PSZ2 G181.06+48.47 III: weak-lensing analysis and merging scenario reconstruction of a low-mass cluster with exceptionally-distant radio relics (2501.09067v2)

Abstract: The galaxy cluster PSZ2 G181.06+48.47 ($z=0.234$) is a post-merging system that exhibits symmetric double radio relics separated by ~2.7 Mpc. We present the first weak-lensing analysis of PSZ2 G181.06+48.47 and propose possible merging scenarios using numerical simulations. Our analysis with Subaru Hyper Suprime-Cam imaging identifies a binary dark matter structure consisting of northern and southern components, separated by ~500 kpc. Assuming Navarro-Frenk-White (NFW) halos, the masses for the northern and southern subclusters are $M_{200c}^{N} = 0.88_{-0.30}^{+0.35} \times 10^{14} M_{\odot}$ and $M_{200c}^{S} = 2.71_{-0.48}^{+0.51} \times 10^{14} M_{\odot}$, respectively. By superposing the two NFW halos, we determine the total mass of the cluster to be $M_{200c} = 4.22_{-1.00}^{+1.10} \times 10^{14} M_{\odot}$ ($M_{500c} = 2.90_{-0.69}^{+0.75} \times 10^{14} M_{\odot}$). Our mass estimate suggests that the two relics are located around the cluster $R_{200c}$, where the density of the intracluster medium is very low. Our idealized simulations find that an off-axis collision of a 3:1 major merger can simultaneously reproduce the observed relic and dark matter halo separations. From these findings, we suggest that the system is observed ~0.9 Gyr after the first pericenter passage and is returning from the first apocenter.

• The paper reveals a binary dark matter configuration with two subclusters separated by 500 kpc using weak-lensing techniques.
• It employs Subaru HSC data and NFW modeling to estimate subcluster masses of approximately 0.88×10^14 M⊙ and 2.71×10^14 M⊙.
• The study reconstructs an off-axis 3:1 merger occurring roughly 0.9 Gyr after pericenter to explain the cluster's exceptionally distant radio relics.

Analysis of Weak-Lensing in the Merging Cluster PSZ2\,G181.06+48.47

The paper conducted by Ahn et al. investigates the post-merging galaxy cluster PSZ2\,G181.06+48.47 at a redshift of $z=0.234$, focusing on its unique double radio relics separated by an extraordinary 2.7 Mpc. This research employs weak gravitational lensing (WL) techniques to analyze the cluster's dark matter distribution and to propose plausible merging scenarios. The combination of Subaru Hyper Suprime-Cam data with numerical simulations allows for detailed observations of the dark matter structure and exploration of the cluster's merging history.

Numerical Results

The WL analysis reveals a binary dark matter configuration divided into northern and southern subclusters, spaced 500 kpc apart. Using the Navarro-Frenk-White (NFW) halo model, the estimated masses for these subclusters are $$M_{200c}^\text{N} = 0.88_{-0.30}^{+0.35} \times 10^{14} M_{\odot}$$ and $$M_{200c}^\text{S} = 2.71_{-0.48}^{+0.51} \times 10^{14} M_{\odot}$$. The total cluster mass, derived from superimposing the two subcluster masses, is determined to be $$M_{200c} = 4.22_{-1.00}^{+1.10} \times 10^{14} M_{\odot}$$. The double radio relics reside around the cluster's $R_{200c}$, a region characterized by a low intracluster medium (ICM) density.

Key Observations

The paper identifies several highly relevant phenomena associated with the cluster:

• Double Radio Relics: The symmetric alignment of the radio relics, each positioned equidistant from the cluster center, provides critical insights into the dynamics and shocks resulting from cluster mergers. Their significant separation suggests advanced propagation in a low-density ICM environment.
• Weak-Lensing Mass Estimates: The combined WL techniques and NFW modeling provide precise mass determinations for the subclusters, revealing insights into the gravitational effects and dark matter distribution affected by the merger process.
• Simulation of Merger Dynamics: The paper leverages idealized simulations to explore possible merger scenarios, concluding that an off-axis 3:1 major merger offers a valid explanation, occurring roughly 0.9 Gyr after the initial pericenter passage. This scenario accurately reproduces both the relic and dark matter separations observed.

Implications and Future Directions

The detailed analysis and conclusion that PSZ2\,G181.06+48.47 is in a late-stage merger offer a deeper understanding of cluster collision dynamics and the role of radio relics as probes of past cosmic events. Practically, these findings might influence observational strategies seeking radio relics in environments with similar ICM conditions. Theoretically, this paper enhances models of shock acceleration of cosmic rays and magnetic field interactions within clusters.

Future research should focus on refining the understanding of the time evolution of merger shocks and the detailed morphology of radio relics through high-resolution radio and X-ray observations. By investigating the discrepancies in relic visibility and enhancing the calibration of WL methods, upcoming studies could potentially sharpen models of cosmic structure formation and dark matter behaviors during cluster mergers.

Overall, this in-depth research highlights the power of combined observational and simulation efforts in unraveling the complexities of large-scale cosmic structures, offering a platform for further exploration into the behavior of matter in the Universe's most massive systems.