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Enhanced radio emission between a galaxy cluster pair

Andrea Botteon, Turgay Caglar, Sibel Döner, Reinout J. van Weeren, Krista Lynne Smith

TL;DR

This study investigates diffuse nonthermal radio emission in the intracluster bridge of the binary cluster PSZ2 G279.79+39.09 ($z = 0.29$) using deep MeerKAT UHF-band observations at 822 MHz and complementary uGMRT Band 3 data. The authors detect faint diffuse synchrotron emission connecting the X-ray clumps, with a projected extent of roughly 1.5 Mpc × 0.8 Mpc and a radio power at 822 MHz of about $1.0$–$1.3\times10^{24}$ W Hz$^{-1}$, while the uGMRT data do not constrain the spectral index. The emission likely results from turbulence generated during the cluster encounter, consistent with turbulent reacceleration scenarios in bridges, though 3D geometry and spectral information remain uncertain. Overall, the work demonstrates the presence of nonthermal phenomena in cluster bridges and underscores the need for multi-band follow-up to fully characterize these diffuse sources, with future SKA surveys poised to advance this field.

Abstract

Interacting pairs of galaxy clusters offer a unique opportunity to study the properties of the gas residing in the intracluster bridge connecting them. As a consequence of the encounter, both the X-ray and radio emission from the gas are expected to be enhanced by shocks and turbulence, facilitating their detection. PSZ2 G279.79+39.09 is likely an off-axis merging system at $z = 0.29$ with its two main cluster components observed at a projected distance of $\sim$1.3 Mpc. In this paper, we investigate the presence of diffuse radio emission associated with the system. We observed this cluster pair with the MeerKAT UHF band (544-1088 MHz) for 7.5 h and with the uGMRT band 3 (300-500 MHz) for 8 h. These are the first targeted radio observations of this system. We discover diffuse synchrotron emission in the system, with indication of enhanced emission in the region bridging the cluster pair. The detection is based on the MeerKAT UHF data, while the uGMRT band 3 observation does not allow us to derive a stringent limit on the spectral index of the source. This emission is likely generated by the turbulence injected as a consequence of the cluster-cluster encounter. However, the study of its physical properties is limited by the observations currently available on the target. If the two clusters have not yet collided, this emission would resemble the radio bridges observed in A399-A401 and A1758N-S. As other systems with multiple cluster components studied in recent years, the analyzed cluster pair represents an appealing target to investigate the presence of nonthermal phenomena beyond the well-studied denser regions of the intracluster medium. While in this work we presented a new detection, our analysis underlines the need for multi-band observations to fully understand these kinds of sources.

Enhanced radio emission between a galaxy cluster pair

TL;DR

This study investigates diffuse nonthermal radio emission in the intracluster bridge of the binary cluster PSZ2 G279.79+39.09 () using deep MeerKAT UHF-band observations at 822 MHz and complementary uGMRT Band 3 data. The authors detect faint diffuse synchrotron emission connecting the X-ray clumps, with a projected extent of roughly 1.5 Mpc × 0.8 Mpc and a radio power at 822 MHz of about – W Hz, while the uGMRT data do not constrain the spectral index. The emission likely results from turbulence generated during the cluster encounter, consistent with turbulent reacceleration scenarios in bridges, though 3D geometry and spectral information remain uncertain. Overall, the work demonstrates the presence of nonthermal phenomena in cluster bridges and underscores the need for multi-band follow-up to fully characterize these diffuse sources, with future SKA surveys poised to advance this field.

Abstract

Interacting pairs of galaxy clusters offer a unique opportunity to study the properties of the gas residing in the intracluster bridge connecting them. As a consequence of the encounter, both the X-ray and radio emission from the gas are expected to be enhanced by shocks and turbulence, facilitating their detection. PSZ2 G279.79+39.09 is likely an off-axis merging system at with its two main cluster components observed at a projected distance of 1.3 Mpc. In this paper, we investigate the presence of diffuse radio emission associated with the system. We observed this cluster pair with the MeerKAT UHF band (544-1088 MHz) for 7.5 h and with the uGMRT band 3 (300-500 MHz) for 8 h. These are the first targeted radio observations of this system. We discover diffuse synchrotron emission in the system, with indication of enhanced emission in the region bridging the cluster pair. The detection is based on the MeerKAT UHF data, while the uGMRT band 3 observation does not allow us to derive a stringent limit on the spectral index of the source. This emission is likely generated by the turbulence injected as a consequence of the cluster-cluster encounter. However, the study of its physical properties is limited by the observations currently available on the target. If the two clusters have not yet collided, this emission would resemble the radio bridges observed in A399-A401 and A1758N-S. As other systems with multiple cluster components studied in recent years, the analyzed cluster pair represents an appealing target to investigate the presence of nonthermal phenomena beyond the well-studied denser regions of the intracluster medium. While in this work we presented a new detection, our analysis underlines the need for multi-band observations to fully understand these kinds of sources.

Paper Structure

This paper contains 8 sections, 7 figures.

Table of Contents

  1. Introduction
  2. Data reduction
  3. MeerKAT
  4. uGMRT
  5. Results
  6. Discussion
  7. Conclusions
  8. Additional radio images

Figures (7)

  • Figure 1: Multiband view of G279+39. Top: optical g,r,z image from the DESI Legacy Imaging Surveys dey19. Center: X-ray adaptively smoothed image in the 0.5--2.0 keV band with point sources removed doner25sub. Bottom: MeerKAT radio image at 822 MHz with a beam of $9.0\hbox{$^{\prime\prime}$} \times 8.0\hbox{$^{\prime\prime}$}$ (shown in the bottom left corner) and a noise of $\sigma_{\rm rms} = 4.7$$\mu$Jy beam$^{-1}$. The cross and diamond markers indicate the position of the galaxies mentioned in the text. The color bar units are counts s$^{-1}$ for the X-ray image and Jansky beam$^{-1}$ for the radio image.
  • Figure 2: Highlighting the diffuse radio emission excess in G279+39. Top panels: MeerKAT images obtained by imaging all baselines (left, same as in Fig. \ref{['fig:multiband']}) and only baselines longer than $1500\lambda$ (right). Bottom panel: fractional difference image, computed by subtracting the top right image from the top left image and dividing the result by the former.
  • Figure 3: Source-subtracted low-resolution MeerKAT images at 822 MHz. Top: image produced by subtracting compact sources in the uv-plane and reimaging the visibilities with a Gaussian taper, yielding a beam of $16.8\hbox{$^{\prime\prime}$} \times 16.3\hbox{$^{\prime\prime}$}$ and a noise of $\sigma_{\rm rms} = 6.3$$\mu$Jy beam$^{-1}$. The yellow ellipse denotes the region used to extract the flux densities of the diffuse emission. Bottom: image obtained by subtracting compact sources in the image-plane at high resolution, then convolving the result to the same $16.8\hbox{$^{\prime\prime}$} \times 16.3\hbox{$^{\prime\prime}$}$ beam. Solid contours are spaced by a factor of 2 from the $3\sigma_{\rm rms}$ level. The $-3\sigma_{\rm rms}$ contour is shown in dashed. The beams are shown in the bottom left corners. The color bar units are Jansky beam$^{-1}$.
  • Figure 4: X-ray image with the MeerKAT low-resolution radio contours with sources subtracted in the uv-plane overlaid. The beam of the radio image is shown in the bottom left corner.
  • Figure 5: Flux density profile across the cluster pair from east to west integrated above the $3\sigma_{\rm rms}$ level and over the entire rectangular regions (see legend). The solid lines are obtained by fitting an exponential decaying model to the data points on the side away from the opposite cluster, starting from the emission peak. The dashed lines represent the mirrored model toward the other cluster.
  • ...and 2 more figures