Observations of the galaxy cluster CL 0217+70 and its surrounding region at 1.4 GHz with the Sardinia Radio Telescope

TL;DR

This study combines Sardinia Radio Telescope and VLA data to analyze the galaxy cluster CL 0217+70 at 1.4 GHz, revealing extended emissions, spectral index variations, and questioning the nature of a candidate relic.

Contribution

First combined SRT and VLA observations of CL 0217+70 providing enhanced imaging and spectral analysis, challenging previous relic identification and revealing extended emissions.

Findings

Cluster radio halo flux density is ~14% higher with combined data.

Spectral index map shows extended region with α~0.6, questioning relic nature.

Detected extended emission possibly related to Galactic foreground rather than cluster.

Abstract

We present the results of observations performed with the Sardinia Radio Telescope (SRT) at 1.3-1.8 GHz of the galaxy cluster CL 0217+70 and a $3^\circ \times 3^\circ$ region around it. We combine the SRT data with archival Very Large Array (VLA) data to obtain images having the VLA angular resolution, but sensitive up to largest scales. The SRT+VLA combination allows us to derive a cluster radio halo flux density higher by $\sim14\%$ compared to the VLA-only data, although consistent within $1\sigma$. We derive a spectral index map between 140 MHz and 1.4 GHz, finding an extended region with spectral index $\alpha\sim0.6$ on the external part of the south-eastern candidate relic, questioning the real nature of this relic. Moreover, we detect an extended emission outside the cluster in the south-eastern area, having an angular extension of $\sim50$ arcmin on the longer side, which would correspond to $\sim10$ Mpc at the cluster distance; the emissivity that this region would have if located at the cluster distance is in line with the one estimated in candidate filaments of the cosmic web; however, the peculiar orientation of this region, not pointed towards the cluster, and the low Galactic latitude of this cluster suggest that its origin can be due to a foreground emission originating in our Galaxy.