[论文解读] Characterization of foreground emission at degree angular scale for CMB B-modes observations. Thermal Dust and Synchrotron signal from Planck and WMAP data

N. Krachmalnicoff, C. Baccigalupi+3 more|arXiv (Cornell University)|Nov 2, 2015

Cosmology and Gravitation Theories参考文献 37被引用 20

一句话总结

本研究利用普朗克和威尔金森微波各向异性探针（WMAP）数据，对度量级角尺度下的偏振同步辐射与热尘埃辐射进行表征，以评估其对宇宙微波背景（CMB）B模信号的污染。研究发现，同步辐射前景在全天空范围内、频率≤100 GHz时仍为显著且不可排除的污染源，前景最小幅度（r_{FG,min}）范围约为0.05至1.5，表明未来CMB实验需依赖更精确的低频数据以实现前景清洁。

ABSTRACT

We quantify the contamination from polarized diffuse Galactic synchrotron and thermal dust emissions to the B-modes of the CMB anisotropies on the degree angular scale, using data from the Planck and WMAP satellites. We compute power spectra of foreground polarized emissions in 352 circular sky patches located at Galactic latitude |b|>20°, each of which covering a fraction of the sky of about 1.5%. We make use of the spectral properties derived from Planck and WMAP data to extrapolate, in frequency, the amplitude of synchrotron and thermal dust B-modes spectra in the multipole bin centered at $\ell\simeq80$. In this way we estimate, for each analyzed region, the amplitude and frequency of the foreground minimum. We detect both dust and synchrotron signal, at degree angular scale and at 3 confidence level, in 28 regions. Here the minimum of the foreground emission is found at frequencies between 60 and 100 GHz with an amplitude,expressed in terms of the equivalent tensor-to-scalar ratio, r_FG, between ~0.06 and ~1. Some of these regions are located at high Galactic latitudes, in areas close to the ones which are being observed by sub-orbital experiments.In all the other sky patches, where synchrotron or dust B-modes are not detectable with the required confidence, we put upper limits on the minimum foreground contamination and find values of r_FG between ~0.05 and ~1.5, in the frequency range 60-90 GHz. Our results indicate that, with the current sensitivity at low frequency, it is not possible to exclude the presence of synchrotron contamination to CMB cosmological B-modes at the level requested to measure a gravitational waves signal with r~0.01, at frequency <100 GHz, anywhere. Therefore, more accurate data are essential in order to better characterize the synchrotron polarized component, and eventually, remove its contamination to CMB signal through foreground cleaning.

研究动机与目标

• 量化与CMB B模探测相关的度量级角尺度下银河系偏振前景发射——同步辐射与热尘埃——的特性。
• 评估这些前景对宇宙学B模信号的污染水平，尤其关注频率≤100 GHz的情况。
• 识别前景污染最小的天区，以供未来旨在探测r ≈ 0.01的CMB实验使用。
• 提供全天空范围内频率依赖的最小前景幅度（r_{FG,min}）估计值，以指导观测策略。

提出的方法

• 在352个圆形天区（|b| > 20°）内计算了偏振同步辐射与热尘埃发射的功率谱，每个天区覆盖约1.5%的天球面积。
• 利用普朗克和WMAP数据的光谱特性，将B模功率谱幅度外推至ℓ ≈ 80的多极数区间。
• 通过识别同步辐射与尘埃B模功率总和最小的频率，对每个天区估算最小前景幅度（r_{FG,min}）。
• 在99%置信水平下评估统计显著性，以确定各区域的检测结果或上限。
• 通过与已知CMB实验观测区域（包括BICEP2/Keck、PolarBear、CLASS和LSPE）的对比，验证结果可靠性。
• 对未在3σ水平检测到同步辐射或尘埃B模的区域，推导出r_{FG,min}的上限。

实验结果

研究问题

• RQ1 在全天空范围内，度量级角尺度下的偏振同步辐射与热尘埃发射水平如何？其随频率的变化特征是什么？
• RQ2 在哪些天区中，最小前景污染（r_{FG,min}）可降低至探测r ≈ 0.01目标水平以下？
• RQ3 当前低频数据能否排除频率≤100 GHz时对CMB B模测量的同步辐射污染？
• RQ4 在前景污染较低的区域，同步辐射贡献在何种频率下降至r_s < 0.005以下？

主要发现

• 在28个天区中，以99%置信水平检测到同步辐射与热尘埃B模信号，其r_{FG,min}在60–100 GHz频率范围内约为0.06至1.0。
• 在其余324个天区中，r_{FG,min}的上限在60–90 GHz频率范围内约为0.05至1.5，表明污染风险持续存在。
• 南银道半球存在一大片区域，其r_{FG,min}上限较低，确认其为CMB B模观测中最洁净的区域之一。
• 在100 GHz以下，同步辐射污染仍是主要关切，因当前低频灵敏度下，任何区域均无法排除其影响，即使目标为r ≈ 0.01。
• 在频率>160 GHz时，所有区域的同步辐射贡献均可忽略（r_s < 0.005），但该频段不足以探测更微弱的引力波信号。
• 结果凸显了亟需更精确的低频数据，以通过先进清洁技术准确表征并去除同步辐射前景。