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Ionization fronts, the sharp radiation fronts behind which H/He ionizingphotons from massive stars and galaxies propagate through space, wereubiquitous in the universe from its earliest times. The cosmic dark ages endedwith the formation of the first primeval stars and galaxies a few hundred Myrafter the Big Bang. Numerical simulations suggest that stars in this era werevery massive, 25 - 500 solar masses, with H II regions of up to 30,000light-years in diameter. We present three-dimensional radiation hydrodynamicalcalculations that reveal that the I-fronts of the first stars and galaxies wereprone to violent instabilities, enhancing the escape of UV photons into theearly intergalactic medium (IGM) and forming clumpy media in which supernovaelater exploded. The enrichment of such clumps with metals by the firstsupernovae may have led to the prompt formation of a second generation oflow-mass stars, profoundly transforming the nature of the first protogalaxies.Cosmological radiation hydrodynamics is unique because ionizing photons coupledstrongly to both gas flows and primordial chemistry at early epochs,introducing a hierarchy of disparate characteristic timescales whose relativemagnitudes can vary greatly throughout a given calculation. We describe theadaptive multistep integration scheme we have developed for the self-consistenttransport of both cosmological and galactic ionization fronts.

Radiation Hydrodynamical Instabilities in Cosmological and Galactic Ionization Fronts