PSZ2 G181.06+48.47 II: radio analysis of a low-mass cluster with exceptionally-distant radio relics

TL;DR

This study presents detailed radio observations of a low-mass galaxy cluster with exceptionally distant and large radio relics, revealing insights into shock-driven particle acceleration during cluster mergers.

Contribution

It provides the first detailed radio analysis of PSZ2 G181.06+48.47, demonstrating the presence of double relics and a faint radio halo, and offers new insights into shock properties and merger dynamics in low-mass clusters.

Findings

Detection of two Mpc-scale radio relics with distinct properties.

Evidence of high Mach number shocks and re-acceleration processes.

Identification of a faint radio halo indicating turbulence.

Abstract

We report upgraded Giant Metrewave Radio Telescope and Karl J. Jansky Very Large Array radio observations of a low-mass merging galaxy cluster PSZ2 G181.06+48.47. This exceptional galaxy cluster hosts two megaparsec-scale diffuse sources, symmetrically located with respect to the cluster center and separated by about 2.6 Mpc in projection. We detect these low surface brightness sources in our new high-frequency observations (0.3-2 GHz) and classify them as radio relics associated with merger-driven shock fronts. The southwest relic exhibits an inverted morphology and shows evidence of spectral steepening in the post-shock region, potentially tracing a high Mach number shock ($\sim 4$) under the framework of diffusive shock acceleration. The northeast relic is found to be highly polarized with a 22% average polarization fraction at 1.5 GHz and aligned magnetic field vectors. Its spectral and polarization properties, along with the presence of a nearby tailed galaxy, support re-acceleration scenarios. The merger axis defined by the two relics is tilted by $\sim 45$ degree with respect to the plane of the sky, which implies an unprecedented physical separation of $\sim 3.5$ Mpc. We also detect a possible faint radio halo, suggesting weak turbulence in the central cluster region. We conclude that the faint double relics can be best explained by two outward moving shock waves in which particles are (re-)accelerated and that the cluster is in an evolved merger state. PSZ2 G181.06+48.47 presents a unique opportunity to investigate particle acceleration in low mass systems characterized by large relic separations.