Systematic effects of foreground removal in 21-cm surveys of reionization
Systematic effects of foreground removal in 21-cm surveys of reionization
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DOI：
10.1111/j.1365-2966.2011.18276.x
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发表时间：
2010-10
影响因子：
4.8
通讯作者：
Nada Petrovic;S. Peng Oh
中科院分区：
文献类型：
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作者：
Nada Petrovic;S. Peng Oh
21 厘米观测有可能彻底改变我们对高红移宇宙的理解。虽然在这些频率上存在极其明亮的无线电连续谱前景，但可以利用它们的频谱平滑度来实现有效的前景扣除。众所周知，无论其他仪器效应如何，这都会消除与测量带宽相当的功率。我们调查相关的系统偏差。我们证明，消除大尺度上的视线波动可以抑制大范围的 3D 功率谱。这种偏差可以通过正确边缘化一维功率谱中的小波数来解决；然而，无偏估计量将不可避免地具有较大的方差。我们还表明，功率谱的高斯实现允许精确且极其快速的蒙特卡罗模拟进行误差分析；完全非高斯场的重复实现是不必要的。我们对前景去除进行蒙特卡罗最大似然模拟，产生与费舍尔矩阵估计一致的功率谱的无偏最小方差估计。前景去除还会扭曲 21 厘米概率分布函数 (PDF)，降低中性区域和电离区域之间的对比度，这可能会给从 PDF 中提取信息的工作带来严重后果。我们表明，大尺度模式的减法是造成这种畸变的原因，并且在再电离的早期阶段，这种畸变不太严重。可以通过使用更大的带宽来减少它。在再电离的后期阶段，最大电离区域（仅由前景发射组成）的识别提供了可能允许恢复大尺度模式的校准点。最后，我们还表明（i）同步加速器和自由发射的宽频率响应将消除电子动量分布中的任何特征，并确保光谱平滑的前景；（ii）河外射电重组线应该是可以忽略不计的前景。
21-cm observations have the potential to revolutionize our understanding of the high-redshift Universe. Whilst extremely bright radio continuum foregrounds exist at these frequencies, their spectral smoothness can be exploited to allow efficient foreground subtraction. It is well known that - regardless of other instrumental effects - this removes power on scales comparable to the survey bandwidth. We investigate associated systematic biases. We show that removing line-of-sight fluctuations on large scales aliases into suppression of the 3D power spectrum across a broad range of scales. This bias can be dealt with by correctly marginalizing over small wavenumbers in the 1D power spectrum; however, the unbiased estimator will have unavoidably larger variance. We also show that Gaussian realizations of the power spectrum permit accurate and extremely rapid Monte Carlo simulations for error analysis; repeated realizations of the fully non-Gaussian field are unnecessary. We perform Monte Carlo maximum likelihood simulations of foreground removal which yield unbiased, minimum variance estimates of the power spectrum in agreement with Fisher matrix estimates. Foreground removal also distorts the 21-cm probability distribution function (PDF), reducing the contrast between neutral and ionized regions, with potentially serious consequences for efforts to extract information from the PDF. We show that it is the subtraction of large-scale modes which is responsible for this distortion, and that it is less severe in the earlier stages of reionization. It can be reduced by using larger bandwidths. In the late stages of reionization, identification of the largest ionized regions (which consist of foreground emission only) provides calibration points which potentially allow recovery of large-scale modes. Finally, we also show that (i) the broad frequency response of synchrotron and free-free emission will smear out any features in the electron momentum distribution and ensure spectrally smooth foregrounds and (ii) extragalactic radio recombination lines should be negligible foregrounds.