紫外背景和恒星局部辐射对H_I柱密度分布的影响
Effects of Ultraviolet Background and Local Stellar Radiation on the H_I Column Density Distribution
摘要：
利用宇宙学光滑粒子流体动力学模拟方法，研究了紫外背景辐射（UVB）和恒星局部辐射对阻尼Ly-alpha系统（DLAs）和亚DLAs在z=3时的H_I柱密度分布f（N_HI）的影响.我们发现，在以前的光学薄近似模拟中，UVB下沉到H_I云中太深，因此我们低估了19 < log（N_HI）< 21.2 compared to the observations. If the UVB is shut off in the high-density regions with n_gas >6 x 10^{-3} cm^{-3}处的f（N_HI），然后我们很好地再现了z=3处观测到的f（N_HI）。我们还通过辐射转移程序对模拟结果进行后处理，研究了恒星局部辐射对f（NHI）的影响，发现恒星局部辐射对f（NHI）的影响不大。结果表明，f（N_HI）的形状主要由UVB决定，而恒星局部辐射的影响要弱得多，并且宇宙学模拟中常用的光学薄近似不足以正确处理DLAs内外中性气体的电离结构.我们的结果还表明，DLA气体是密切相关的过渡区从光学厚的中性气体光学薄的电离气体的暗物质晕。
英文摘要：
We study the impact of ultraviolet background (UVB) radiation field and the local stellar radiation on the H_I column density distribution f(N_HI) of damped Ly-alpha systems (DLAs) and sub-DLAs at z=3 using cosmological smoothed particle hydrodynamics simulations. We find that, in the previous simulations with an optically thin approximation, the UVB was sinking into the H_I cloud too deeply, and therefore we underestimated the f(N_HI) at 19 < log(N_HI) < 21.2 compared to the observations. If the UVB is shut off in the high-density regions with n_gas > 6 x 10^{-3} cm^{-3}, then we reproduce the observed f(N_HI) at z=3 very well. We also investigate the effect of local stellar radiation by post-processing our simulation with a radiative transfer code, and find that the local stellar radiation does not change the f(N_HI) very much. Our results show that the shape of f(N_HI) is determined primarily by the UVB with a much weaker effect by the local stellar radiation and that the optically thin approximation often used in cosmological simulation is inadequate to properly treat the ionization structure of neutral gas in and out of DLAs. Our result also indicates that the DLA gas is closely related to the transition region from optically-thick neutral gas to optically-thin ionized gas within dark matter halos.
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