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

TL;DR

This study uses simulations of galaxy cluster mergers to explore signatures of dark matter self-interactions, finding that galaxy-dark matter offsets are small but BCG oscillations could provide tighter constraints on the self-interaction cross section.

Contribution

The paper presents the first detailed simulations of equal mass galaxy cluster mergers with self-interacting dark matter, revealing new signatures like BCG oscillations for constraining dark matter properties.

Findings

Galaxy-dark matter offsets are smaller than observed in real mergers.

BCG oscillations around the merger center persist for several Gyr.

Potential for tighter constraints on dark matter self-interaction cross section from BCG offsets.

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.