PSZ2 G181.06+48.47. II. Radio Analysis of a Low-mass Cluster with Exceptionally Distant Radio Relics

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 (∼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 reacceleration scenarios. The merger axis defined by the two relics is tilted by ∼45° with respect to the plane of the sky, which implies an unprecedented physical separation of ∼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.

Keywords

• Non-thermal radiation sources
• Radio continuum emission
• Galaxy clusters
• Large-scale structure of the universe
• Extragalactic radio sources