DESTRUCTION OF MOLECULAR-HYDROGEN DURING COSMOLOGICAL REIONIZATION

We investigate the ability of primordial gas clouds to retain molecula r hydrogen CPI,) during the initial phase of the reionization epoch. W e find that before the Stromgren spheres of the individual ionizing so urces overlap, the UV background below the ionization threshold is abl e to penetrate large clouds and suppress their H-2 abundance. The cons equent lack of H-2 cooling could prevent the collapse and fragmentatio n of clouds with virial temperatures T-vir less than or similar to 10( 4) K (or masses less than or similar to 10(8) M.[(1 + z(vir))/10](-3/2 )). This negative feedback on structure formation arises from the very first ionizing sources and precedes the feedback due to the photoioni zation heating.