The LOFAR sub-arcsecond view of the high-redshift radio relic in PSZ2 G091.83+26.11

Enhanced inverse Compton losses at high redshift are expected to steepen the spectrum of diffuse radio sources in galaxy clusters, making low-frequency (ν MHz) observations favourable. However, observations at these low frequencies suffer from limitations due to resolution, which affects our ability to determine the exact location of the particle acceleration and the correct separation from the compact emission from radio galaxies. In this work, we aim to unveil the properties of the radio relic in the distant galaxy cluster PSZ2,G091.83+26.11 (z=0.822) by resolving the location of the particle acceleration site and by carefully inspect the downstream region. We made use of the full European LOw Frequency Array (LOFAR) at 145 MHz, which enabled us to study a radio relic at (sub-)arcsecond resolutions for the first time at frequencies below 1 GHz. We complement our analysis with the corresponding arcsecond-resolution observations at higher frequencies, taken with the Karl-Jansky Very Large Array (VLA). We confirm that the diffuse radio emission is not associated with a radio galaxy, and the same spectral index gradient towards the cluster centre is found as in the previous 5'' resolution maps. The 0.4'' and 1.9'' resolution images also reveal hints of emission ahead of the shock, with a stronger brightness in a bridge of emission connecting the relic and a radio galaxy. The 1.9'' profiles across the relic's downstream at both 145 MHz and 3.0 GHz are well described with a log-normal distribution of magnetic fields. The shock surface at 145 MHz presents a sharp discontinuity, in correspondence with a change in electron density, rotation measure, and fractional polarisation values. This is possibly related to a change in the magnetic fields. Finally, we find hints of redshift evolution of the radio power versus cluster mass correlation. The impressive angular resolution achievable by the LOFAR long baselines provides an unprecedented view of the low-energy plasma in galaxy clusters. This is extremely significant in the case of high-redshift clusters, where radio emission at low frequencies is less affected by energy losses, but its detection is strongly limited by poor resolution.