  To elucidate the effects of the UV background radiation on the collapse of
pregalactic clouds, we implement a radiation-hydrodynamical calculation,
combining one-dimensional spherical hydrodynamics with an accurate treatment of
the radiative transfer of ionizing photons. Both absorption and scattering of
UV photons are explicitly taken into account. It turns out that a gas cloud
contracting within the dark matter potential does not settle into hydrostatic
equilibrium, but undergoes run-away collapse even under the presence of the
external UV field. The cloud center is shown to become self-shielded against
ionizing photons by radiative transfer effects before shrinking to the rotation
barrier. Based on our simulation results, we further discuss the possibility of
H2 cooling and subsequent star formation in a run-away collapsing core. The
present results are closely relevant to the survival of subgalactic Population
III objects as well as to metal injection into intergalactic space.
