The influence of Ωb 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, Omega(b) = 0.02, 0.05, and 0.125. Increasing Omega(b) 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 Lyalpha forest absorption are almost unaffected by changes in Omega(b) 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 Omega(b)(1.7), and the higher photoionization rate increases the gas temperature in low-density regions. Damped Lyalpha absorption and Lyman limit absorption both increase with increasing Omega(b), 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 Omega(b) but slower than Omega(b)(2), and the global star formation rate scales approximately as Omega(b)(1.5). In the higher Omega(b) 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 Omega(b) on high-redshift structure, and we provide scalings that can be used to estimate the impact of Omega(b) uncertainties on the predictions of hydrodynamic simulations.