Discovery of a nearby radio relic in the low-mass, merging cluster Abell 4067

Abstract

Shock waves generated during cluster mergers offer a powerful probe of how large-scale structure grows and evolves in the Universe. As part of the MeerKAT-South Pole Telescope (SPT) survey, we report the discovery of a single arc-like radio relic in the galaxy cluster Abell 4067 ($z=0.099$), one of the lowest-mass clusters known to host such a structure. MeerKAT UHF-band (0.58--1.09 GHz) observations reveal a relic with a largest linear size of $\sim 1.48 \pm 0.02$ Mpc, located at a projected distance of 0.95 Mpc from the cluster centre. XMM-Newton X-ray data show that the relic's position and orientation relative to the intracluster medium (ICM) elongation are consistent with a merger-driven shock-wave scenario. The relic has an estimated radio power of $3.10 \pm 0.03 \times 10^{24}$ W Hz$^{-1}$ at 150 MHz. When placed in the $P_{150\,\mathrm{MHz}}$--$M_{500}$ scaling relation, the Abell 4067 relic appears less luminous compared to relics in more massive clusters, suggesting an association with weak merger shocks. This finding supports the idea that relics in low-mass clusters may form through less energetic merger events, leading to weak merger shocks. This is further supported by the absence of a detectable central radio halo in Abell 4067, reinforcing the idea that luminous radio halos are not a universal outcome of cluster mergers and highlighting the role of cluster mass, merger energetics, and evolutionary stage in shaping diffuse radio emission in the intracluster medium.

Figures (3)

• Figure 1: Left: MeerKAT 816 MHz image of Abell 4067 at Robust 0.0, with a resolution of 11.7 arcsec $\times$ 11.7 arcsec, position angle 0.0 degree and rms noise, $\sigma$ = 6.20 $\mu$Jy beam$^{-1}$. The labels S1 - S10 indicate the compact sources within the $R_{500}$ cluster region (dashed orange circle) and those near the extended source labelled ‘Relic’ are A, B, C and FRII. The X-ray cluster centre is marked by ‘X’. The synthesized beam is shown in orange in the lower-left corner. Right: DESI Legacy Survey grz image of the region of the labelled sources with the MeerKAT 816 MHz contours overlaid. Contour levels are drawn at [5, 9, 13, 17] $\times$ 1$\sigma$, where $\sigma$ = 6.20 $\mu$Jy beam$^{-1}$.
• Figure 2: Left: XMM-Newton X-ray image of Abell 4067 (smoothed to 12 arcsec) in the 0.5 - 2.0 keV band. The orientation of the radio relic is perpendicular to the major axis of the X-ray emission from the ICM. The symbol ‘X’ mark the position of the cluster according to the XMM-Newton and the yellow dashed circle indicate the $R_{500}$ radius. Right: Zoomed-in view of the region around the compact, bullet-like galaxy S2 (2MASX J23590416$-$6036344) as it penetrates the low-density outer ICM Chon_2015. Red contour levels show the X-ray emission from the 12 arcsec smoothed XMM-Newton image, while black contour levels show the radio emission from the MeerKAT 816 MHz full-resolution image (8.9 arcsec $\times$ 8.9 arcsec, Robust$-0.5$).
• Figure 3: The $P_{150\,\mathrm{MHz}}$ - $M_{500}$ scaling relation of radio relics, colour-coded by redshift. The solid line represents the best-fit relation, based on the correlation reported by Jones_2023. The darker shaded region indicates the $\pm1\sigma$ intrinsic scatter (0.55 dex), while the lighter shaded region shows the corresponding 95$\%$ intrinsic scatter envelope. The radio relic in Abell 4067 is marked with a blue star and lies $\sim2.8\sigma$ above the best-fitting relation. Low-mass single relic systems are highlighted with blue diamonds.