Wiggling Through the ICM: Multi-Resolution Radio Imaging of a Tailed Radio Galaxy in MACS J1354.6+7715

TL;DR

This study uses LOFAR 144 MHz and uGMRT 400 MHz observations to analyze a dominant NAT in MACS J1354.6+7715 (z=0.3967). The high-resolution imaging reveals a ~300 kpc tail whose spectral index steepens from about −0.46 near the core to −2.43 outward, and spectral modelling yields a radiative age of 150 ± 10 Myr, corresponding to a galaxy velocity of 1956 ± 130 km s⁻¹. The X-ray morphology is bimodal but relatively undisturbed, and no diffuse halos or relics are detected, supporting a pre-merger, first-infall scenario. The inferred high velocity and lack of cluster-scale turbulence imply strong ICM-AGN interaction during early assembly, making multi-frequency radio imaging a powerful probe of both AGN lifecycles and early cluster dynamics. The study also highlights uncertainties in magnetic-field assumptions and ageing models, suggesting that deeper, broader-frequency data will improve dynamical inferences for infalling galaxies in clusters.

Abstract

Tailed radio galaxies are powerful tracers of interactions between active galactic nuclei (AGN) and the intracluster medium (ICM), providing unique insights into cluster dynamics. We present LOw Frequency ARray (LOFAR) 144 MHz and uGMRT 400 MHz observations of the cluster MACS J1354.6+7715 (z = 0.3967) to investigate the radio emission associated with its member galaxies and the cluster environment. The dominant tailed radio galaxy in the cluster exhibits a sharply bent tail extending over approximately 300 kpc, with the spectral index steepening from approximately -0.46 +/- 0.21 near the AGN core to approximately -2.43 +/- 0.30 in the outermost regions. Synchrotron modelling of the tail yields a radiative age of 150 +/- 10 Myr, implying a galaxy velocity of 1956 +/- 130 km s^-1, which is of order ~ 0.9 times the escape velocity. We find no evidence of relics or halos in our radio images, and the X-ray morphology from Chandra appears relatively undisturbed, suggesting that the system is a pre-merging candidate. Our results indicate that the radio galaxy is undergoing its first infall into the cluster, providing an excellent laboratory for studying the impact of the ICM on AGN activity and galaxy evolution, and demonstrating how multi-frequency radio observations of tailed galaxies can uniquely probe both AGN lifecycles and the early stages of cluster assembly.

Figures (9)

• Figure 1: High-resolution LOFAR HBA image (beam size: $0.30"\times 0.21"$, $\sigma_{\mathrm{rms}} = 0.031\mathrm{mJy\,beam^{-1}}$) of the tailed radio galaxy (C) in the galaxy cluster MACS J1354.6+7715, observed at 144 MHz. A and B denotes two compact radio sources, while D marks an additional radio source detected in the field. The colour scale (square-root scaling, $\gamma = 0.5$) spans from $10^{-7}\,\mathrm{Jy\,beam^{-1}}$ up to $10^{-3}\,\mathrm{Jy\,beam^{-1}}$, revealing both faint diffuse emission and compact features along the tail. Contours from the intermediate-resolution image (beam size: $1.56"\times 1.04"$, $\sigma_{\mathrm{rms}} = 0.099\,\mathrm{mJy\,beam^{-1}}$) are overlaid in cyan, starting at $5\sigma_{\mathrm{rms}}$ and increase by successive factors of $\sqrt{2}$. The white bar in the bottom-right corner corresponds to a physical scale of 100 kpc.
• Figure 2: Left: uGMRT Band-3 image (centered at 400 MHz) of the tailed radio galaxy in MACS J1354.6+7715 with a synthesized beam of $9.53" \times 5.73"$ and an rms noise level of $0.034\,\mathrm{ mJy\,beam^{-1}}$. The colour scale (square-root scaling, $\gamma = 0.5$) spans from $5\times10^{-9}\,\mathrm{Jy\,beam^{-1}}$ to $3\times10^{-2}\,\mathrm{Jy\,beam^{-1}}$. Right: LOFAR HBA image (centered at 144 MHz) convolved to match the uGMRT resolution, with an rms noise level of $0.323 \,\mathrm{mJy\,beam^{-1}}$. The colour scale (square-root scaling, $\gamma = 0.5$) spans from $5\times10^{-9}\,\mathrm{Jy\,beam^{-1}}$ to $5\times10^{-2}\,\mathrm{Jy\,beam^{-1}}$. The white ellipse in the bottom-left corner of each panel indicates the synthesized beam, and the white bar in the bottom-right corner of both panels corresponds to a physical scale of 100 kpc. The white circles (radius $=$ 90 kpc) mark the regions along the tail used for spectral ageing analysis (see Section \ref{['sec:4.2']} ).
• Figure 3: Spectral index map of the radio emission in the cluster region (Left), with the corresponding uncertainty plot (Right), derived between 144 MHz and 400 MHz. The head of the tailed radio galaxy shows a spectral index of $\alpha \approx -0.46 \pm 0.21$, while the spectral index steepens along the tail to values as low as $\alpha \approx -2.43 \pm 0.30$. The median spectral index uncertainty is $\sigma_{\alpha} \approx 0.21$, increasing to values of $\sim 0.35$ toward the edges of the detected emission. The contours show the 144 MHz radio emission as detected by LOFAR (right panel of Fig. \ref{['fig:2.2']}), matched to the uGMRT resolution. The black contours are drawn at levels starting at $5\sigma_{\mathrm{rms}}$ ($\sigma_{\mathrm{rms}} = 0.323\,\mathrm{mJy\,beam^{-1}}$) and increase by multiplicative factors of $\sqrt{2}$. The synthesized beam is shown as a black ellipse in the bottom-left corner, and the black bar in the bottom-right corner indicates a physical scale of 100 kpc. The black circles (radius = 90 kpc) mark the regions along the tail used for spectral ageing analysis (see Section \ref{['sec:4.2']} ).
• Figure 4: Zoom-in view of the Intermediate-resolution image (Left; beam size $1.56"\times1.04"$, $\sigma_{\mathrm{rms}} = 0.099\,\mathrm{ mJy\,beam^{-1}}$) and High-resolution image (Right; beam size $0.30"\times0.21"$, $\sigma_{\mathrm{rms}} = 0.031\,\mathrm{ mJy\,beam^{-1}}$) highlighting detailed features of the radio jet and tail. The colour scale (square-root scaling, $\gamma = 0.5$) spans from $3\times10^{-5}\,\mathrm{Jy\,beam^{-1}}$ to $2.5\times10^{-2}\,\mathrm{Jy\,beam^{-1}}$ in the high-resolution image and from $5\times10^{-5}\,\mathrm{Jy\,beam^{-1}}$ to $2\times10^{-2}\,\mathrm{Jy\,beam^{-1}}$ in the intermediate-resolution image. In both panels, the white ellipse in the bottom-left corner indicates the synthesized beam, and the white bar in the bottom-right corner corresponds to a physical scale of 100 kpc.
• Figure 5: Spectral ageing fits using two different models: shows the Jaffe–Perola (JP) model (left), and the Tribble model (right). The curves represent model spectra for different spectral ages, with the observed LOFAR and uGMRT flux density measurements (over the region mentioned in Fig. \ref{['fig:2.2']}) overlaid for comparison. The magnetic field assumed in both the fittings is $B=\frac{B_{\mathrm{cmb}}}{\sqrt{3}}$.