Low-frequency radio emission in the massive galaxy cluster MACS J0717.5 + 3745

¹ Université de Lyon, 69003 Lyon, France
e-mail: mamta.pommier@univ-lyon1.fr
² Université Lyon 1, Observatoire de Lyon, 9 avenue Charles André, 69230 Saint-Genis Laval, France
³ CNRS, UMR 5574, Centre de Recherche Astrophysique de Lyon, 69007 Lyon, France
⁴ École Normale Supérieure de Lyon, 69007 Lyon, France
⁵ LERMA, Observatoire de Paris, 61 avenue de l’Observatoire, 75014 Paris, France
⁶ Raman Research Institute, 560 080 Bangalore, India
⁷ Laboratoire Lagrange, UMR7293, Université de Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d’Azur, 06300 Nice, France
⁸ National Centre for Radio Astrophysics, TIFR, Pune University Campus, Post Bag 3, 411007 Pune, India
⁹ Theoretical Astrophysics Group, Tata Institute of Fundamental Research, Homi Bhabha Road, 400 005 Bombay, India

Received: 1 May 2013
Accepted: 18 June 2013

Abstract

Aims. To investigate the nonthermal emission mechanism and their interaction during cluster mergers, we analyze multiple low-frequency radio data for the X-ray luminous massive galaxy cluster MACS J0717.5 + 3745, located at z = 0.5548. Large-scale structure-formation models in the Universe suggest that galaxy clusters grow via constant accretion of gas and the merger of galaxy groups and smaller clusters. Low-frequency radio observations trace these mergers in the form of relics and halos. The dual frequency observations were performed on MACS J0717.5 + 3745 to investigate the spectral index pattern of the nonthermal emission and its interaction within the intracluster medium (ICM), during merger process.

Methods. Continuum observations were carried out using GMRT at 0.235 and 0.61 GHz on MACS J0717.5 + 3745 and archival data from the VLA (0.074 and 1.42 GHz) and WSRT (0.325 GHz) was used to complement the results. Furthermore, to explore the thermal and nonthermal interactions within the ICM and the morphological distribution, Chandra X-ray and HST data were used.

Results. A highly complex nonthermal radio emission distribution is seen in the cluster at very low frequencies, with a global spectral index . We have detected a giant radio halo within the cluster system with a linear size of 1.58 Mpc and a “Chair-shaped” filament structure between the merging subclusters of linear size 853 kpc at 0.235 GHz. This is the most powerful halo ever observed with P_1.4 = 9.88 × 10²⁵ WHz^-1 and an equipartition magnetic field estimate of ~6.49 μG. The bright filament structure is well located in the central merging region of subclusters with enhanced temperature, as shown by Chandra and HST data analysis, further indicating the formation of this structure due to shock waves encountered within the ICM during the merger events.