The LOFAR sub-arcsecond view of the high-redshift radio relic in PSZ2G091.83+26.11

Abstract

Enhanced inverse Compton (IC) losses at high redshift steepen diffuse radio spectra in galaxy clusters, making low-frequency (~100 MHz) observations favorable. However, low-frequency studies often lack the resolution needed to locate particle acceleration sites or separate diffuse emission from radio galaxies. In this paper, we unveil the properties of the radio relic in the distant cluster PSZ2G091.83+26.11 (z=0.822) by resolving the acceleration site and inspecting the downstream region. Using the European LOFAR (ILT) at 145 MHz, we study a radio relic at (sub-)arcsecond resolution for the first time below 1 GHz, complemented by arcsecond-resolution VLA data at higher frequencies. We confirm the diffuse emission is not a radio galaxy. A spectral index gradient toward the cluster center matches previous 5'' maps. High-resolution 0.4'' and 1.9'' images reveal emission ahead of the shock, connecting the relic to a radio galaxy. 1.9'' profiles across the downstream at 145 MHz and 3.0 GHz follow a log-normal magnetic field distribution. The 145 MHz shock surface shows a sharp discontinuity at the same location of a change in electron density, Rotation Measure, and fractional polarization, likely tied to magnetic field changes. 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 is opening an unprecedented view of the low energetic 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.

Figures (11)

• Figure 1: Comparison of the uv-coverage for the LOFAR 145 MHz long-baseline (left panel) and the LOFAR 145 MHz Dutch array (right panel).
• Figure 2: Full-resolution (i.e. $\Theta=0.4"\times0.3"$) ILT 145 MHz images of PSZ2G091 centred on the radio relic. On the left panel, we show the whole radio relic area, while in the right panel we show a zoom on R2, with radio contours drawn starting from $2{\sigma_{\rm rms}}$ (where ${\sigma_{\rm rms}}=25.4$$\mu$${\rm Jy~beam}^{-1}$) and the new features highlighted.
• Figure 3: Lower-resolution ILT 145 MHz images of PSZ2G091. From left to right: $\Theta=1.8"\times1.1"$ (${\sigma_{\rm rms}}=91.2$$\mu$${\rm Jy~beam}^{-1}$); $\Theta\sim3.0"\times1.7"$ (${\sigma_{\rm rms}}=120.5$$\mu$${\rm Jy~beam}^{-1}$); $\Theta\sim3.6"\times2.9"$ (${\sigma_{\rm rms}}=148.2$$\mu$${\rm Jy~beam}^{-1}$); $\Theta\sim5.0"\times4.3"$ (${\sigma_{\rm rms}}=134.0$$\mu$${\rm Jy~beam}^{-1}$). Radio contours are drawn at $2.5{\sigma_{\rm rms}}\times[-1,1,2,4,8,16,32]$, with ${\sigma_{\rm rms}}$ the noise level in each image (negative contours are drawn with dashed line). Source labelling follow digennaro+21c.
• Figure 4: grz image from the 10th data release of DESI Legacy Survey (DR10) of PSZ2G091 with the $1.8"$ ILT radio contours. In the inset, we show the zoom-in on R2, with radio contours at $0.4"$ resolution. For both radio images, white contours are at the $2.5{\sigma_{\rm rms}}\times[1,2,4,8,16]$ levels (see Tab. \ref{['tab:images']} for the map noise at these resolutions), and the beam size is displayed in the lower left corners.
• Figure 5: Spectral analysis of PSZ2G091. Left: spectral index map at $1.9"$, using 145 MHz and 3.0 GHz observations. Central: spectral index map and $3.0"$, using 145 MHz, 1.5 GHz and 3.0 GHz observations. Right: spectral curvature map at $3.0"$. Radio contours in each panel are taken from the ILT 145 MHz at the corresponding resolution, and are drawn starting from the $3{\sigma_{\rm rms}}$ level.