[论文解读] Reassessment of the dipole in the distribution of quasars on the sky

Arefe Abghari, Emory F. Bunn+5 more|arXiv (Cornell University)|May 16, 2024

Cosmology and Gravitation Theories被引用 5

一句话总结

论文重新评估类星体密度偶极子，显示来自大尺度各向异性和掩蔽的巨大系统误差，这些误差阻碍对与CMB偶极子一致性的稳健排除。

ABSTRACT

We investigate recent claims by Secrest et al. of an anomalously large amplitude of the dipole in the distribution of CatWISE-selected quasars on the sky. Two main issues indicate that the systematic uncertainties in the derived quasar-density dipole are underestimated. Firstly, the spatial distribution of the quasars is not a pure dipole, possessing low-order multipoles of comparable size to the dipole. These multipoles are unexpected and presumably caused by unknown systematic effects; we cannot be confident that the dipole amplitude is not also affected by the same systematics until the origin of these fluctuations is understood. Secondly, the 50 percent sky cut associated with the quasar catalogue strongly couples the multipoles, meaning that the power estimate at ell=1 contains significant contributions from ell>1. In particular, the dominant quadrupole mode in the Galactic mask strongly couples the dipole with the octupole, leading to a large uncertainty in the dipole amplitude. Together these issues mean that the dipole in the quasar catalogue has an uncertainty large enough that consistency with the cosmic microwave background (CMB) dipole cannot be ruled out. More generally, current data sets are insufficiently clean to robustly measure the quasar dipole and future studies will require samples that are larger (preferably covering more of the sky) and free of systematic effects to make strong claims regarding their consistency with the CMB dipole.

研究动机与目标

• 评估在潜在系统误差存在的情况下，类星体密度偶极子是否与CMB偶极子一致。
• 识别并量化可能偏置偶极子估计的大尺度系统误差。
• 评估掩蔽如何将高阶多极与偶极耦合并放大不确定性。
• 使用子样本和替代方法检验偶极子估计器的鲁棒性。
• 为未来具有更大、数据更清洁的类星体偶极子研究提供指导。

提出的方法

• 使用 CatWISE 基于的类星体样本，应用 W1–W2 颜色 cut 和按 Secrest 等人制定的银河掩蔽来形成密度图。
• 分析大掩蔽对偶极估计的影响，使用球谐回归并通过耦合矩阵 M 耦合多极。
• 在掩蔽下模拟加入更高的多极（四极、八极）以研究偶极在掩蔽下的偏差。
• 使用原地相关函数方法（PolSpice）从被掩蔽的图像估计偶极及更高的多极。
• 研究日心黄道梯度校正对类星体计数的影响及其对偶极解释的影响。

Figure 1 : All-sky CMB map from Planck with the monopole subtracted. We specifically show the Public Release 4 data [ 41 ] , employing the Commander component-separation method. A mask covering $22\text{\,}\mathrm{\char 37\relax}$ of the sky has been applied ( $f_{\mathrm{sky}}=$78\text{\,}\mathrm{\

实验结果

研究问题

• RQ1 在考虑系统误差和掩蔽后，类星体密度偶极子是否与CMB偶极子一致？
• RQ2 大尺度系统误差和恒星污染如何影响推断的偶极子幅度和方向？
• RQ3 天空掩蔽如何将高阶多极耦合到偶极子，以及对不确定性的影响？
• RQ4 在现实系统误差下，不同的偶极子/与偶极相关的估计器（基于回归、相关函数）的鲁棒性如何？

主要发现

• 类星体密度图呈现出大尺度结构和超过纯偶极子的多极，削弱了对干净偶极测量的信心。
• 掩蔽导致偶极和高阶多极之间的强耦合（特别是四极和八极），从而偏置偶极幅度。
• 明亮–昏暗分割表明恒星污染偏置偶极及其与CMB偶极的一致性取决于样本选择。
• 包含更高多极的仿真显示偶极估计器可能被偏置；在简单假设下的 p 值从接近5σ降至约2σ，当考虑更高多极和掩蔽时。
• PolSpice 原样空间分析给出 D ≈ 0.010，与较大的统计和系统误差共存。
• 总体而言，当前数据尚不够干净，无法对是否与CMB偶极一致做出稳健的确认或排除；需要更大、数据更清洁的样本。

Figure 2 : Left: density map of quasars selected from CatWISE , with the masked region shown in grey. Right: same map smoothed with a top-hat filter of area $1\text{\,}\mathrm{steradian}$ and with the colour scale chosen to show the contrast relative to the average density. The white color represent