Destruction of Molecular Hydrogen during Cosmological Reionization

We investigate the ability of primordial gas clouds to retain molecular hydrogen (H 2) during the initial phase of the reionization epoch. We nd that before the Strr omgren spheres of the individual ionizing sources overlap, the UV background below the ionization threshold is able to penetrate large clouds and suppress their H 2 abundance. The consequent lack of H 2 cooling could prevent the collapse and fragmentation of clouds with virial temperatures T vir < 10 4 K (or masses < 10 8 M (1 + z vir)=10] 3=2). This negative feedback on structure-formation arises from the very rst ionizing sources, and precedes the feedback due to the photoionization heating.