Deep insights into Abell 2163: unveiling the treasure trove of ICM plasma physics

TL;DR

This work presents a deep, multi-frequency analysis of Abell 2163, revealing an exceptionally extended radio halo reaching $\sim r_{500}$ with a frequency-dependent e-folding radius and a bright central ridge, plus a NE relic and several peripheral sources. By combining high-sensitivity uGMRT data (400 and 650 MHz) with archival VLA and XMM-Newton observations, the authors map the spatial and spectral properties of the nonthermal ICM and correlate them with the thermal X-ray emission, finding a sublinear but robust radio–X-ray correlation that varies with radius and frequency. Spatially resolved spectral-index maps show fluctuations and radial steepening consistent with inhomogeneous turbulent re-acceleration and magnetic-field variations, while seed electrons from tailed galaxies and peripheral lobes can sustain the halo with modest acceleration efficiency (~0.1–0.2%). The results underscore the role of merger-driven turbulence in powering extended nonthermal plasma across cluster volumes, highlight the complexities of magnetic-field scaling and particle acceleration, and point to SKA-era observations as crucial for probing peripheral cluster regions in greater detail.

Abstract

Nonthermal emission observed in galaxy clusters provides a direct probe into the plasma physics of the intra-cluster medium (ICM) under extreme conditions. We report the first detailed analysis of the giant radio halo in the merging galaxy cluster Abell 2163, using upgraded Giant Metrewave Radio Telescope (uGMRT) and Very Large Array (VLA) observations. Combining radio data (300-1400 MHz) with archival X-ray data offers a unique opportunity to study the complex ICM physics of the cluster. The sensitive uGMRT observations map the halo emission for the first time out to an extent of ~3.3 Mpc, up to $r_{500}$, and also effectively recover other diffuse sources. The radio surface brightness profile is well fitted with an exponential function up to r$_{500}$, with an evolution of e-folding radius over frequencies (larger at low frequencies). The spatially resolved spectral index map reveals fluctuations and outward radial steepening of the average spectral index. Radio and X-ray surface brightness are well correlated, with a correlation slope of $\sim$0.70 for the halo, and $\sim$0.40 for the ridge. The correlation slope varies from cluster centre to outskirts, suggesting the magnetic field and thermal gas density scaling relation (B $\propto n_{e}^{0.5}$) should be reassessed, provided that the re-acceleration efficiency is constant. We propose that diffuse lobes at the periphery could serve as a reservoir for seed electrons, behind the radio halo emission, with an estimated acceleration efficiency reaching $\sim$0.1% in the external regions. Additionally, a major E-W merger is suggested, leading to turbulence in the ICM and generating the halo.

Figures (20)

• Figure 1: Left:uGMRT band 3 full resolution image IMG1 obtained with robust$=0.5$ is shown here. The rms is $\sigma_{\rm rms}$$=$ 25 $\mu$Jybeam$^{-1}$ and the beam is 8$"$$\times$ 7$"$ . We have labelled the relevant extended sources: the relic at the NE outskirts, and other diffuse sources and tailed galaxies. The low-surface brightness gigantic radio halo emission is seen in the central region. Right:uGMRT band 4 full resolution image IMG5 obtained with robust$=0.5$ is shown here. The rms is $\sigma_{\rm rms}$$=$ 8.5 $\mu$Jybeam$^{-1}$ and the beam is 5$"$$\times$ 4$"$.
• Figure 2: Multi-wavelength view of the central region of the A2163 is shown. The white intensity shows the optical image of the A2163 field in the DSS2 red. The intensity in blue shows the XMM-Newton X-ray image at 0.3$-$2.5 keV, which is smoothed to 5$"$ using a Gaussian kernel. The radio emission from the uGMRT band 3 image is shown in red, with resolution 16$"$$\times$ 16$"$.
• Figure 3: High (left) and low (right) resolution uGMRT band3 (top), uGMRT band4 (middle), and VLA 1.4 GHz (bottom) images of Abell 2163 are shown. The low-resolution images are created at a common 35$"$ resolution (IMG4, IMG8, IMG10). The beam size is indicated in the bottom left corner of each image. The yellow circle indicates the r$_{500}$ of the cluster, and the ridge emission is also labelled. The presence of a sharp edge at the SW outskirts is also shown (see the middle panel, right image). The contour levels starts with 3$\sigma_{rms}$$\times$ [1,2,4...], where $\sigma_{rms}$ is reported in Table. \ref{['img_summary']}.
• Figure 4: DESI i-band image (greyscale) cutout of the A2163 field shown, overlaid with the radio emission at 650 MHz, with a beam size of 5$"$$\times$ 4$"$ (IMG5). The contour levels are drawn at [90, 270]$\times$ 3$\sigma_{\rm rms}$, where $\sigma_{\rm rms} = 8.5$$\mu$Jybeam$^{-1}$, to show the overlay more effectively. The magenta circle indicates the position of the peak of the tailed radio sources and the coincidence of a galaxy in optical.
• Figure 5: Integrated spectrum of the radio halo and relics from literature data and our work is shown. The magenta sector indicates the observations discussed in this work. The dashed line shows a single power-law fit to the spectrum. The error bars are estimated using the equation. \ref{['eq-flux-err']} for our analysis.