PSZ2 G181.06+48.47 I: X-ray exploration of a low-mass cluster with exceptionally-distant radio relics (2501.07651v3)

Abstract: Relics are diffuse, highly-polarized radio sources that trace merger-driven shocks at the periphery of merging galaxy clusters. The LOFAR survey recently discovered a rare example of double relics in the low-mass cluster PSZ2 G181.06+48.47. Through a detailed exploration of new Chandra and XMM-Newton observations, we reveal that PSZ2 G181.06+48.47 has a lower mass ($M_{500,X}=2.32^{+0.29}_{-0.25}\times10^{14}$ M${\odot}$) than previously thought. Despite its cool global temperature of $kT{500}=3.62^{+0.15}_{-0.07}$ keV, PSZ2 G181.06+48.47 is one of the most disturbed clusters in the Planck sample, with a complex morphological and thermodynamic structure. We discover a set of three discontinuities within <500 kpc of the cluster center, and, from a surface brightness analysis, place $5\sigma$ upper limits of $M_{NE}<1.43$ and $M_{SW}<1.57$ for any shock associated with the relic locations. We also revise established scaling relations for double radio-relics by adding 12 new systems not included in previous work. The PSZ2 G181.06+48.47 relics have the widest separation (scaled for $r_{500}$) of all known double-relic systems. The exceptional distance from the cluster center ($>r_{200}$), indicates the relics may be associated with shocks in the ``run-away" phase. We propose that this late-stage, post-apocenter merger is captured as the two subclusters with a mass ratio of 1.2-1.4 fall back into each other. The outer relic shocks were likely produced at the first core passage, while the inner discontinuities are associated with the second infall.

• The paper determines significant discrepancies in mass estimates between X-ray and Planck measurements using advanced Chandra and XMM-Newton data.
• It identifies two merging subclusters with distinct X-ray peaks and a gas bridge, offering insights into post-merger dynamics.
• The study reveals weak shock signatures and distant double radio relics that challenge existing models of relic formation in low-mass clusters.

Analysis of PSZ2\,G181.06+48.47: A Low-Mass Galaxy Cluster with Double Radio Relics

The paper, "PSZ2\,G181.06+48.47 I: X-ray exploration of a low-mass cluster with exceptionally-distant radio relics," conducts a detailed investigation of the galaxy cluster PSZ2\,G181.06+48.47, significantly characterized by the presence of double radio relics at large separations from the cluster center. The research employs advanced observations from Chandra and XMM-Newton to unpack the complexities of a low-mass cluster, distinguishing it in the field of radio astronomy and X-ray astrophysics.

Cluster Characteristics and Discrepancies in Mass Estimates

Through X-ray observations, the research determines the mass of PSZ2\,G181.06+48.47 to be $$M_{500,X}=2.57^{+0.37}_{-0.38}\times10^{14} \;{\rm M_{\odot}}$$, indicating a significant deviation from the previous Planck mass measurements. This discovery underscores a central finding of the paper: discrepancies in mass estimation approaches highlight the necessity for combined multiplex analysis to avoid the biases inherent in singular observational methods. The cluster boasts a mean temperature of $kT_{500}=3.86^{+0.27}_{-0.29}$\,keV and reveals a highly disturbed state relative to other Planck samples—a critical insight for models studying post-merger dynamics in galaxy clusters.

Subclusters and Merger Dynamics

The analysis identifies two subclusters with a mass ratio of approximately 1.2-1.4, engaged in a merger along the northeast-southwest axis. These substructures display distinct X-ray peaks, each aligning closely with a Brightest Cluster Galaxy (BCG). The paper describes a bridge of lower entropy and stripped gas between the subclusters, hinting at partial disruption of original cluster cores due to merger activity. This scenario offers a practical case paper in understanding subcluster interactions and their thermodynamic evolution.

Inner Discontinuities and Radio Relics

X-ray analysis reveals complex morphological and thermodynamic structures marked by inner discontinuities consistent with weak shocks. Interestingly, the Mach numbers derived from X-ray observations ($\mathcal{M}_{X,\rho}\sim1.3-1.4$) are in stark contrast to those inferred from radio data, suggesting a component of the merger activity occurring off the plane-of-sky. The presence of radio relics at such large clustercentric distances (>r_{200}) is noted as an atypical feature, providing a compelling scenario for understanding late-stage post-merger phenomena and re-acceleration processes in low-mass systems.

Implications for Double Radio Relic Systems

The paper's contribution to the scaling relations of double radio relics is notable, adding 12 new systems to existing models and asserting that the PSZ2\,G181.06+48.47 observes typical LLS and radio power, albeit with unique separation attributes. Parallel investigations into the cluster's radio characteristics, as discussed, posited an inclination to the merger, potentially offering new opportunities to redefine relationships observed in relic systems when correlated with low-mass cluster dynamics.

Conclusion and Future Research

This research provides innovative insights into the paper of galaxy clusters with double relics, proffering significant revisions to our understanding of particle acceleration and cluster evolution in low-mass regimes. The discovery of PSZ2\,G181.06+48.47—characterized by wide relic separation and low mass—challenges preconceptions about the limits of radio relic formation, particularly in weak shock conditions. Prospective studies and upcoming missions, such as the Hot Universe Baryon Surveyor (HUBS) or Athena, stand to gain substantially from insights offered by this paper. Future efforts should focus on simulations and multi-frequency observations to further illuminate the intricate dynamics and formation scenarios of such systems.