[论文解读] Fossil Groups Origins: I. RX J105453.3+552102 a very massive and relaxed system at z~0.5

J. A. L. Aguerri, M. Girardi+12 more|arXiv (Cornell University)|Jan 12, 2011

Galaxies: Formation, Evolution, Phenomena参考文献 99被引用 31

一句话总结

本研究利用深度光学成像和116个星系的红移数据，调查了红移z ≈ 0.5处一个大质量、动力学松弛的星系团RX J105453.3+552102。尽管其质量极高（M ≈ 1 × 10¹⁵ h₇₀⁻¹ M☉）且处于动力学松弛状态，但其未满足经典星等差距标准（Δm₁₂ = 1.9 mag < 2 mag），挑战了其作为化石星系群的分类，尽管它表现出类似化石的特性，如双峰星等函数和中心星系中较低的恒星形成率。

ABSTRACT

The most accepted scenario for the origin of fossil groups (FGs) is that they are galaxy associations in which the merging rate was fast and efficient. These systems have assembled half of their mass at early epoch of the Universe, subsequently growing by minor mergers. They could contain a fossil record of the galaxy structure formation. We have started a project in order to characterize a large sample of FGs. In this paper we present the analysis of the fossil system RX J105453.3+552102. Optical deep images were used for studying the properties of the brightest group galaxy and for computing the photometric luminosity function of the group. We have also performed a detail dynamical analysis of the system based on redshift data for 116 galaxies. This galaxy system is located at z=0.47, and shows a quite large line-of-sight velocity dispersion σ_{v}~1000 km/s. Assuming the dynamical equilibrium, we estimated a virial mass of M ~ 10^{15} h_{70} M_{\odot}. No evidence of substructure was found within 1.4 Mpc radius. We found a statistically significant departure from Gaussianity of the group members velocities in the most external regions of the group. This could indicate the presence of galaxies in radial orbits in the external region of the group. We also found that the photometrical luminosity function is bimodal, showing a lack of M_{r} ~ -19.5 galaxies. The brightest group galaxy shows low Sersic parameter (n~2) and a small peculiar velocity. Indeed, our accurate photometry shows that the difference between the brightest and the second brightest galaxies is 1.9 mag in the r-band, while the classical definition of FGs is based on a magnitude gap of 2. We conclude that this fossil system does not follow the empirical definition of FGs. Nevertheless, it is a massive, old and undisturbed galaxy system with little infall of L^{*} galaxies since its initial collapse.

研究动机与目标

• 利用多波段数据表征化石星系群候选体RX J105453.3+552102。
• 检验其是否满足基于星等差距和动力学特性的经验化石星系群定义。
• 研究z ≈ 0.5处大质量、动力学松弛星系团的形成历史与演化状态。
• 利用Millennium模拟评估此类系统在ΛCDM框架下的稀有性。

提出的方法

• 获取深度光学成像，以测量最亮星系群星系（BGG）的光度函数和Sersic轮廓。
• 收集116个星系的红移数据，进行动力学分析并识别出78个星系群成员。
• 在动力学平衡假设下，计算速度 dispersion（σᵥ ≈ 1000 km s⁻¹）和virial质量（M(<R₂₀₀) ≈ 1 × 10¹⁵ h₇₀⁻¹ M☉）。
• 利用空间和运动学数据，分析速度分布的非高斯性及外区的径向各向异性。
• 将观测到的光度函数与模拟及以往星系团研究进行比较，检测双峰性和Mr ≈ -19.5处的凹陷。
• 利用Millennium模拟估算在z = 0.5处、σ₈ = 0.8条件下，M₂₀₀ > 1 × 10¹⁵ h₇₀⁻¹ M☉的高质晕的预期空间密度。

实验结果

研究问题

• RQ1 RX J105453.3+552102是否满足化石星系群分类的经典星等差距标准（Δm₁₂ > 2 mag）？
• RQ2 RX J105453.3+552102的动力学状态如何？是否存在子结构或径向各向异性？
• RQ3 该系统的光度函数如何反映其形成历史与星系群的特性？
• RQ4 RX J105453.3+552102是否与ΛCDM模拟中对z ≈ 0.5处大质量、动力学松弛星系团的预测一致？

主要发现

• 该系统的红移为⟨z⟩ = 0.47，速度 dispersion 为σᵥ ≈ 1000 km s⁻¹，由此得出virial质量为M(<R₂₀₀) ≈ 1 × 10¹⁵ h₇₀⁻¹ M☉。
• 在1.4 Mpc范围内未检测到子结构，表明其处于动力学松弛状态，但外区速度分布的非高斯性提示存在径向各向异性。
• 光度函数呈双峰性，且在Mr ≈ -19.5处存在凹陷，表明中间光度星系的缺失。
• BGG的Sersic指数n ≈ 2，且呈现盘状等高线，表明其形成可能源于富含气体的并合，而非干式、等质量的星系团并合。
• 两颗最亮星系之间的星等差距在r波段为1.9 mag，未达到经典化石星系群定义的2 mag阈值。
• 在σ₈ = 0.8的Millennium模拟中，z = 0.5处每模拟体积仅预期存在约1个此类大质量星系团，表明此类系统极为稀有。