Free access article

A&A 503, 707-720 (2009)
DOI: 10.1051/0004-6361/200912520

Revealing the magnetic field in a distant galaxy cluster: discovery of the complex radio emission from MACS J0717.5 +3745

A. Bonafede^{1, 2}, L. Feretti², G. Giovannini^{1, 2}, F. Govoni³, M. Murgia^{2, 3}, G. B. Taylor⁴, H. Ebeling⁵, S. Allen⁶, G. Gentile^{7, 8}, and Y. Pihlström<sup>4

1</sup>  Università di Bologna, Dip. di Astronomia, via Ranzani 1, 40126 Bologna, Italy
    e-mail: bonafede@ira.inaf.it
²  INAF, Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
³  INAF, Osservatorio Astronomico di Cagliari, Strada 54, Loc. Poggio dei Pini, 09012 Capoterra (Ca), Italy
⁴  Department of Physics and Astronomy, University of New Mexico, Albuquerque, NM 87131 and Adjunct Astronomer at the National Radio Astronomy Observatory, USA
⁵  Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
⁶  Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, 382 Via Pueblo Mall, Stanford, CA 94305-4060, USA
⁷  Institut d'Astronomie et d'Astrophysique, Faculté des Sciences, Université Libre de Bruxelles, CP 226, Bvd du Triomphe, 1050 Bruxelles, Belgium
⁸  Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281, 9000 Gent, Belgium

Received 18 May 2009 / Accepted 2 July 2009

Abstract
Aims. To study at multiple frequencies the radio emission arising from the massive galaxy cluster MACS J0717.5+3745 (z=0.55). Known to be an extremely complex cluster merger, the system is uniquely suited for an investigation of the phenomena at work in the intra-cluster medium (ICM) during cluster collisions.
Methods. We use multi-frequency and multi-resolution data obtained with the Very Large Array radio telescope, and X-ray features revealed by Chandra, to probe the non-thermal and thermal components of the ICM, their relations and interactions.
Results. The cluster shows highly complex radio emission. A bright, giant radio halo is detected at frequencies as high as 4.8  GHz. MACS J0717.5+3745 is the most distant cluster currently known to host a radio halo. This radio halo is also the most powerful ever observed, and the second case for which polarized radio emission has been detected, indicating that the magnetic field is ordered on large scales.

Key words: galaxies: clusters: general -- galaxies: clusters: individual: MACS J0717+3745 -- radiation mechanisms: non-thermal -- polarization -- magnetic fields

© ESO 2009

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