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

Abstract

We explore possibilities of collapse and star formation in Population III objects exposed to the external ultraviolet background (UVB) radiation. Assuming spherical symmetry, we solve self-consistently radiative transfer of photons, non-equilibrium H2 chemistry, and gas hydrodynamics. Although the UVB does suppress the formation of low mass objects, the negative feedback turns out to be weaker than previously suggested. In particular, the cut-off scale of collapse drops significantly below the virial temperature 10^4 K at weak UV intensities, due to both self-shielding of the gas and H2 cooling. Clouds above this cut-off tend to contract highly dynamically, further promoting self-shielding and H2 formation. For plausible radiation intensities and spectra, the collapsing gas can cool efficiently to temperatures well below 10^4 K before rotationally supported and the final H2 fraction reaches 10^{-3}. Our results imply that star formation can take place in low mass objects collapsing in the UVB. The threshold baryon mass for star formation is \sim 10^9 solar mass for clouds collapsing at redshifts z \simlt 3, 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 links to dwarf spheroidal/elliptical galaxies and faint blue objects. These results suggest that the UVB can play a key role in regulating the star formation history of the Universe.