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The Breakdown Scale of H I Bias Linearity

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