The Planck clusters in the LOFAR sky VI. LoTSS-DR2: Properties of radio relics

TL;DR

This study provides the first statistical analysis of radio relics in galaxy clusters using homogeneous LOFAR data, revealing new insights into relic properties, their relation to cluster mass, and the connection with radio halos.

Contribution

It introduces a uniform method to analyze relic properties in a large, unbiased sample, and compares relic-hosting clusters with and without radio halos.

Findings

Approximately 10% of clusters host radio relics.

Confirmed correlation between relic size, distance from cluster center, and cluster mass.

Relics are wider than theoretical predictions, indicating gaps in current models.

Abstract

Context. It is well-established that shock waves in the intracluster medium launched by galaxy cluster mergers can produce synchrotron emission, which is visible to us at radio frequencies as radio relics. However, the particle acceleration mechanism producing these relics is still not fully understood. It is also unclear how relics relate to radio halos, which trace merger-induced turbulence in the intracluster medium. Aims. We aim to perform the first statistical analysis of radio relics in a mass-selected sample of galaxy clusters, using homogeneous observations. Methods. We analysed all relics observed by the Low Frequency Array Two Metre Sky Survey Data Release 2 (LoTSS DR2) at 144 MHz, hosted by galaxy clusters in the second Planck catalogue of SZ sources (PSZ2). We measured and compared the relic properties in a uniform, unbiased way. In particular, we developed a method to describe the characteristic downstream width in a statistical manner. Additionally, we searched for differences between radio relic-hosting clusters with and without radio halos. Results. We find that, in our sample, $\sim$ 10% of galaxy clusters host at least one radio relic. We confirm previous findings, at higher frequencies, of a correlation between the relic-cluster centre distance and the longest linear size, as well as the radio relic power and cluster mass. However, our findings suggest that we are still missing a population of low-power relics. We also find that relics are wider than theoretically expected, even with optimistic downstream conditions. Finally, we do not find evidence of a single property that separates relic-hosting clusters with and without radio halos.