A portion of light scalar dark matter, especially axions, may organize into gravitationally bound clumps (stars) and be present in large number in the galaxy today. It is therefore of utmost interest to determine if there are novel observational signatures of this scenario. Work has shown that for moderately large axion-photon couplings, such clumps can undergo parametric resonance into photons, for clumps above a critical mass $M^{\star}_c$ determined precisely by some of us in Ref. [1]. In order to obtain a clump above the critical mass in the galaxy today would require mergers. In this work we perform full 3-dimensional simulations of pairs of axion clumps and determine the conditions under which mergers take place through the emission of scalar waves, including analyzing head-on and non-head-on collisions, phase dependence, and relative velocities. Consistent with other work in the literature, we find that the final mass from the merger $M^{\star}_{\text{final}}\approx 0.7(M^{\star}_1+M^{\star}_2)$ is larger than each of the original clump masses (for $M^{\star}_1\sim M^{\star}_2$). Hence, it is possible for sub-critical mass clumps to merge and become super-critical and therefore undergo parametric resonance into photons. We find that mergers are expected to be kinematically allowed in the galaxy today for high Peccei-Quinn scales, which is strongly suggested by unification ideas, although the collision rate is small. While mergers can happen for axions with lower Peccei-Quinn scales due to statistical fluctuations in relative velocities, as they have a high collision rate. We estimate the collision and merger rates within the Milky Way galaxy today. We find that a merger leads to a flux of energy on earth that can be appreciable and we mention observational search strategies.

中文翻译：

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暗物质轴子团块和共振光子发射的合并

一部分光标量暗物质，尤其是轴子，可能会组织成受引力束缚的团块（恒星），并在今天的星系中大量存在。因此，确定这种情况是否有新的观察特征是最重要的。工作表明，对于中等大的轴子 - 光子耦合，这种团块可以通过参数共振转化为光子，对于超过临界质量 $M^{\star}_c$ 的团块，我们中的一些人在参考文献中精确确定。[1]。为了在今天的星系中获得超过临界质量的团块，需要合并。在这项工作中，我们对成对的轴子团进行了全 3 维模拟，并确定了通过发射标量波发生合并的条件，包括分析正面和非正面碰撞、相位相关性、和相对速度。与文献中的其他工作一致，我们发现合并的最终质量 $M^{\star}_{\text{final}}\approx 0.7(M^{\star}_1+M^{\star} _2)$ 大于每个原始团块质量（对于 $M^{\star}_1\sim M^{\star}_2$）。因此，亚临界质量团块有可能合并并变成超临界质量，从而经历参数共振成为光子。我们发现，对于高 Peccei-Quinn 尺度的星系，预计今天在运动学上允许合并，尽管碰撞率很小，但统一思想强烈暗示了这一点。由于相对速度的统计波动，具有较低 Peccei-Quinn 标度的轴子可能会发生合并，因为它们具有较高的碰撞率。我们估计了今天银河系内的碰撞和合并率。