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

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 ($M_{500}\gtrsim 5\times 10^{14}$ 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 $M_{500}=(1.9\pm 0.2)\times 10^{14}$ 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.

8 pages, 5 figures, 1 table (including appendix). Accepted for publication in ApJL

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