Discovery of a Radio Halo (and Relic) in a M 500 < 2 × 1014 M ⊙ Cluster

Radio halos are diffuse synchrotron sources observed in dynamically unrelaxed galaxy clusters. Current observations and models suggest that halos trace turbulent regions in the intracluster medium where mildly relativistic particles are reaccelerated during cluster mergers. Due to the higher luminosities and detection rates with increasing cluster mass, radio halos have been mainly observed in massive systems ( M500≳5×1014 M⊙ ). Here, we report the discovery of a radio halo with a largest linear scale of ≃750 kpc in PSZ2G145.92-12.53 (z = 0.03) using LOw Frequency ARray (LOFAR) observations at 120–168 MHz. With a mass of M500=(1.9±0.2)×1014 M⊙ and a radio power at 150 MHz of P150=(3.5±0.7)×1023 W Hz−1, 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 ∼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 (∼5%) based on turbulent reacceleration 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.