用星系团中弓形激波的静止距离代替马赫数
Standoff Distance of Bow Shocks in Galaxy Clusters as Proxy for Mach Number
摘要：
对合并星团的X射线观测提供了许多弓形激波引导合并子星团的例子。虽然激波的马赫数可以用朗肯-雨果尼奥条件从观测到的密度跳跃中估算出来，但它只反映了激波本身的速度，通常不等于下落的子团暗物质晕的速度，也不等于分离两个子团的气态大气的接触不连续的速度。在这里，我们系统地分析额外的信息，可以通过测量的间隔距离，即之间的距离的冲击和接触不连续性，驱动这个冲击的前缘。间隔距离受到许多其他效应的影响，例如（1）主星团的引力（导致下落的子星团加速/减速），（2）主星团中大气的密度和压力梯度，（3）子星团的非球形形状，以及（4）投影效应。前两种效应倾向于使离距偏向同一方向，在合并前（后）阶段推动弓形激波更接近（远离）子团。特别是，在合并后的阶段，激波可能比在均匀介质中以恒定速度运动的物体模型所预测的更远离子集群。这就意味着，将偏离距离与密度/温度跃变的马赫数测量值结合起来，可以提供关于合并的重要信息，例如，区分合并前和合并后阶段。
英文摘要：
X-ray observations of merging clusters provide many examples of bow shocks leading merging subclusters. While the Mach number of a shock can be estimated from the observed density jump using Rankine-Hugoniot condition, it reflects only the velocity of the shock itself and is generally not equal to the velocity of the infalling subcluster dark matter halo or to the velocity of the contact discontinuity separating gaseous atmospheres of the two subclusters. Here we systematically analyze additional information that can be obtained by measuring the standoff distance, i.e. the distance between the leading edge of the shock and the contact discontinuity that drives this shock. The standoff distance is influenced by a number of additional effects, e.g. (1) the gravitational pull of the main cluster (causing acceleration/deceleration of the infalling subcluster), (2) the density and pressure gradients of the atmosphere in the main cluster, (3) the non-spherical shape of the subcluster, and (4) projection effects. The first two effects tend to bias the standoff distance in the same direction, pushing the bow shock closer to (farther away from) the subcluster during the pre- (post-)merger stages. Particularly, in the post-merger stage, the shock could be much farther away from the subcluster than predicted by a model of a body moving at a constant speed in a uniform medium. This implies that a combination of the standoff distance with measurements of the Mach number from density/temperature jumps can provide important information on the merger, e.g. differentiating between the pre- and post-merger stages.
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