In the wake of dark giants: new signatures of dark matter self-interactions in equal-mass mergers of galaxy clusters

Abstract

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