红移21厘米观测中色束效应的前景扣除极限
Limits on Foreground Subtraction from Chromatic Beam Effects in Global Redshifted 21 cm Measurements
摘要：
在全球红移21厘米测量的前景减法是有限的天线波束方向图中的频率相关的（色）结构。彩色光束将银河系前景发射中的角结构耦合到光谱结构，这些光谱结构可能无法通过平滑函数形式去除。我们报告了基于两个偶极天线的模拟结果，用于检测全球EoR签名实验（EDGES）。在模拟前景减去频谱的残留水平被发现两个天线之间的差异很大，这表明天线设计必须仔细考虑。阻抗也高度依赖于天线指向的赤经赤经和经赤经，RMS值在不同指向上相差20倍。对于EDGES和其他使用天顶指向天线的地面实验，赤经和赤经直接对应于当地恒星时和部署地点的纬度，因此在选择地点时应考虑到色波束效应。我们介绍了“刀片”偶极子天线，并显示，通过模拟，它具有更好的色度性能比以前用于EDGE的“四点”天线。从模拟的光谱中去除5项多项式后，叶片天线在整个天空中产生1-5 mK的残差，而四点天线通常需要6项多项式才能产生可比的残差。对于这两个天线，恢复的21 cm输入信号的信噪比对于给定实际热噪声水平的5项多项式前景拟合达到峰值。
英文摘要：
Foreground subtraction in global redshifted 21 cm measurements is limited by frequency-dependent (chromatic) structure in antenna beam patterns. Chromatic beams couple angular structures in Galactic foreground emission to spectral structures that may not be removed by smooth functional forms. We report results for simulations based on two dipole antennas used by the Experiment to Detect the Global EoR Signature (EDGES). The residual levels in simulated foreground-subtracted spectra are found to differ substantially between the two antennas, suggesting that antenna design must be carefully considered. Residuals are also highly dependent on the right ascension and declination of the antenna pointing, with RMS values differing by as much as a factor of 20 across pointings. For EDGES and other ground-based experiments with zenith pointing antennas, right ascension and declination correspond directly to the local sidereal time and the latitude of the deployment site, hence chromatic beam effects should be taken into account when selecting sites. We introduce the "blade" dipole antenna and show, via simulations, that it has better chromatic performance than the "fourpoint" antenna previously used for EDGES. The blade antenna yields 1-5~mK residuals across the entire sky after a 5-term polynomial is removed from simulated spectra, whereas the fourpoint antenna typically requires a 6-term polynomial for comparable residuals. For both antennas, the signal-to-noise ratio of recovered 21 cm input signals peaks for a 5-term polynomial foreground fit given realistic thermal noise levels.
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