Early Structure Formation and Reionization in a Cosmological Model with a Running Primordial Power Spectrum
Early Structure Formation and Reionization in a Cosmological Model with a Running Primordial Power Spectrum
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DOI：
10.1086/378852
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发表时间：
2003-05
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通讯作者：
N. Yoshida;Aaron Sokasian;L. Hernquist;V. Springel
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作者：
N. Yoshida;Aaron Sokasian;L. Hernquist;V. Springel
在假设基元密度涨落的光谱功率指数是长度尺度函数的前提下，研究了Λ清洁发展机制宇宙中高红移结构的形成和再电离。通过对最近的Wilkinson微波各向异性探测器(WMAP)数据和另外两个大尺度结构观测数据的综合分析，我们采用了一种特殊的“运行”谱指数(RSI)模型。我们进行了高分辨率的宇宙学模拟，并用它们来研究原始气体云的形成，在那里可能会形成第一批恒星。虽然早期结构在RSI模型中是分层形成的，与标准的幂规律Λ清洁发展机制模型非常相似，但小尺度上的功率减少导致低质量(~106M☉)“迷你晕”的形成时间比Λ清洁发展机制模型有相当大的延迟。这两个模型在z>；15时，这些晕圈中形成恒星的原始气体云的丰度也相差了一个数量级以上。RSI模型中极少量的气体云表明，再电离开始的时间晚于z<；15，这通常导致总的汤姆森光学厚度比Λ模型中的要小。通过进行辐射传输计算，我们还研究了星系中形成的恒星群的再电离。结果表明，为了使宇宙再电离z~7，在红移5<；z<；18范围内，RSI模型中星系的紫外光光子逃逸分数必须高达0.6，才能形成类似于局部宇宙的恒星群。即使有一个代表早期大质量恒星群的重顶初始质量函数和/或来自星系的极高的光子发射率，对于合理的早期恒星形成模型，总光学厚度也只能达到τe~0.1。因此，RSI模型与WMAP卫星推断的大汤姆逊光学厚度是冲突的。
We study high-redshift structure formation and reionization in a ΛCDM universe under the assumption that the spectral power index of primordial density fluctuations is a function of length scale. We adopt a particular formulation of the "running" spectral index (RSI) model as suggested by the combined analysis of the recent Wilkinson Microwave Anisotropy Probe (WMAP) data and two other large-scale structure observations. We carry out high-resolution cosmological simulations and use them to study the formation of primordial gas clouds where the first stars are likely to form. While early structure forms hierarchically in the RSI model, quite similar to the standard power-law ΛCDM model, the reduced power on small scales causes a considerable delay in the formation epoch of low-mass (~106 M☉) "minihalos" compared with the ΛCDM model. The abundance of primordial star-forming gas clouds in such halos also differs by more than an order of magnitude at z > 15 between the two models. The extremely small number of gas clouds in the RSI model indicates that reionization is initiated later than z < 15, generally resulting in a smaller total Thomson optical depth than in the ΛCDM model. By carrying out radiative transfer calculations, we also study reionization by stellar populations formed in galaxies. We show that, in order to reionize the universe by z ~ 7, the escape fraction of ultraviolet photons from galaxies in the RSI model must be as high as 0.6 throughout the redshift range 5 < z < 18 for a stellar population similar to that of the local universe. Even with a top-heavy initial mass function representing an early population of massive stars and/or an extraordinarily high photon emission rate from galaxies, the total optical depth can only be as large as τe ~ 0.1 for reasonable models of early star formation. The RSI model is thus in conflict with the large Thomson optical depth inferred by the WMAP satellite.