[论文解读] The influence of diffuse scattered light II. Observations of galaxy haloes and thick discs and hosts of BCGs

C. Sandín|arXiv (Cornell University)|Feb 25, 2015

Galaxies: Formation, Evolution, Phenomena参考文献 94被引用 31

一句话总结

本研究表明，地面望远镜的漫射散射光可在包括侧向和正面螺旋星系、BCG宿主星系及椭圆星系在内的多种星系类型中，模拟出星系表面亮度和颜色轮廓中的微弱晕和厚盘结构。作者利用径向扩展的点扩散函数（PSFs），证明散射光会在大半径区域产生人工晕和红化现象，意味着许多已报告的晕可能并非物理结构，而是仪器效应所致。

ABSTRACT

Studies of deep photometry of galaxies have presented discoveries of excess light in surface-brightness and colour profiles at large radii in the form of diffuse faint haloes and thick discs. In a majority of the cases, it has seemed necessary to use exotic stellar populations or alternative physical solutions to explain the excess. Few studies have carefully scrutinized the role of scattered light in this context. I explore the influence of scattered light on ground-based observations of haloes and thick discs around edge-on galaxies, haloes around face-on disc galaxies, host galaxies around blue compact galaxies (BCGs), and haloes around elliptical galaxies. Surface-brightness structures of all considered types of galaxies are modelled and analysed to compare scattered-light haloes and thick discs with measurements. I simulate the influence of scattered light and accurate sky subtraction on simplified Sérsic-type and face-on disc galaxy models. All galaxy models are convolved with both lower-limit and brighter point spread functions (PSFs); for a few galaxies it was possible to use dedicated PSFs. The results show bright scattered-light haloes and high amounts of red excess at large radii and faint surface brightnesses for nearly all types of galaxies; exceptions are the largest elliptical-type galaxies where the influence of scattered light is smaller. Studies have underestimated the role of scattered light to explain their surface-brightness profiles. My analysis shows surface-brightness profiles that include scattered light that are very similar to and overlap measurements at all radii. The derivation of physical properties of haloes, thick discs, and BCG hosts from diffuse data is misleading since accurate and radially extended PSFs are non-existent. Significantly improved analyses that include new measurements of PSFs are required to study diffuse haloes further.

研究动机与目标

• 调查地面望远镜的散射光是否能解释星系表面亮度和颜色轮廓中观测到的微弱晕和厚盘结构。
• 评估当前数据处理方法（尤其是天空背景扣除和PSF精度）在探测真实物理晕方面的局限性。
• 证明许多已报告的晕和颜色梯度可能实为散射光引起的仪器效应所致。
• 主张必须使用精确且径向扩展的PSF，才能区分物理晕与散射光造成的晕。
• 倡导改进PSF的测量与建模，以实现对星系微弱结构的可靠分析。

提出的方法

• 使用Sérsic和盘面轮廓模型，对侧向和正面星系、BCG宿主星系及椭圆星系的星系表面亮度轮廓进行建模。
• 将星系模型与下限PSF（King 1971）及更亮的PSF（M02的V,0m；V,3m；i,0m；i,3m）进行卷积，以模拟散射光效应。
• 在可用时使用专用PSF，并在多个波段内比较结果，以评估PSF依赖的颜色效应。
• 分析散射光与天空背景扣除精度的联合影响对径向表面亮度和颜色轮廓的作用。
• 将建模轮廓与已发表的观测结果进行比较，检验散射光是否能再现观测到的微弱结构。
• 通过识别散射光主导于物理信号的区域，评估可靠测量的径向范围。

实验结果

研究问题

• RQ1 地面望远镜的散射光在多大程度上能再现星系表面亮度轮廓中观测到的微弱晕和厚盘结构？
• RQ2 PSF形状和径向扩展的变化如何影响深空测光中人工晕和颜色梯度的外观？
• RQ3 为何低表面亮度星系和遥远星系中报告的晕和颜色梯度很可能主要由散射光而非物理结构造成？
• RQ4 在低表面亮度下，天空背景扣除和PSF精度的极限是什么，如何影响真实物理晕的探测？
• RQ5 外区观测到的红化现象是否可由PSF引起的颜色梯度解释，而非物理恒星种群效应？

主要发现

• 散射光产生的表面亮度轮廓与所有星系类型中观测到的微弱晕和厚盘结构高度吻合，包括侧向螺旋星系、正面盘状星系、BCG宿主星系及椭圆星系。
• 散射光的影响导致大半径区域出现显著红化，尤其在晕区，产生可模拟物理恒星种群变化的人工颜色梯度。
• 即使天空背景扣除准确，散射光在低表面亮度区域仍占主导，若无精确且扩展的PSF，无法区分物理晕与仪器伪影。
• PSF必须扩展至至少观测结构径向范围的1.5倍，才能避免低估散射光效应。
• 当前深空巡天很可能使用了过亮且扩展不足的PSF，加剧了散射光效应，进一步掩盖了真实的物理晕。
• 在获得精确且径向扩展的PSF之前，探测真实物理晕的唯一可靠方法是通过不对称潮汐特征或邻近星系中的分辨恒星。