The history of the baryon budget: Cosmic logistics in a hierarchical universe

Using a series of high-resolution N-body hydrodynamical numerical simulations, we investigate several scenarios for the evolution of the baryon budget in galactic halos. We derive individual halo star formation history (SFH), as well as the global star formation rate in the universe. We develop a simple analytical model that allows us to compute surprisingly accurate predictions, when compared to our simulations, but also to other simulations presented in Springel & Hernquist (2003b, MNRAS, 339, 312). The model depends on two main parameters: the star formation time scale t* and the wind efficiency η_w. We also compute, for halos of a given mass, the baryon fraction in each of the following phases: cold disc gas, hot halo gas, and stars. Here again, our analytical model predictions are in good agreement with simulation results, if one correctly takes finite resolution effect into account. We compare predictions of our analytical model to several observational constraints and conclude that a very narrow range of the model parameters is allowed. The important role played by galactic winds is outlined, as well as a possible "superwind" scenario in groups and clusters. The "anti-hierarchical" behavior of observed SFH is reproduced well by our best model with t* = 3 Gyr and η_w = 1.5. We obtain in this case a present-day cosmic baryon budget of Ω* ≃ 0.004, Ω_cold ≃ 0.0004, Ω_hot ≃ 0.01 and Ω_back ≃ 0.02 (diffuse background).