The diffuse radio emission in the high-redshift cluster PSZ2 G091.83+26.11: total intensity and polarisation analysis with Very Large Array 1-4 GHz observations

Abstract

We present the peculiar case of PSZ2G091.83+26.11 at z=0.822. This cluster hosts a Mpc-scale radio halo and an elongated radio source, whose location with the respect to the intracluster medium (ICM) distribution and to the cluster centre is not consistent with a simple merger scenario. We use VLA data at 1-4 GHz to investigate the spectral and polarisation properties of the diffuse radio emission. We combine them with previously published data from LOFAR n the 120-168 MHz band, and from the uGMRT at 250-500 and 550-900 MHz. We also complement the radio data with Chandra X-ray observations to compare the thermal and non-thermal emission of the cluster. The elongated radio emission is visible up to 3.0 GHz and has an integrated spectral index of $-1.24\pm 0.03$, with a steepening from $-0.89\pm 0.03$ to $-1.39\pm 0.03$. These values correspond to Mach numbers ${\mathcal{M}}_{\mathrm{radio},\mathrm{int}}=3.0\pm 0.19$ and ${\mathcal{M}}_{\mathrm{radio},\mathrm{inj}}=2.48\pm 0.15$. Chandra data reveals a surface brightness discontinuity at the location of the radio source, with a compression factor of $\mathcal{C}=2.22_{-0.30}^{+0.39}$ (i.e. ${\mathcal{M}}_{\mathrm{Xray}}=1.93_{-0.32}^{+0.42}$). We also find that the source is polarised at GHz frequencies. We estimate an intrinsic polarisation fraction of $\sim 0.2$, a Rotation Measure of $\sim 50\mathrm{rad}{\mathrm{m}}^{-2}$ (including the Galactic contribution) and an external depolarisation of $\sim 60\mathrm{rad}{\mathrm{m}}^{-2}$. The $B$-vectors are aligned with the major axis of the source, suggesting magnetic field compression. Hence, we classify this source as a radio relic. We also find a linear/super-linear correlation between the non-thermal and thermal emission. We propose an off-axis merger and/or multiple merger events to explain the position and orientation of the relic. Given the properties of the radio relic, we speculate that PSZ2G091.83+26.11 is in a fairly young merger state.

21 pages, 17 figures, 9 tables; accepted for publication in Astronomy & Astrophysics