The high-redshift 21 cm signal from the Epoch of Reionization (EoR) is a promising observational probe of the early universe. Current- and next-generation radio interferometers such as the Hydrogen Epoch of Reionization Array (HERA) and Square Kilometre Array (SKA) are projected to measure the 21 cm auto power spectrum from the EoR. Another observational signal of this era is the kinetic Sunyaev-Zel'dovich (kSZ) signal in the cosmic microwave background (CMB), which will be observed by the upcoming Simons Observatory (SO) and CMB-S4 experiments. The 21 cm signal and the contribution to the kSZ from the EoR are expected to be anti-correlated, the former coming from regions of neutral gas in the intergalactic medium and the latter coming from ionized regions. However, the naive cross-correlation between the kSZ and 21 cm maps suffers from a cancellation that occurs because ionized regions are equally likely to be moving toward or away from the observer and so there is no net correlation with the 21 cm signal. We present here an investigation of the 21 cm-kSZ-kSZ bispectrum, which should not suffer the same cancellation as the simple two-point cross-correlation. We show that there is a significant and non-vanishing signal that is sensitive to the reionization history, suggesting the statistic may be used to confirm or infer the ionization fraction as a function of redshift. In the absence of foreground contamination, we forecast that this signal is detectable at high statistical significance with HERA and SO. The bispectrum we study suffers from the fact that the kSZ signal is sensitive only to Fourier modes with long-wavelength line-of-sight components, which are generally lost in the 21 cm data sets owing to foreground contamination. We discuss possible strategies for alleviating this contamination, including an alternative four-point statistic that may help circumvent this issue.

中文翻译：

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再电离时期的 21 cm kSZ–kSZ 双谱

来自再电离时代 (EoR) 的高红移 21 厘米信号是对早期宇宙的有希望的观测探测器。当前和下一代无线电干涉仪，例如再电离阵列的氢时代 (HERA) 和平方公里阵列 (SKA)，预计将测量来自 EoR 的 21 厘米自动功率谱。这个时代的另一个观测信号是宇宙微波背景 (CMB) 中的动力学 Sunyaev-Zel'dovich (kSZ) 信号，即将进行的西蒙斯天文台 (SO) 和 CMB-S4 实验将观测到该信号。预计 21 cm 信号和 EoR 对 kSZ 的贡献是反相关的，前者来自星系际介质中的中性气体区域，后者来自电离区域。然而，kSZ 和 21 cm 地图之间的朴素互相关受到抵消的影响，因为电离区域同样可能朝向或远离观察者移动，因此与 21 cm 信号没有净相关性。我们在此展示了对 21 cm-kSZ-kSZ 双谱的研究，它不应遭受与简单两点互相关相同的抵消。我们表明存在对再电离历史敏感的显着且非消失的信号，这表明该统计数据可用于确认或推断作为红移函数的电离分数。在没有前景污染的情况下，我们预测该信号可通过 HERA 和 SO 以高统计显着性被检测到。我们研究的双谱受到这样一个事实的影响，即 kSZ 信号仅对具有长波长视线分量的傅立叶模式敏感，由于前景污染，这些分量通常在 21 cm 数据集中丢失。我们讨论了减轻这种污染的可能策略，包括可能有助于规避此问题的替代四点统计数据。