[论文解读] Fossil groups in the Millennium simulation. From the brightest to the faintest galaxies during the past 8 Gyr

María José Kanagusuku, Eugenia Díaz-Giménez+1 more|Americanae (AECID Library)|Nov 18, 2015

Galaxies: Formation, Evolution, Phenomena参考文献 46被引用 6

一句话总结

本研究利用 Millennium Simulation II 的半解析星系模型，探究过去 80 亿年中，化石星系群与非化石星系群内明亮及暗淡星系的演化。研究发现，化石星系群约在 35 亿年前因早期快速并合而形成显著的星等差距，导致至今仍保持独特的光度采样特征；暗淡星系数在化石星系群中早期即趋于稳定，随后迅速增加，表明其形成历史由早期并合速率所主导。

ABSTRACT

We investigate the evolution of bright and faint galaxies in fossil and non-fossil groups. We used mock galaxies constructed based on the Millennium run simulation II. We identified fossil groups at redshift zero according to two different selection criteria, and then built reliable control samples of non-fossil groups that reproduce the fossil virial mass and assembly time distributions. The faint galaxies were defined as having r-band absolute magnitudes in the range [-16,-11]. We analysed the properties of the bright and faint galaxies in fossil and non-fossil groups during the past 8 Gyr. We observed that the brightest galaxy in fossil groups is typically brighter and more massive than their counterparts in control groups. Fossil groups developed their large magnitude gap between the brightest galaxies around 3.5 Gyr ago. The brightest galaxy stellar masses of all groups show a notorious increment at that time. By analysing the behaviour of the magnitude gap between the first and the second, third, and fourth ranked galaxies, we found that at earlier times, fossil groups comprised two large brightest galaxies with similar magnitudes surrounded by much fainter galaxies, while in control groups these magnitude gaps were never as large as in fossils. At early times, fossil groups in the faint population were denser than non-fossil groups, then this trend reversed, and finally they became similar at the present day. The mean number of faint galaxies in non-fossil systems increases in an almost constant rate towards later times, while this number in fossil groups reaches a plateau at $z\sim0.6$ that lasts $\sim 2$ Gyr, and then starts growing again more rapidly. The formation of fossil groups is defined at the very beginning of the groups according to their galaxy luminosity sampling, which could be determined by their merging rate at early times.

研究动机与目标

• 通过对比化石星系群与非化石对照组的星系族群，理解化石星系群的形成与演化机制。
• 探究化石星系群中星系的光度采样是否由早期并合历史决定。
• 分析化石与非化石星系群中明亮星系（星等差距）与暗淡星系（M_r ∈ [-16,-11]）随宇宙时的演化。
• 检验不同的化石星系群选择标准（Jones et al. 2003 与 Dariush et al. 2010）是否导致显著不同的演化趋势。
• 评估环境与吸积在塑造化石系统中暗淡星系数密度与分布中的作用。

提出的方法

• 使用 Millennium Run Simulation II 中的模拟星系，结合星系形成的半解析模型。
• 在 z=0 时，依据两种标准识别化石星系群：经典 ΔM12 > 2 标准，以及 Dariush et al. (2010) 基于早期组装时间的标准。
• 构建与化石星系群在 virial 质量与组装时间分布上匹配的非化石星系群对照样本。
• 从 z ≈ 1 至 z = 0，追踪化石与对照组中第一星系（BGG）与暗淡星系（M_r ∈ [-16,-11]）的演化。
• 分析第一星系与第二、三、四星系之间星等差距（ΔM12, ΔM13, ΔM14）随时间的变化。
• 比较不同红移区间内化石与对照组中暗淡星系数密度分布与平均星系数的差异。

实验结果

研究问题

• RQ1 化石星系群是否比非化石星系群更早形成显著的星等差距？这一过程的触发机制是什么？
• RQ2 在宇宙早期，化石与非化石星系群中明亮与暗淡星系的光度采样有何差异？
• RQ3 早期并合速率在塑造化石系统中独特星系族群演化中扮演了何种角色？
• RQ4 为何化石星系群中暗淡星系数在 z ≈ 0.6 之前几乎保持不变，之后迅速上升，而这一现象在非化石星系群中并不明显？
• RQ5 不同的化石星系群选择标准在多大程度上影响了星系族群演化历史的推断？

主要发现

• 化石星系群中第一与第二星系之间的星等差距约在 35 亿年前形成，与第一星系恒星质量的显著增加相吻合。
• 在早期（z ≳ 0.7），化石星系群的星等差距大于对照组，表明从一开始即表现出更显著的光度采样特征。
• 在 z ≳ 0.7 时，化石星系群中的暗淡星系数密度高于对照组，但该趋势在 z ≈ 0.3–0.5 时发生反转，对照组的密度反而更高。
• 化石星系群中暗淡星系的平均数量在 z ≈ 0.6 达到平台期，并在约 20 亿年内保持近乎恒定，随后迅速上升，而对照组则呈现稳定增长，表明演化路径显著不同。
• Dariush et al. (2010) 的选择标准识别出的化石系统比 Jones et al. (2003) 的经典标准所识别的系统更早组装，且具有更高质量的第一星系。
• 化石与对照组中暗淡星系数的演化存在显著差异：化石系统因吸积减少而早期趋于稳定，而对照组则持续稳定增长，表明其环境或吸积历史存在根本不同。