[论文解读] Radio halos in a mass-selected sample of 75 galaxy clusters: II. Statistical analysis

Cuciti, V., Cassano, R.+9 more|arXiv (Cornell University)|Jan 1, 2021

Galaxies: Formation, Evolution, Phenomena被引用 67

一句话总结

本研究基于普朗克SZ星表中质量选定的75个星系团样本，首次对射电晕进行了统计分析，结合深度射电与X射线数据。研究证实射电晕光度与星系团质量之间存在强烈相关性，显示超过90%的射电晕出现在并合星系团中，并表明星系团动力学显著影响射电功率-质量关系中的离散度，且发射率表现出明显的双峰分布。观测到的射电晕分数从高质质量星系团的约70%下降至低质量星系团的约35%，与湍流再加速模型一致。

ABSTRACT

Context. Many galaxy clusters host megaparsec-scale difiuse radio sources called radio halos. Their origin is tightly connected to the processes that lead to the formation of clusters themselves. In order to reveal this connection, statistical studies of the radio properties of clusters combined with their thermal properties are necessary. For this purpose, we selected a sample of galaxy clusters with M500≤ 6 × 1014M and z = 0.08-0.33 from the Planck Sunyaev-Zel'dovich catalogue. In Paper I, we presented the radio and X-ray data analysis that we carried out on the clusters of this sample. Aims. In this paper we exploit the wealth of data presented in Paper I to study the radio properties of the sample, in connection to the mass and dynamical state of clusters. Methods.We used the dynamical information derived from the X-ray data to assess the role of mergers in the origin of radio halos.We studied the distribution of clusters in the radio power-mass diagram, the scaling between the radio luminosity of radio halos and the mass of the host clusters, and the role of dynamics in the radio luminosity and emissivity of radio halos. We measured the occurrence of radio halos as a function of the cluster mass and we compared it with the expectations of models developed in the framework of turbulent acceleration. Results. We find that more than the 90% of radio halos are in merging clusters and that their radio power correlates with the mass of the host clusters. The correlation shows a large dispersion. Interestingly, we show that cluster dynamics contributes significantly to this dispersion, with more disturbed clusters being more radio luminous. Clusters without radio halos are generally relaxed, and the upper limits to their diffiuse emission lie below the correlation. Moreover, we show that the radio emissivity of clusters exhibits an apparent bimodality, with the emissivity of radio halos being at least ~5 times larger than the non-emission associated with more relaxed clusters. We find that the fraction of radio halos drops from ~70% in high-mass clusters to ~35% in the lower mass systems in the sample and we show that this result is in good agreement with the expectations from turbulent re-acceleration models.

研究动机与目标

• 研究射电晕特性与星系团质量及动力学状态之间的统计关联。
• 确定并合活动如何影响射电晕光度与发射率。
• 测量射电晕在不同星系团质量下的发生率，并与理论模型进行比较。
• 评估星系团动力学在射电功率-质量相关性离散度中的作用。
• 利用具有深度射电观测的完整质量选定样本，检验湍流再加速模型的预测。

提出的方法

• 从普朗克SZ星表中选取75个星系团，其M500 ≥ 6 × 10^14 M⊙，红移范围为0.08 ≤ z ≤ 0.33。
• 结合深度射电观测（610 MHz 和 1.4 GHz）与X射线推导的动力学信息，将星系团分类为并合或宁静状态。
• 在射电功率-质量（P1.4GHz–M500）与射电发射率-质量图中绘制星系团，以研究相关性与离散度。
• 整合Cassano等（2013）与Martinez Aviles等（2016）的数据，以更精确约束射电功率-质量关系的斜率。
• 在两个质量区间内测量射电晕的发生率：M < 8 × 10^14 M⊙ 与 M ≥ 8 × 10^14 M⊙。
• 将观测到的射电晕分数与Cassano & Brunetti（2005）湍流再加速模型的预测进行比较。

实验结果

研究问题

• RQ1 射电晕光度对宿主星系团质量的依赖关系如何？该关系中的离散度如何与星系团动力学相关？
• RQ2 射电晕的发生率如何随星系团质量变化？是否与湍流再加速模型的预测一致？
• RQ3 并合事件在射电晕形成中起多大作用？并合星系团中有多大比例拥有此类晕？
• RQ4 射电发射率是否存在双峰分布？其对具有与不具有晕的星系团物理条件有何启示？
• RQ5 观测到的射电晕特性能否由湍流再加速机制解释，尤其是在低质量系统中？

主要发现

• 超过90%的射电晕位于并合星系团中，仅有10%出现在宁静系统中。
• 射电晕的光度与星系团质量相关，但该关系存在显著离散度，且离散度强烈受星系团动力学状态的影响。
• 未检测到射电晕的星系团，其非热辐射的上限低于射电功率-质量关系，表明它们无法维持可探测的射电晕。
• 射电发射率表现出明显的双峰分布，拥有射电晕的星系团其发射率至少是宁静星系团的五倍。
• 拥有射电晕的星系团比例从高质量系统（M ≥ 8 × 10^14 M⊙）的约70%下降至低质量系统（M < 8 × 10^14 M⊙）的约35%，与湍流再加速模型一致。
• 观测到的射电晕发生率对质量与并合活动的依赖关系与湍流再加速模型的理论预期高度一致，表明低质量并合可能不足以激发吉赫兹频段的射电辐射。