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 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.

Analyzing the Impact of Self-Interacting Dark Matter in Equal Mass Mergers of Galaxy Clusters

In the scholarly paper titled "In the Wake of Dark Giants: New Signatures of Dark Matter Self Interactions in Equal Mass Mergers of Galaxy Clusters," the authors explore the complexities inherent in galaxy cluster mergers when considering self-interacting dark matter (SIDM). The traditional paradigm in cosmology has depicted dark matter as cold and collisionless, but emerging observations and numerical simulations have raised questions about this assumption, especially at sub-galactic scales. This study primarily focuses on the ramifications of SIDM in the context of equal mass mergers of massive galaxy clusters, serving as a proxy for investigating dark matter collisionality.

The authors employed $N$-body simulation techniques to analyze collisions involving 10$^{15}$ M$_\odot$ galaxy clusters, reminiscent of real-world examples like the El Gordo and Sausage clusters. Their simulations incorporated SIDM with velocity-independent interactions and investigated scenarios with cross-sections ranging from 1 to 10 cm$^2$/g. A significant focus of the study revolves around measuring offsets between galaxies and dark matter post-merger—a potential signature of SIDM. Traditionally, these offsets have been anticipated to be substantial in SIDM scenarios; however, the study’s simulations reveal otherwise. Offsets observed were notably smaller, often less than 20 kpc for a cross-section of 1 cm$^2$/g.

Notably, the analysis indicates that observed galaxy-dark matter offsets in equal mass mergers are not sufficient to constrain the self-interaction cross-section effectively. This revelation challenges previous assumptions that merging clusters could be straightforward observational probes for measuring SIDM properties. Due to the transient nature of these offsets—most prominent immediately after the pericenter—but quickly diminishing after—measuring them requires precise timing and consideration of various merger parameters which can significantly influence outcomes.

An intriguing aspect of this study is its focus on alternative SIDM signatures, especially post-coalescence behavior. After the merger and relaxation of dark matter halos, a core sloshing phenomenon emerges—a stable oscillation of galaxies around the core of the remnant cluster. These oscillations persist on scales of 100 kpc even several Gyr post-merger, suggesting a robust signature potentially leading to tighter constraints on $\sigma_\text{SI}/m_\chi$. Such persistent miscentering could serve as a lucrative observational target; current BCG misalignment hints at $\sigma_\text{SI}/m_\chi \lesssim 0.1\hbox{ cm}^2/\hbox{g}$, thus suggesting far stronger constraints on SIDM—the most stringent to date.

The implications of these findings underscore the complexity in leveraging galaxy cluster mergers as probes for SIDM. While traditional measures like offsets might not yield the expected constraints, the study paves the way for deeper inquiry into SIDM effects that could be observationally verified through miscentering in relaxed clusters. However, realizing these aims necessitates further exploration through hydrodynamic simulations and examining more complex merger histories. This could open pathways for comprehensive cosmological analyses that incorporate gas dynamics, unequal mass mergers, and diverse interaction models to mimic realistic intergalactic conditions better.

In conclusion, this paper provides a significant reevaluation of SIDM's impact during equal mass galaxy cluster mergers. While posing challenges to traditional offset measurement methods, it uncovers promising avenues for future investigations into dark matter properties through BCG miscentering and core dynamics post-merger. This work calls for revisited observational strategies and refined simulation efforts to deepen our understanding of dark matter's fundamental nature and interactions within the cosmic landscape.