[论文解读] The influence of diffuse scattered light I. The PSF and its role to observations of the edge-on galaxy NGC 5907

C. Sandín|Jun 20, 2014

Galaxies: Formation, Evolution, Phenomena参考文献 56被引用 48

一句话总结

本研究表明，散射光，尤其是点扩散函数（PSF）的展宽尾部，是星系NGC 5907中观测到的微弱光晕的主要成因。通过重新分析PSF测量数据并建模卷积后的星系轮廓，作者发现即使对背景水平和PSF归一化进行微小调整，也会显著改变表面亮度的感知，从而将光晕解释为PSF引起的伪影，而非物理结构。

ABSTRACT

All telescopes and instruments are to some degree affected by scattered light. It is possible to estimate the amount of such scattered light, and even correct for it, with a radially extended point spread function (PSF). The outer parts of the PSF have only rarely been determined, since they are faint and therefore difficult to measure. A mostly complete overview of existing properties and measurements of radially extended PSFs is presented, to both show their similarities and to indicate how bright extended objects can be used to measure the faintest regions. The importance of the far wings of the PSF and their possible temporal variations are demonstrated in three edge-on galaxy models. The same study is applied to the first edge-on galaxy where earlier observations reveal a halo, NGC 5907. All PSFs were collected in two diagrams, after they were offset or normalized, when that was possible. Surface-brightness structures of edge-on galaxies were modelled and analysed to study scattered-light haloes that result with an exponential disc. The models were convolved with both a lower-limit PSF and a more average PSF. The PSF of the observed data could be used in the case of NGC 5907. The comparison of the PSFs demonstrates a lower-limit $r^{-2}$ power-law decline at larger radii. The analysis of the galaxy models shows that also the outer parts of the PSF are important to correctly model and analyse observations and, in particular, fainter regions. The reassessed analysis of the earlier measurements of NGC 5907 reveals an explanation for the faint halo in scattered light, within the quoted level of accuracy.

研究动机与目标

• 重新评估在NGC 5907中观测到的微弱恒星光晕的起源，挑战其为物理结构的假设。
• 研究PSF，特别是其展宽外缘，对侧向星系表面亮度测量的影响。
• 量化背景水平不确定性与PSF归一化对微弱外区结构解释的影响。
• 证明散射光可在颜色轮廓中产生强烈的红移过量，从而模仿物理特征。
• 认为在深空成像中，PSF相关效应被系统性低估，尤其在扩展天体的微弱区域。

提出的方法

• 收集并比较了来自不同来源的多个PSF，包括MBH94、K71以及新数据，以评估其径向范围和归一化。
• 使用比MBH94原始使用值低3 ADU的背景水平重新构建PSF，对PSF进行重新校准以更准确反映天空条件。
• 将模型星系轮廓（指数盘）与下限PSF（r⁻²幂律）及更真实的PSF进行卷积，以模拟观测到的表面亮度。
• 分析卷积模型的R-i颜色轮廓，检测PSF形状引起的红移过量，尤其在i波段。
• 使用NGC 5907的现有深空成像数据，并将PSF衍生的光晕与观测到的表面亮度和颜色结构进行比较。
• 评估时间与波长相关的PSF变化对观测结构的影响，特别是在微弱区域。

实验结果

研究问题

• RQ1 NGC 5907中的微弱光晕是否可由散射光而非物理恒星光晕来解释？
• RQ2 背景天空水平和PSF归一化的变化如何影响侧向星系外区测量到的表面亮度？
• RQ3 PSF的展宽尾部，特别是具有r⁻²幂律衰减时，对观测到的表面亮度轮廓贡献有多大？
• RQ4 为何光晕的R-i颜色轮廓表现出强烈的红移过量？这是由PSF形状还是物理特性引起的？
• RQ5 PSF相关效应在量级上与其它解释（如潮汐流、奇异恒星种群）相比如何，用于解释NGC 5907光晕？

主要发现

• 背景水平降低3 ADU（0.17%）导致PSF显著变亮，但仍在MBH94原始PSF的误差范围内。
• 重新评估的PSF分析表明，散射光，特别是来自PSF展宽尾部的光，足以完全解释NGC 5907中观测到的光晕。
• PSF卷积模型的R-i颜色轮廓表现出强烈的红移过量，这完全由PSF形状引起，而非物理颜色梯度。
• PSF的外缘，具有r⁻²幂律衰减，对外观测表面亮度贡献了显著且系统性的成分，尤其在微弱区域。
• 本研究确认，必须仔细考虑PSF随时间与波长的变化，因为即使在天空水平或PSF归一化上存在微小误差，也会在微弱结构解释中导致巨大误差。
• 光晕为PSF伪影的结论与其他观测结果一致，包括LFD96和ZSS99的结果，无需引入奇异恒星种群或潮汐流即可解释数据。