The Global 21 cm Absorption from Cosmic Dawn with Inhomogeneous Gas Distribution

We make an analytical estimate of the maximum 21 cm absorption signal from the cosmic dawn, taking into account the inhomogeneity of gas distribution in the intergalactic medium (IGM) due to non-linear structure formation. The gas located near halos is over-dense but adiabatically heated, while the gas far from halos is under-dense and hence cooler. The cumulative effect of adiabatic heating and cooling from this gas inhomogeneity results in a reduction in the maximum global 21 cm absorption depth by about 40% as compared with homogeneous IGM model, assuming saturated coupling between the spin temperature of neutral hydrogen (HI) and the adiabatic gas kinetic temperature.