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arXiv 1904.03052astro-ph.GAastro-ph.HE
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星系团外围和射电遗迹的失控合并冲击

Runaway Merger Shocks in Galaxy Cluster Outskirts and Radio Relics

Congyao Zhang, Eugene Churazov, William R. Forman, Natalia Lyskova

备注：
9 pages, 8 figures; published in MNRAS
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摘要：

当两个子集群合并时，自然会产生中等强度的冲击。例如，当一个较小的子星系团落入一个更大质量星系团的引力势中时，就会形成弓形激波，并与子星系团一起运动。然而，在近心通过后，子星系团被主星系团的重力减速，而激波继续远离星系团中心。这些冲击被认为是在许多集群中发现的无线电遗迹供电的有前途的候选人。本文的目的是探讨的命运，这样的冲击，当他们旅行到集群的郊区，远离的地方，冲击发起。在均匀介质中，这种“逃逸”激波会随着距离的增加而减弱。然而，当激波在星系团中移动到大半径时，激波沿着陡峭的密度梯度移动，这有助于激波在很长的距离上保持其强度。观测和数值模拟表明，在$R_{500}$之外，气体密度剖面与$\sim r^{-3}$一样陡，甚至比它更陡，这表明在星团外围存在一个中强地震的“宜居带“，在那里，地震强度可以保持甚至放大。失控激波的一个特征是，相对于初始状态，强压缩仅限于激波后面的一个狭窄区域。因此，如果这样一个激波经过一个预先存在相对论粒子的区域，那么由于纯绝热压缩，无线电发射率的增加也将被限制在一个狭窄的径向壳层内。

英文摘要：

Moderately strong shocks arise naturally when two subclusters merge. For instance, when a smaller subcluster falls into the gravitational potential of a more massive cluster, a bow shock is formed and moves together with the subcluster. After pericenter passage, however, the subcluster is decelerated by the gravity of the main cluster, while the shock continues moving away from the cluster center. These shocks are considered as promising candidates for powering radio relics found in many clusters. The aim of this paper is to explore the fate of such shocks when they travel to the cluster outskirts, far from the place where the shocks were initiated. In a uniform medium, such a "runaway" shock should weaken with distance. However, as shocks move to large radii in galaxy clusters, the shock is moving down a steep density gradient that helps the shock to maintain its strength over a large distance. Observations and numerical simulations show that, beyond $R_{500}$, gas density profiles are as steep as, or steeper than, $\sim r^{-3}$, suggesting that there exists a "Habitable zone" for moderately strong shocks in cluster outskirts where the shock strength can be maintained or even amplified. A characteristic feature of runaway shocks is that the strong compression, relative to the initial state, is confined to a narrow region just behind the shock. Therefore, if such a shock runs over a region with a pre-existing population of relativistic particles, then the boost in radio emissivity, due to pure adiabatic compression, will also be confined to a narrow radial shell.

机构、总结、中文摘要均由AI分析而来，可能存在错误，请以论文正文为准。

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