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

K. Knowles, H. T. Intema, A. J. Baker, V. Bharadwaj, J. R. Bond, C. Cress, N. Gupta, A. Hajian, M. Hilton, A. D. Hincks, R. Hlozek, J. P. Hughes, R. R. Lindner, T. A. Marriage, F. Menanteau, K. Moodley, M. D. Niemack, E. D. Reese, J. Sievers, C. SifónR. Srianand, E. J. Wollack

Research output: Contribution to journalArticlepeer-review


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 and 610 MHz. We find this cluster to host a faint (S610 = 5.6 ± 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, M500, SZ = (5.0 ± 1.2) × 1014 M, found to host a GRH. We measure the GRH at lower significance at 325 MHz (S325 = 10.3 ± 5.3 mJy), obtaining a spectral index measurement of α 325610 = 1.0-0.9+0.7. This result is consistent with the mean spectral index of the population of typical radio haloes, α = 1.2 ± 0.2. Adopting the latter value, we determine a 1.4 GHz radio power of P1.4 GHz = (1.0 ± 0.3) × 1024 W Hz-1, placing this cluster within the scatter of known scaling relations. Various lines of evidence, including the intracluster medium 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⊥ = 1880 ± 210 km s-1. We construct a simple merger model to infer relevant time-scales in the merger. From its location on the P1.4 GHz-LX scaling relation, we infer that we observe ACT-CL J0256.5+0006 just before first core crossing.

Original languageEnglish (US)
Pages (from-to)4240-4258
Number of pages19
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
StatePublished - Jul 11 2016


  • Galaxies: clusters: individual: (ACT-CL J0256.5+0006)
  • Galaxies: clusters: intracluster medium
  • Radio continuum: general
  • X-rays: individual: ACT-CL J0256.5+0006

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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