研究成果・発表論文

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

Kitayama, T.,   Susa, H.,   Umemura, M.,   & Ikeuchi, S.


要旨
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 H$_2$ 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 T$_vir$åisebox-0.5ex~10$^4$K at weak UV intensities (J$_21$<\i̊sebox-0.5ex~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-shielding and H$_2$ 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 H$_2$ fraction reaches \rs̊ebox-0.5ex~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 \rae̊box-0.5ex~10$^9$M$_solar$ for clouds collapsing at redshifts z<\raib̊ox-0.5ex~3, but drops significantly at higher redshifts. In a conventional cold dark matter universe, the latter coincides roughly with that of the 1\ensuremathσ 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.






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