshift structure

We analyze high-redshift structure in three hydrodynamic simulations that have identical initial conditions and cosmological parameters and differ only in the value of the baryon density parameter, Ωb = 0.02, 0.05, and 0.125. Increasing Ωb does not change the fraction of baryons in the diffuse (unshocked) phase of the intergalactic medium, but it increases cooling rates and therefore transfers some baryons from the shocked intergalactic phase to the condensed phase associated with galaxies. Predictions of Lyα forest absorption are almost unaffected by changes in Ωb at velocity scales greater than 5 km s-1 (our resolution limit), provided that the UV background intensity is adjusted so that the mean opacity of the forest matches the observed value. The required UV background intensity scales as Ω, and the higher photoionization rate increases the gas temperature in low-density regions. Damped Lyα absorption and Lyman limit absorption both increase with increasing Ωb, although the impact is stronger for damped absorption and is weaker at z = 4 than at z = 2-3. The mass of cold gas and stars in high-redshift galaxies increases faster than Ωb but slower than Ω, and the global star formation rate scales approximately as Ω. In the higher Ωb models, the fraction of baryonic material within the virial radius of dark matter halos is usually higher than the universal fraction, indicating that gasdynamics and cooling can lead to an overrepresentation of baryons in virialized systems. On the whole, our results imply a fairly intuitive picture of the influence of Ωb on high-redshift structure, and we provide scalings that can be used to estimate the impact of Ωb uncertainties on the predictions of hydrodynamic simulations.