[论文解读] Shape and connectivity of groups and clusters: Impact of dynamical state and accretion history

Gouin, C., Tony Bonnaire+1 more|arXiv (Cornell University)|Jan 12, 2021

Galaxies: Formation, Evolution, Phenomena参考文献 94被引用 4

一句话总结

本研究利用IllustrisTNG模拟，探究星系群和星系团的动力学状态与吸积历史如何塑造其形态及与宇宙网的连通性。研究发现，大质量晕因各向异性吸积而呈现更椭圆的形态且连通性更强；未松弛、近期形成的晕也更椭圆且连通性更强，而松弛、古老的晕则呈球形且连通性较弱——表明各向异性和连通性是形成历史的稳健示踪指标，其信息超越质量本身。

ABSTRACT

Matter distribution around clusters is highly anisotropic from their being the nodes of the cosmic web. Clusters' shape and the number of filaments they are connected to, i.e., their connectivity, should reflect their level of anisotropic matter distribution and must be, in principle, related to their physical properties. We investigate the influence of the dynamical state and the formation history on both the morphology and local connectivity of about 2400 groups and clusters of galaxies from the large hydrodynamical simulation IllustrisTNG at z=0. We find that the mass of groups and clusters mainly influences the geometry of the matter distribution: massive halos are significantly more elliptical, and more connected to the cosmic web than low-mass ones. Beyond the mass-driven effect, ellipticity and connectivity are correlated and are imprints of the growth rate of groups and clusters. Both anisotropy measures appear to trace different dynamical states, such that unrelaxed groups and clusters are more elliptical and more connected than relaxed ones. This relation between matter anisotropies and dynamical state is the sign of different accretion histories. Relaxed groups and clusters are mostly formed long time ago, and slowly accreting matter at the present time. They are rather spherical and weakly connected to their environment, mostly because they had enough time to relax and, hence, lost the connection with their preferential directions of accretion and merging. In contrast, late-formed unrelaxed objects are highly anisotropic with large connectivities and ellipticities. These groups and clusters are in formation phase and must be strongly affected by the infalling of materials from filaments.

研究动机与目标

• 理解星系群和星系团的动力学状态与吸积历史如何影响其形态及与宇宙网的连通性。
• 分离质量与形成历史及松弛状态对晕各向异性影响的效应。
• 检验椭圆率与连通性是否可作为动力学状态与吸积率的独立示踪指标。
• 评估连通性作为晕生长历史代理指标的重要性，尤其在大质量星系团中。
• 为利用模拟的动力学状态解释观测数据中的星系团形态与环境提供框架。

提出的方法

• 利用z = 0时的大型流体动力学模拟IllustrisTNG，分析约2400个星系群和星系团。
• 通过R200内暗物质粒子的惯性张量定义晕的椭圆率，测量形状各向异性。
• 利用密度场上的分水岭变换量化每个晕连接的丝状结构数量，以表征连通性。
• 基于子结构与速度 dispersion 的动力学状态估计器，将晕分类为松弛与未松弛两类。
• 在质量区间（M200 > 10^13.5 M⊙/h）内进行统计分析，以分离质量效应与动力学状态效应。
• 将连通性与椭圆率与吸积率及形成红移相关联，以推断生长历史。

实验结果

研究问题

• RQ1 星系群或星系团的动力学状态如何影响其椭圆率与与宇宙网的连通性？
• RQ2 椭圆率与连通性在多大程度上反映相同或不同的晕形成历史方面？
• RQ3 是否存在连通性与吸积率之间的相关性，且该相关性独立于质量？
• RQ4 在不同质量区间内，松弛与未松弛晕在形态与环境连通性方面有何差异？
• RQ5 连通性与椭圆率能否作为晕动力学状态与生长速率的独立指标？

主要发现

• 质量是主导晕几何形态的因素：质量越大，晕的椭圆率越高，与宇宙网的连通性越强。
• 除质量外，椭圆率与连通性显著相关，且反映相同的生长速率，表明其根源均在于吸积动力学。
• 在控制质量后，未松弛晕的平均椭圆率与连通性仍高于松弛晕，表明各向异性反映了动力学状态。
• 松弛晕通常年龄较大、呈球对称且连通性弱，因其有足够时间松弛并消除方向性吸积的痕迹。
• 未松弛晕通常年轻、快速吸积且高度各向异性，与丝状结构有强连接，表明其通过丝状吸积持续形成。
• 在固定质量下，位于宇宙网节点的晕比位于丝状结构中的晕具有更高的吸积率，证实连通性与生长率相关。