射电阵列大天区面积巡天中 21 cm 信号的提取和系统误差的分析

Observations of the 21 cm signal provide a new way to measure the baryon acoustic oscillations in the large scale structure and to probe the dark energy. However, since the foreground emission is about four orders of magnitude stronger than the signal, the measurement of such the weak signal thus is significantly difficult. Due to the assumption of the spectral smoothness, the commonly used techniques for foreground removal have some limitation. In addition, the study of the impact of instrumental systematic errors is still at the beginning stage. In this project, we aim to: (1) extend our recently proposed foreground removal technique (HIEMICA), which is the Bayesian semi-blind source separation approach but without any prior assumptions about foregrounds, such that it can be applicable to the case of the curved sky in large sky area survey and can reliably recover the 21 cm signal under the strong foreground contamination; (2) build an end-to-end simulation of interferometric observations for dish and cylinder arrays in the Tianlai and CHIME experiments to test various approaches for signal recovery; (3) explore and develop the Gibbs sampling and the Hamiltonian Monte Carlo techniques to accurately subtract the foreground contamination by fitting more complicated foreground models; (4) quantitatively assess the impact of the instrumental systematic errors. All of the above results will be directly applied to the real data analysis in the Tianlai and CHIME observations.

21 cm信号的观测提供了一种测量大尺度结构中的重子声波振荡和探测暗能量的新途径。然而，由于前景辐射比 21 cm信号约强四个数量级，致使观测该信号存在极大的困难。普遍使用的前景扣除算法因依赖频谱的光滑性假设存在一定局限性。另外，对仪器造成的系统误差的分析研究还处于起步阶段。在本项目中，我们将：（1）扩展我们最近提出的一种贝叶斯框架下不依赖于先验假设的源分离半盲算法（HIEMICA），使其适用于大天区面积巡天中的曲面天场并在强前景干扰下稳定可靠地重建 21 cm 信号；（2） 针对天籁和 CHIME 实验中蝶形和柱形天线干涉阵列，建立全过程仿真模拟观测以检验各种信号提取算法；（3） 探索和发展吉布斯采样和哈密顿蒙特卡洛技术，通过拟合更复杂的前景模型精确扣除前景污染；（4）基于解析计算和时间流仿真数据分析，量化估算仪器系统误差的影响。以上研究结果将直接应用于天籁和 CHIME 的观测。

21 cm宇宙学信号是宇宙学研究中最后的一块重要拼图，提供了测量宇宙大尺度结构和暗能量的新途径。我们针对21cm观测的难点，做了如下研究：.1. 开展了银河系极化前景辐射的分析，构建银河系前景辐射模型，完整建立了前景同步辐射的模拟程序，为理解和精确扣除 21cm 前景做准备。 .2. 前景扣除方面，开展了射电信号前景扣除算法的研究，发展和测试了前景扣除的解析盲算法，具有高度的数值稳定性。 .3. 在仿真模拟方面，我们考虑了各种观测效应，严格建立了针对天籁等实验装置的21cm 数据仿真和信号分析的数据管线，发展了角功率谱、三维功率谱等互关联谱测量算法。整个项目成为了DESI的外部合作项目。.4. 针对模拟的观测数据，我们测量了天籁 21cm 辐射场和DESI 星系密度场的互关联成团信噪比，提取中性氢成团性信号，并限制了宇宙结构增长历史，发现基于天籁pathfinder 和DESI 的交叉相关限制精度可达2.7%。