Criteria for the formation of Population III objects in the ultraviolet background radiation
Criteria for the formation of Population III objects in the ultraviolet background radiation
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紫外线背景辐射中第三族天体的形成标准
DOI：
10.1111/j.1365-2966.2001.04669.x
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发表时间：
2001
影响因子：
4.8
通讯作者：
S. Ikeuchi
中科院分区：
文献类型：
--
作者：
T. Kitayama;H. Susa;M. Umemura;S. Ikeuchi
我们探索了暴露在外部紫外线背景(UVB)辐射下的种群III天体坍塌和恒星形成的可能性。在球对称的假设下，我们解决了光子的自洽辐射传输、非平衡态氢气化学和气体流体力学。尽管UVB确实抑制了低质量天体的形成，但负反馈比之前提出的要弱。特别是，由于气体的自屏蔽和氢的冷却，在弱紫外光强度下(J21∼10≲2)，崩塌的截止尺度显著下降到维里温度TEIR−104K以下。这一截止点上方的云层倾向于高度动态地收缩，进一步促进了自我屏蔽和H2的形成。对于可信的辐射强度和光谱，坍塌气体可以有效地冷却到远低于104K的温度，然后旋转支撑，最终氢分数达到∼-10−3。
我们的结果表明，恒星的形成可以发生在在UVB中坍塌的低质量物体中。恒星形成的门槛重子质量是红移z∼3处塌缩的云的重子质量为109m⊙，但在红移较高时显著下降。在传统的冷暗物质宇宙中，后者与1σ密度涨落大致重合。因此，在这个阈值附近和以上的物体可以构成发光结构的‘积木’，我们讨论了它们与矮球/椭圆形星系和暗蓝色物体的联系。这些结果表明，UVB可以在调节宇宙恒星形成历史中发挥关键作用。
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 Tvir∼104 K at weak UV intensities (J21≲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 1σ 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.