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

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 ${\sigma }_{\text{SI}}\mathrm{/}{m}_{\chi }$ = 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 ${\sigma }_{\text{SI}}\mathrm{/}{m}_{\chi }=1$~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 ${\sigma }_{\text{SI}}\mathrm{/}{m}_{\chi }\lesssim$ 0.1 cm${}^2$/g—the tightest constraint yet.