A giant radio halo in a low-mass SZ-selected galaxy cluster: ACT-CL J0256.5+0006

Abstract

We present the detection of a giant radio halo (GRH) in the Sunyaev-Zel'dovich (SZ)-selected merging galaxy cluster ACT-CL J0256.5+0006 ($z=0.363$), observed with the Giant Metrewave Radio Telescope at 325 MHz and 610 MHz. We find this cluster to host a faint ($S_{610}=5.6\pm 1.4$ mJy) radio halo with an angular extent of 2.6 arcmin, corresponding to 0.8 Mpc at the cluster redshift, qualifying it as a GRH. J0256 is one of the lowest-mass systems, $M_{500,\mathrm{SZ}}=(5.0\pm 1.2)\times 10^{14}{M}_{\odot }$, found to host a GRH. We measure the GRH at lower significance at 325 MHz ($S_{325}=10.3\pm 5.3$ mJy), obtaining a spectral index measurement of ${\alpha }_{325}^{610}=1.0_{-0.9}^{+0.7}$. This result is consistent with the mean spectral index of the population of typical radio halos, $\alpha =1.2\pm 0.2$. Adopting the latter value, we determine a 1.4 GHz radio power of $P_{1.4\text{GHz}}=(1.0\pm 0.3)\times 10^{24}$ W Hz${}^{-1}$, placing this cluster within the scatter of known scaling relations. Various lines of evidence, including the ICM morphology, suggest that ACT-CL J0256.5+0006 is composed of two subclusters. We determine a merger mass ratio of 7:4, and a line-of-sight velocity difference of $v_{\perp }=1880\pm 280$ km s${}^{-1}$. We construct a simple merger model to infer relevant time-scales in the merger. From its location on the $P_{1.4GHz}-L_{\mathrm{X}}$ scaling relation, we infer that we observe ACT-CL J0256.5+0006 approximately 500 Myr before first core crossing.

Updated version: 21 pages, 17 figures, published in MNRAS

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• Diffuse Radio Emission from Galaxy Clusters
• The Atacama Cosmology Telescope: Dynamical masses for 44 SZ-selected galaxy clusters over 755 square degrees
• Radio fossils, relics, and haloes in Abell 3266: cluster archaeology with ASKAP-EMU and the ATCA
• Mystery solved: discovery of extended radio emission in the merging galaxy cluster Abell 2146
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• A Massive Cluster at z = 0.288 Caught in the Process of Formation: The Case of Abell 959
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