星系21厘米观测的最佳光谱叠加：精度评估和对称叠加
Optimizing spectral stacking for 21-cm observations of galaxies: accuracy assessment and symmetrized stacking
摘要：
我们提出了一个评估的$21$ -厘米谱线叠加实验中进行的常见操作的准确性。为此，我们生成模拟干涉测量数据，测量频率为$1310<ν<1420$ MHz（$0.005<z<0.084$）的21厘米发射，并覆盖天空的区域$\sim 6$度$^2$，模仿真实MeerKAT观测的观测特性。我们发现，原光束校正占百分之几（$\sim8\%$在0初级光束功率，$\sim 3\%$在0.6初级光束功率）偏离真正的$M_{\rm HI}$信号，并根据噪声特性的加权方案提供公正的结果。相反，基于距离的加权方案在应用于通量受限样品（研究案例中为$ΔM_{\rm HI}\sim 40-50\%$）时可以解释显著的系统质量差异。我们发现没有显着的差异时，光谱红移的不确定性，在堆叠过程中（$ Δz\sim 0.00035$，即$Δv \sim 100\,{\rm km\, s}^{-1}$）占最终获得的$\braket{M_{\rm HI}}$。我们还提出了一种新的技术，以增加有效尺寸的星系样本，利用星系立方体的几何对称性，潜在地提高SNR的一个因素$\sim\sqrt{2}$时，分析最终堆叠的光谱（一个因素4立方体）。这个过程被发现是鲁棒无偏的，同时有效地提高信噪比，如预期的。我们认为，需要一个适当的框架，采用详细和现实的模拟，利用即将到来的数据集从SKA探路者在一个准确和可靠的方式。
英文摘要：
We present an assessment of the accuracy of common operations performed in $21$-cm spectral line stacking experiments. To this end, we generate mock interferometric data surveying the 21-cm emission at frequency $1310<ν<1420$ MHz ($0.005<z<0.084$) and covering an area $\sim 6$ deg$^2$ of the sky, mimicking the observational characteristics of real MeerKAT observations. We find that the primary beam correction accounts for just few per cent ($\sim8\%$ at 0 primary beam power, $\sim 3\%$ at 0.6 primary beam power) deviations from the true $M_{\rm HI}$ signal, and that weighting schemes based on noise properties provide unbiased results. On the contrary, weighting schemes based on distance can account for significant systematic mass differences when applied to a flux-limited sample ($ΔM_{\rm HI}\sim 40-50\%$ in the studied case). We find no significant difference in the final $\braket{M_{\rm HI}}$ obtained when spectroscopic redshift uncertainties are accounted for in the stacking procedure ($ Δz\sim 0.00035$, i.e. $Δv \sim 100\,{\rm km\, s}^{-1}$). We also present a novel technique to increase the effective size of the galaxy sample by exploiting the geometric symmetries of galaxy cubelets, potentially enhancing the SNR by a factor $\sim\sqrt{2}$ when analyzing the final stacked spectrum (a factor 4 in a cubelet). This procedure is found to be robustly unbiased, while efficiently increasing the SNR, as expected. We argue that an appropriate framework employing detailed and realistic simulations is required to exploit upcoming datasets from SKA pathfinders in an accurate and reliable manner.
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