Criteria for the formation of Population III objects in the ultraviolet background radiation

We explore possibilities of collapse and star formation in Population III o bjects exposed to the external ultraviolet background (UVB) radiation. Assu ming spherical symmetry, we solve self-consistently radiative transfer of p hotons, non-equilibrium H-2 chemistry and gas hydrodynamics. Although the U VB does suppress the formation of low-mass objects, the negative feedback t urns out to be weaker than previously suggested. In particular, the cut-off scale of collapse drops significantly below the virial temperature T-vir s imilar to 10(4) K at weak UV intensities (J(21) less than or similar to 10( -2)), Owing to both self-shielding of the gas and H-2 cooling. Clouds above this cut-off tend to contract highly dynamically, further promoting self-s hielding and H2 formation. For plausible radiation intensities and spectra, the collapsing gas can cool efficiently to temperatures well below 104 K b efore rotationally supported and the final H2 fraction reaches similar to 1 0(-3). Our results imply that star formation can take place in low-mass objects co llapsing in the UVB. The threshold baryon mass for star formation is simila r to 10(9) M circle dot for clouds collapsing at redshifts z less than or s imilar to3, but drops significantly at higher redshifts. In a conventional cold dark matter universe, the latter coincides roughly with that of the 1 sigma density fluctuations. Objects near and above this threshold can thus constitute 'building blocks' of luminous structures, and we discuss their l inks to dwarf spheroidal/elliptical galaxies and faint blue objects. These results suggest that the UVB can play a key role in regulating the star for mation history of the Universe.