[论文解读] A Catalog of Extended Clusters and Ultra-Compact Dwarf Galaxies - An Analysis of their Parameters in Early- and Late-Type Galaxies

R. C. Bruens, Pavel Kroupa|arXiv (Cornell University)|Sep 19, 2012

Galaxies: Formation, Evolution, Phenomena参考文献 117被引用 20

一句话总结

本文汇编了迄今为止最大的扩展恒星天体（EOs）目录，包括扩展星团（ECs）和超紧凑矮星系（UCDs），分析了其在早型与晚型星系中的结构参数。研究发现，EOs在$ r_{\rm eff} $与$ M_{\rm V} $平面中与球状星团形成连续序列，具有依赖于光度的大小限制，并在$ M_{\rm V} \approx -6.5$等效星等处出现光度函数峰值，强烈支持其起源于星团复合体合并的形成机制。

ABSTRACT

Extended stellar clusters with effective radii larger than 10 pc have been found in various environments. Objects with masses comparable to globular clusters (GCs) are called extended clusters (ECs), while objects with masses in the dwarf galaxy regime are called ultra-compact dwarf galaxies (UCDs). The paper analyses the observational parameters luminosity, effective radius, and projected distance to the host galaxy, of all known ECs and UCDs and the dependence of these parameters on the type and the luminosity of their host galaxy. We searched the available literature to compile a catalog of star clusters larger than 10 pc. As there is no clear distinction between ECs and UCDs, both types of objects will be called extended stellar objects (EOs). In total, we found 813 EOs of which 171 are associated with late-type and 642 with early-type galaxies. EOs cover a luminosity range from about MV = -4 to -14 mag. However, the vast majority of EOs brighter than -10 mag are associated with elliptical galaxies. At each magnitude EOs are found with effective radii between 10 pc and an upper size limit, which shows a clear trend: the more luminous the object the larger is the upper size limit. For EOs associated with early-and late-type galaxies, the luminosity functions peak at -6.40 and -6.47 mag, respectively, which is about one magnitude fainter than the peak of the GC luminosity function. EOs and GCs form a coherent structure in the reff vs. MV parameter space, while there is a clear gap between EOs and early type dwarf galaxies. However, there is a small potential overlap at the high-mass end, where the most extended EOs are close to the parameters of compact elliptical galaxies. We compare the EO sample with numerical models and conclude that the parameters of the EO sample as a whole can be very well explained by a star cluster origin, where EOs are the results of merged cluster complexes.

研究动机与目标

• 汇编一份全面、最新的扩展恒星天体（EOs）目录，有效半径$ r_{\rm eff} > 10 $ pc，包括ECs和UCDs。
• 分析EO参数——光度（$ M_{\rm V} $）、有效半径（$ r_{\rm eff} $）和投影距离（$ R_{\rm proj} $）——与宿主星系类型和光度的依赖关系。
• 检验EO观测到的结构参数是否可由星团起源解释，特别是通过星团复合体（CCs）的合并。
• 评估现有巡天中对暗淡和扩展EOs的完整性和检测偏差。

提出的方法

• 系统性文献检索，汇编来自多样化星系环境的813个EOs，其$ r_{\rm eff} > 10 $ pc。
• 根据宿主星系形态和光度，将EOs分类为与早型星系或晚型星系相关。
• 对光度函数、$ r_{\rm eff} $与$ M_{\rm V} $分布以及与宿主星系光度的关系进行统计分析。
• 将观测到的EO参数与星团复合体（CCs）合并的数值模拟预测进行比较，以检验形成情景。
• 使用来自多个来源的参考目录条目，采用标准化参数（如$ r_{\rm eff} $、$ M_{\rm V} $、$ R_{\rm proj} $），并进行一致性质量控制。
• 对确认的EOs（175个）和候选EOs（638个）分别进行分析，以评估结构趋势的稳健性。

实验结果

研究问题

• RQ1 在早型与晚型星系中，EOs的有效半径与绝对星等的分布如何？
• RQ2 EOs的光度函数与球状星团和矮星系的光度函数相比有何异同？
• RQ3 宿主星系光度与相关EOs的光度或大小之间是否存在系统性趋势？
• RQ4 EOs观测到的结构参数是否可由星团复合体合并的形成情景解释？
• RQ5 在$ r_{\rm eff} $与$ M_{\rm V} $平面中，EOs与早型矮星系之间的间隙具有何种意义？

主要发现

• 该目录包含813个EOs，其中642个与早型星系相关，171个与晚型星系相关，其V波段绝对星等范围为$ M_{\rm V} = -4 $至$ -14 $等。
• 绝大多数光度高于$ M_{\rm V} = -10 $等的EOs与巨型椭圆星系相关，表明在高光度下存在强烈的宿主光度依赖性。
• EOs的上限尺寸随光度增加而增大，从$ M_{\rm V} = -5 $等时的约30 pc上升至$ M_{\rm V} = -14 $等时的约100 pc。
• EOs的光度函数在早型宿主中峰值位于$ M_{\rm V} \approx -6.40 $等，在晚型宿主中为$ -6.47 $等，比球状星团光度函数的峰值约暗一个星等。
• EOs与球状星团在$ r_{\rm eff} $与$ M_{\rm V} $平面中形成连贯序列，明显与早型矮星系分离，仅在高质质量端可能与致密椭圆星系存在重叠。
• EOs的观测结构参数，包括极端天体如VUCD7的核-晕表面亮度分布，可被星团复合体合并的数值模型良好再现，支持EOs起源于星团的形成机制。