Title: In the Wake of Dark Giants: New Signatures of Dark Matter Self Interactions in Equal Mass Mergers of Galaxy Clusters Show Chinese title

Title: 在黑暗巨人的阴影下：星团等质量合并中暗物质自相互作用的新特征

Abstract: Merging galaxy clusters have been touted as one of the best probes for constraining self-interacting dark matter, but few simulations exist to back up this claim. We simulate equal mass mergers of 10$^{15}$ M$_\odot$ halos, like the El Gordo and Sausage clusters, with cosmologically-motivated halo and merger parameters, and with velocity-independent dark-matter self-interactions. Although the standard lore for merging clusters is that self-interactions lead to large separations between the galaxy and dark-matter distributions, we find that maximal galaxy-dark matter offsets of $\lesssim~20$~kpc form for a self-interaction cross section of $\sigma_\text{SI}/m_\chi$ = 1 cm$^2$/g. This is an order of magnitude smaller than those measured in observed equal mass and near equal mass mergers, and is likely to be even smaller for lower-mass systems. While competitive cross-section constraints are thus unlikely to emerge from offsets, we find other signatures of self-interactions which are more promising. Intriguingly, we find that after dark matter halos coalesce, the collisionless galaxies (and especially the Brightest Cluster Galaxy [BGC]) oscillate around the center of the merger remnant on stable orbits of 100 kpc for $\sigma_\text{SI}/m_\chi = 1$~cm$^2$/g for at least several Gyr, well after the clusters have relaxed. If BCG miscentering in relaxed clusters remains a robust prediction of SIDM under the addition of gas physics, substructure, merger mass ratios (e.g., 10:1 like the Bullet Cluster), and complex cosmological merger histories, the observed BCG offsets may constrain $\sigma_\text{SI}/m_\chi \lesssim$ 0.1 cm$^2$/g---the tightest constraint yet. Show Chinese abstract

Abstract: 正在合并的星系团被认为是限制自相互作用暗物质的最佳探针之一，但支持这一说法的模拟却很少。 我们模拟了两个质量相等的$^{15}$M$_\odot$晕之间的合并，类似于El Gordo和香肠星系团，并使用宇宙学上有动机的晕参数和合并参数，以及速度无关的暗物质自相互作用。 尽管合并星系团的标准观点是自相互作用会导致星系和暗物质分布之间产生大分离，但我们发现对于自相互作用截面为$\sigma_\text{SI}/m_\chi$= 1 cm$^2$/g时，最大星系-暗物质偏移量仅为$\lesssim~20$~kpc。 这比观测到的质量相等和接近质量相等的合并所测量到的偏移量小一个数量级，并且对于较低质量系统可能更小。 因此，竞争性的截面限制不太可能从偏移量中出现，但我们发现了其他更有前景的自相互作用特征。 有趣的是，我们发现，在暗晕合并之后，无碰撞的星系（尤其是最亮的星系团星系[BGC]）在合并残余物中心周围以100 kpc的稳定轨道振荡，持续时间至少有几Gyr，远超过星系团的放松时间，时间为$\sigma_\text{SI}/m_\chi = 1$~cm$^2$/g。 如果在松散星系团中BCG（星系团中心星系）的偏心现象在加入了气体物理、亚结构、合并质量比（例如子弹星系团那样的10:1）、以及复杂的宇宙学合并历史后仍然是SIDM（自相互作用暗物质）的一个稳健预测，那么观测到的BCG偏移量可能会限制 $\sigma_\text{SI}/m_\chi \lesssim$ 0.1 cm$^2$/g---这是迄今为止最严格的限制。