Discovery of a radio halo (and relic) in a $M_{500} < 2 \times 10^{14}$ M$_\odot$ cluster

TL;DR

This paper reports the discovery of a low-power radio halo and relic in a low-mass galaxy cluster using LOFAR, challenging previous assumptions that such features are only found in massive clusters and enabling new constraints on their formation.

Contribution

The discovery of the least powerful radio halo in a low-mass cluster demonstrates LOFAR's capability to detect faint radio features, expanding understanding of radio halo occurrence in smaller clusters.

Findings

Radio halo with 750 kpc scale in a $1.9 imes 10^{14} M_\odot$ cluster

Detection of a radio relic 1.7 Mpc from the cluster center

LOFAR can detect radio halos in low-mass clusters, informing models of their formation

Abstract

Radio halos are diffuse synchrotron sources observed in dynamically unrelaxed galaxy clusters. Current observations and models suggest that halos trace turbulent regions in the intra-cluster medium where mildly relativistic particles are re-accelerated during cluster mergers. Due to the higher luminosities and detection rates with increasing cluster mass, radio halos have been mainly observed in massive systems ($M_{500} \gtrsim 5 \times10^{14}$ M$_\odot$). Here, we report the discovery of a radio halo with a largest linear scale of $\simeq$750 kpc in PSZ2G145.92-12.53 ($z=0.03$) using LOFAR observations at 120$-$168 MHz. With a mass of $M_{500} = (1.9\pm0.2) \times 10^{14}$ M$_\odot$ and a radio power at 150 MHz of $P_{150} = (3.5 \pm 0.7) \times 10^{23}$ W/Hz, this is the least powerful radio halo in the least massive cluster discovered to date. Additionally, we discover a radio relic with a mildly convex morphology at $\sim$1.7 Mpc from the cluster center. Our results demonstrate that LOFAR has the potential to detect radio halos even in low-mass clusters, where the expectation to form them is very low ($\sim$5%) based on turbulent re-acceleration models. Together with the observation of large samples of clusters, this opens the possibility to constrain the low end of the power-mass relation of radio halos.