[论文解读] Fossil Groups in the Millennium Simulation: Their environment and its evolution

Eugenia Díaz-Giménez, Ariel Zandivarez+4 more|Scientific Electronic Library Online (São Paulo Research Foundation, Latin American and Caribbean Center on Health Sciences Information, Conselho Nacional de Desenvolvimento Científico e Tecnológico)|Dec 28, 2010

Galaxies: Formation, Evolution, Phenomena参考文献 36被引用 10

一句话总结

本研究利用Millennium模拟分析了化石星系群与普通星系群，发现化石系统形成更早，并在约2.5 rvir处表现出明显的局部密度峰，这是由于更早的组装所致，即使在控制了质量和形成时间后依然如此。这种环境差异表明，环境在化石系统形成中起着关键作用，而不仅仅是早期坍缩的结果。

ABSTRACT

Fossil systems are defined to be X-ray bright galaxy groups with a 2-magnitude difference between their two brightest galaxies within half the projected virial radius,and represent an interesting extreme of the population of galaxy agglomerations.However,the physical conditions and processes leading to their formation are still poorly constrained.We compare the outskirts of fossil systems with that of normal groups to understand whether environmental conditions play a significant role in their formation.We study galaxy groups in both,numerical simulations and observations.We use a variety of statistical tools including the spatial cross-correlation function and the local density parameter Δ_5 to probe differences in the density and structure of the environments of normal and fossil systems in the Millennium simulation.We find that the number density of galaxies surrounding fossil systems evolves from greater than that observed around normal systems at z=0.69, to lower than the normal systems by z=0.Both fossil and normal systems exhibit an increment in their otherwise radially declining local density measure (Δ_5) at distances of order 2.5r_{vir} from the system centre.We show that this increment is more noticeable for fossil systems than normal systems and demonstrate that this difference is linked to the earlier formation epoch of fossil groups.Despite the importance of the assembly time, we show that the environment is different for fossil and non-fossil systems with similar masses and formation times along their evolution.We also confirm that the physical characteristics identified in the Millennium simulation can also be detected in SDSS observations.Our results confirm the commonly held belief that fossil systems assembled earlier than normal systems but also show that the surroundings of fossil groups could be responsible for the formation of their large magnitude gap.

研究动机与目标

• 探究环境条件是否在早期坍缩之外对化石星系群的形成产生影响。
• 在宇宙学模拟中比较化石星系群与普通星系群的大尺度和局部环境。
• 评估观测到的化石系统中星等差距是否源于固有的环境差异，还是仅仅由于早期形成。
• 利用SDSS的观测数据验证模拟结果，确认环境特征的可探测性。
• 使用统计工具在z=0时基于环境结构区分化石系统，即使其质量和形成时间已匹配。

提出的方法

• 应用空间交叉相关函数，量化化石群与普通群之间星系团聚差异。
• 使用局部密度参数Δ₅，测量以星系群中心为中心、延伸至~5 rvir范围内的径向密度分布。
• 追踪从z=0.69到z=0的红移快照中局部密度分布的演化，观察其随时间的变化。
• 构建化石群与普通群的子样本，使其具有匹配的virial质量与组装时间，以隔离环境影响。
• 执行统计检验，以区分形成时间与固有环境差异的独立影响。
• 将模拟的局部密度分布与SDSS中四组化石群和四组普通群的投影观测结果进行比较，以验证发现。

实验结果

研究问题

• RQ1 化石系统是否存在于与质量与形成时间相近的普通群不同的环境中？
• RQ2 从z=0.69到z=0，化石系统的局部密度分布如何演化？其在~2.5 rvir处观测到的隆起由何驱动？
• RQ3 化石系统中增强的局部密度特征是由于形成时间还是固有的环境结构？
• RQ4 局部密度分布能否作为统计工具，用于在z=0时识别星系系统的形成 epoch？
• RQ5 模拟中观察到的环境特征在真实观测数据中是否可探测？

主要发现

• 化石系统在~2.5 rvir处表现出比普通系统更尖锐、更显著的局部密度峰，且在所有红移下均持续存在。
• 局部密度峰的幅度与形成时间强相关：形成越早的系统，其密度峰越大，即使在控制质量后依然如此。
• 即使化石群与普通群的质量与组装时间完全匹配，化石群在其外围区域仍表现出更高的局部密度，表明存在固有的环境差异。
• 局部密度分布的演化显示，随着红移增加，密度峰向外移动且幅度减小，表明星系正动态地向群的外围区域聚集。
• 交叉相关函数证实，化石系统周围增强的团聚并非仅由质量或形成时间引起，而是反映了独特的环境结构。
• SDSS观测的化石群投影局部密度分布与模拟结果相似，均表现出类似隆起，证实该特征在真实数据中可测量，尽管统计量较低。