The Contribution of the Intergalactic Medium to Cosmic Microwave Background Anisotropies

We compute the power spectrum of the cosmic microwave background (CMB) temperature anisotropies generated by the intergalactic medium (IGM). To estimate the electron pressure along the line of sight and its contribution to the Sunyaev-Zel'dovich component of the CMB anisotropies, we assume that the nonlinear baryonic density contrast is well described by a lognormal distribution. For model parameters in agreement with observations and for an experiment operating in the Rayleigh-Jeans regime, the largest IGM contribution corresponds to scales l ≈ 2000. The amplitude is rather uncertain and could be as large as 100-200 μK2, comparable to the contribution of galaxy clusters. The actual value is strongly dependent on the gas polytropic index γ, the amplitude of the matter power spectrum σ8, namely, C ~ (γ2σ8)12. At all redshifts, the largest contribution comes from scales very close to the baryon Jeans length. This scale is not resolved in numerical simulations that follow the evolution of gas on cosmological scales. The anisotropy generated by the IGM could make compatible the excess of power measured by Cosmic Background Imager (CBI) on scales of l ≥ 2000 with σ8 = 0.9. Taking the CBI result as an upper limit, the polytropic index can be constrained to γ < 1.5 at 2 σ level at redshifts z ~ 0.1-0.4. With its large frequency coverage, the Planck satellite will be able to measure the secondary anisotropies coming from hot gas. Cluster and IGM contributions could be separated by cross-correlating galaxy/cluster catalogs with CMB maps. This measurement will determine the state of the gas at low and intermediate redshifts.