The morphology of cosmological reionization by means of Minkowski functionals

The morphology of the total gas and the neutral hydrogen (H I) distributions during the cosmological epoch of reionization can be quantified with Minkowski functionals (MFs) of isodensity surfaces. We compute the MFs from the output of a high-resolution numerical simulation which includes explicit treatment of transfer of ultraviolet (UV) ionizing radiation. 'Galaxies' identified in the simulation using semi-analytic models of galaxy formation are assumed to be the sole sources of UV photons. The MFs of the total gas distribution are well described by the analytic expressions derived for lognormal random fields. The statistical properties of the diffuse H I depend on the gas distribution and on the way ionized regions propagate in the intergalactic medium (IGM). The deviations of the MFs of the H I distribution from those of a lognormal random field are, therefore, caused by reionization. We use the MFs to discriminate between the various stages of reionization in the simulation. We suggest a simple model of reionization which reproduces the MFs derived from this simulation. Using random realizations of lognormal density fields, we also assess the ability of MFs to distinguish between different reionization scenarios. Our results are relevant to the analysis of future cosmological 21-cm maps.