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Early in the reionization process, the intergalactic medium (IGM) would havebeen quite inhomogeneous on small scales, due to the low Jeans mass in theneutral IGM and the hierarchical growth of structure in a cold dark matterUniverse. This small-scale structure acted as an important sink during theepoch of reionization, impeding the progress of the ionization fronts thatswept out from the first sources of ionizing radiation. Here we present resultsof high-resolution cosmological hydrodynamics simulations that resolve thecosmological Jeans mass of the neutral IGM in representative volumes severalMpc across. The adiabatic hydrodynamics we follow are appropriate in anunheated IGM, before the gas has had a chance to respond to the photoionizationheating. Our focus is determination of the resolution required in cosmologicalsimulations in order to sufficiently sample and resolve small-scale structureregulating the opacity of an unheated IGM. We find that a dark matter particlemass of m_dm < 50 Msun and box size of L > 1 Mpc are required. With ourconverged results we show how the mean free path of ionizing radiation andclumping factor of ionized hydrogen depends upon the ultraviolet background(UVB) flux and redshift. We find, for example at z = 10, clumping factorstypically of 10 to 20 for an ionization rate of Gamma ~ 0.3 - 3 x 1e-12 s^-1,with corresponding mean free paths of ~ 3 - 15 Mpc, extending previous work onthe evolving mean free path to considerably smaller scales and earlier times.

The Opacity of the Intergalactic Medium During Reionization: Resolving Small-Scale Structure