Colliding clusters and dark matter self-interactions

Abstract: When a dark matter halo moves through a background of dark matter particles, self-interactions can lead to both deceleration and evaporation of the halo and thus shift its centroid relative to the collisionless stars and galaxies. We study the magnitude and time evolution of this shift for two classes of dark matter self-interactions, viz. frequent self-interactions with small momentum transfer (e.g. due to long-range interactions) and rare self-interactions with large momentum transfer (e.g. contact interactions), and find important differences between the two cases. We find that neither effect can be strong enough to completely separate the dark matter halo from the galaxies, if we impose conservative bounds on the self-interaction cross-section. The majority of both populations remain bound to the same gravitational potential and the peaks of their distributions are therefore always coincident. Consequently any apparent separation is mainly due to particles which are leaving the gravitational potential, so will be largest shortly after the collision but not observable in evolved systems. Nevertheless the fraction of collisions with large momentum transfer is an important characteristic of self-interactions, which can potentially be extracted from observational data and provide an important clue as to the nature of dark matter.

Colliding Clusters and Dark Matter Self-Interactions

In the study of galaxy clusters and their interactions, dark matter (DM) self-interactions present an intriguing aspect that offers potential insights into the properties of DM particles. The paper "Colliding clusters and dark matter self-interactions" investigates how self-interacting dark matter (SIDM) influences cluster collisions, specifically considering the separation between DM halos and their associated collisionless galaxies.

Overview of Dark Matter Self-Interactions

The prevalent cosmological model suggests that DM is collisionless and cold, aligning with the constraints imposed by various experimental observations of DM-nucleon scattering cross-sections. However, models proposing a self-interacting dark sector, potentially with strong internal couplings but weak interactions with Standard Model particles, remain viable and are motivated as a mechanism to resolve certain small-scale structure formation issues observed in CDM cosmology.

The paper differentiates between two paradigms of DM self-interactions: frequent self-interactions characterized by small momentum transfers and rare interactions involving substantial momentum transfers. This distinction is pivotal, as it corresponds to differing physical processes and observational consequences in cluster collisions.

Frequent Self-Interactions and Drag Forces

In scenarios where frequent self-interactions occur, the DM particles undergo many small-angle scatterings, collectively imposing a drag effect on the DM halo. The paper employs numerical simulations to study this effect quantitatively. For clusters like the Bullet Cluster, stringent constraints are placed on the interaction cross-section to align with observed halo properties. The analysis reveals that drag forces arising from frequent interactions lead to a systematic deceleration of the DM halo. As a result, loosely bound galaxies may escape the halo's gravitational potential, forming a detectable separation manifesting as galaxies moving ahead of the decelerating DM halo.

The paper emphasizes that while frequent interactions can cause cumulative halo evaporation and a drag force, both effects are proportional to the momentum transfer cross-section. Numerical results indicate that even in the presence of frequent interactions, the central densities remain sufficient to prevent complete separation from galaxies, predicting observable separations below current detection thresholds.

Rare Self-Interactions and Momentum Transfers

In contrast, rare self-interactions involve significant momentum transfers, yet their infrequency implies that many DM particles traverse the cluster interaction region unscathed. During such rare interactions, scattered particles can attain velocities allowing them to escape; this modifies the tail of the DM distribution without affecting the central peak. Consequently, whereas the peak of the DM halo remains coincident with the galaxy distribution peak, a centroid shift might be observed in systems shortly after the collision due to the asymmetric distribution of scattered and unscattered particles.

The paper's analytical and numerical exploration of rare interactions highlights that the separation of DM halos and galaxies occurs primarily from the dispersion of scattered particles, creating observable variations in halo shapes post-collision. The authors outline a simulation framework and simple analytical models capturing these dynamics, emphasizing that the magnitude of separations in typical mergers such as the Bullet Cluster or Abell 520 are modest yet potentially detectable with improved observational techniques.

Observational Implications and Future Prospects

Detecting and measuring separations in colliding clusters could offer robust constraints on DM self-interaction cross-sections, significantly informing DM particle physics beyond the SM. The work presented in the paper outlines the necessity of distinguishing interaction types, emphasizing the observational signatures of frequent versus rare interactions. As future advances in astronomical observations, such as enhancements in weak lensing techniques, improve the precision of galaxy cluster surveys, the detection of such separations will play a crucial role in testing the hypothesis of self-interacting DM.

This paper contributes significantly to theoretical predictions of SIDM properties in astrophysical contexts, stimulating further research into how such interactions might manifest across cosmological structures, ultimately providing a pathway to unlocking the mysteries of the dark sector.