关闭潘多拉之盒——Abell 2744 最深度的 X 射线观测与多波段并合图景
Closing Pandora's Box -- The deepest X-ray observations of Abell 2744 and a multi-wavelength merger picture
AI总结：
本文利用 Chandra 超深 X 射线观测结合并合模拟，揭示 Abell 2744 中三个次团冷核的瓦解与西北激波，排除了西北次团的引力弹弓情景，提出南北正面并合后西北次团首次落入的演化图景，并指出大尺度超密度区域对观测的贡献。
摘要：
Abell 2744，又称潘多拉星系团，是一个复杂的并合星系团。虽然沿南北轴方向存在一次明显的主并合，但西北次团（northwest subcluster）的动力学状态一直高度不确定。我们利用 Chandra X 射线天文台获得了 Abell 2744 的超深（约 2.1 Ms）X 射线观测数据，并结合一套理想化的星系团并合模拟重新解读了多波段图景。新数据以前所未有的细节揭示了三个 X 射线明亮次团中冷核的瓦解，并证实了西北方向存在激波。对团成员星系的位置-速度聚类分析显示出一个清晰分离的 S2 成分，其 Δz 意味着 53 Mpc 的间距或 4500 km/s 的视线速度，或很可能是两者的某种组合。虽然双体并合模拟允许西北次团在第一次近心点通过后经历引力弹弓效应，但三体并合模拟排除了这一情景，因为两次并合必须相隔约 0.5 Gyr 发生，而两次并合激波的联合冲击会完全瓦解东南和西北冷核；这些冷核只有在 1-2 Gyr 后才能重新形成，届时核心间距将远超观测值。最能描述 Abell 2744 的情景是 0.5-0.6 Gyr 前发生的一次南北方向正面并合，随后西北次团首次落入。此外，我们注意到，一个包含三个星系团尺度暗物质晕、质量与引力透镜约束一致的模型，所产生的透镜会聚度和表面亮度在大部分视场中仍低于观测值，而温度则与观测一致。这表明存在一个大尺度超密度区域，它在不加热最致密气体的情况下对弥散发射和总面密度有贡献。
英文摘要：
Abell 2744, also known as Pandora's Cluster, is a complex merging galaxy cluster. While a major merger is clear along the north-south axis, the dynamical state of the northwest subcluster has been highly uncertain. We present ultra-deep ($\approx$2.1 Ms) X-ray observations of Abell 2744 obtained with the Chandra X-ray Observatory and reinterpret the multi-wavelength picture with a suite of idealised simulations of galaxy cluster mergers. The new data reveal in unprecedented detail the disruption of cool cores in the three X-ray luminous subclusters and confirm the presence of a shock to the NW. A position-velocity clustering of the cluster member galaxies shows a clearly separated S2 component, with a $Δz$ implying a separation of 53 Mpc or a line-of-sight velocity of $4500\ \rm{km \ s^{-1}}$, or likely some combination of the two. While binary simulations allow NW to have undergone a gravitational slingshot after the first pericenter passage, triple merger simulations rule out this scenario, because the two mergers would have had to occur $\sim$0.5 Gyr apart, and the joint impact of the shocks from the two mergers would completely disrupt the SE and NW cool cores; they only reform after 1-2 Gyr, by which point the core separations greatly exceed observations. The scenario that best describes Abell 2744 is a head-on N-S merger $0.5-0.6$ Gyrs ago followed by a first infall of the NW subcluster. Furthermore, we note that a model with three cluster-size halos, with masses consistent with gravitational lensing constraints, nevertheless produces a lensing convergence and surface brightness lower than observed in most of the field of view, whereas the temperatures are consistent with observations. This suggests the presence of a large-scale overdensity, which contributes to the diffuse emission and total surface density without heating the densest gas.
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