Universal gas density and temperature profile |
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Authors: | E. Komatsu U. Seljak |
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Affiliation: | 1Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan; 2Center for Computational Physics, University of Tsukuba, Tsukuba 305-8577, Japan; 3Department of Physics, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan |
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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 T vir∼104 K at weak UV intensities ( J 21≲10−2) , owing 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 104 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 ∼ 109 M⊙ for clouds collapsing at redshifts z ≲3 , but drops significantly at higher redshifts. In a conventional cold dark matter universe, the latter coincides roughly with that of the 1 σ 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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Keywords: | molecular processes radiative transfer galaxies: formation cosmology: theory diffuse radiation |
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