The 21 cm intensity mapping experiments promise to obtain the large-scale distribution of HI gas at the post-reionization epoch. In order to reveal the underlying matter density fluctuations from the HI mapping, it is important to understand how HI gas traces the matter density distribution. Both nonlinear halo clustering and nonlinear effects modulating HI gas in halos may determine the scale below which the HI bias deviates from linearity. We employ three approaches to generate the mock HI density from a large-scale N-body simulation at low redshifts, and demonstrate that the assumption of HI linearity is valid at the scale corresponding to the first peak of baryon acoustic oscillations, but breaks down at $k \gtrsim 0.1\,h\, {\rm Mpc}^{-1}$. The nonlinear effects of halo clustering and HI content modulation counteract each other at small scales, and their competition results in a model-dependent"sweet-spot"redshift near $z$=1 where the HI bias is scale-independent down to small scales. We also find that the linear HI bias scales approximately linearly with redshift for $z\le 3$.

中文翻译：

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HI 偏置线性的细分尺度

21 cm 强度映射实验有望在再电离后时期获得 HI 气体的大规模分布。为了从 HI 映射中揭示潜在的物质密度波动，了解 HI 气体如何追踪物质密度分布非常重要。非线性光晕聚类和非线性效应调制光晕中的 HI 气体都可以确定 HI 偏差偏离线性的比例。我们采用三种方法从低红移的大规模 N 体模拟中生成模拟 HI 密度，并证明 HI 线性的假设在对应于重子声学振荡的第一个峰值的尺度上是有效的，但在$k \gtrsim 0.1\,h\, {\rm Mpc}^{-1}$。光晕聚类和 HI 含量调制的非线性效应在小尺度上相互抵消，并且它们的竞争导致了接近 $z$=1 的依赖于模型的“甜蜜点”红移，其中 HI 偏差在小范围内与尺度无关。我们还发现线性 HI 偏差与 $z\le 3$ 的红移近似线性地缩放。