Formation of Subgalactic Clouds under Ultraviolet Background Radiation

The effects of the UV background radiation on the formation of sub-galacticclouds are studied by means of one-dimensional hydrodynamical simulations. Theradiative transfer of the ionizing photons due to the absorption by HI, HeI andHeII, neglecting the emission, is explicitly taken into account. We find thatthe complete suppression of collapse occurs for the clouds with circularvelocities typically in the range V_c \sim 15-40 km/s and the 50% reduction inthe cooled gas mass with V_c \sim 20-55 km/s. These values depend mostsensitively on the collapse epoch of the cloud, the shape of the UV spectrum,and the evolution of the UV intensity. Compared to the optically thin case,previously investigated by Thoul & Weinberg (1996), the absorption of theexternal UV photon by the intervening medium systematically lowers the abovethreshold values by \Delta V_c \sim 5 km/s. Whether the gas can contract orkeeps expanding is roughly determined by the balance between the gravitationalforce and the thermal pressure gradient when it is maximally exposed to theexternal UV flux. Based on our simulation results, we discuss a number ofimplications on galaxy formation, cosmic star formation history, and theobservations of quasar absorption lines. In Appendix, we derive analyticalformulae for the photoionization coefficients and heating rates, whichincorporate the frequency/direction-dependent transfer of external photons.Comment: 38 pages, 16 figures, accepted for publication in Ap