Astron. Astrophys. 321, 55-63 (1997)

The large-scale structure of the diffuse radio halo of the Coma cluster at 1.4 GHz

B.M. Deiss ¹, W. Reich ², H. Lesch ³ and R. Wielebinski <sup>2

1</sup> Institut für Theoretische Physik, J.W. Goethe-Universität, Robert-Mayer-Straße 10, D-60054 Frankfurt am Main, Germany
² Max-Planck-Insitut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
³ Institut für Astronomie und Astrophysik der Universität München, Scheinerstraße 1, D-81679 München, Germany

Received 23 October 1995 / Accepted 9 August 1996

Abstract

We present new measurements of the diffuse radio emission from the Coma cluster of galaxies at 1.4 GHz using the Effelsberg 100-m-telescope. Even at that high frequency, the halo source Coma C has an extent down to noise of corresponding to 3 Mpc () in Coma. The radio map reveals clear similarities with images of the extended X-ray halo of the Coma cluster. However, the radio halo appears to be displaced from the X-ray halo by arcmin. After subtracting the contributions from point sources we obtained an integrated diffuse flux density of mJy from Coma C.

We derive relations between the various observationally determined spectral indices and the spectral index of the synchrotron emissivity, which allow one to achieve a rough estimate concerning the consistency of the presently available data at different frequencies and to place constraints on the emissivity index distribution. In the halo's core region, the inferred emissivity index between 0.3 GHz and 1.4 GHz appears to be in the range 0.4 - 0.75 implying that there must be some very effective mechanism for particle acceleration operating in the intracluster medium. We discuss implications of our measurements and of the spectral index information on current theories for radio halo formation. We stress the importance of having more measurements of Coma C at frequencies above 1.4 GHz, in order to be able to derive constraints on the physics of the formation process of the radio halo.

Key words: clusters: individual: Coma clusters intergalactic medium radio continuum: galaxies acceleration of particles diffusion

Send offprint requests to: B.M. Deiss

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Contents

• 1. Introduction
• 2. Radio continuum observations
• 3. Results
  • 3.1. Morphological structure of the radio halo
  • 3.2. Integrated diffuse flux from Coma C
  • 3.3. Scale-size-vs.-frequency relation
• 4. Discussion
  • 4.1. Consistency of data and the spectral index distribution
  • 4.2. Implications on theories of halo formation
• 5. Summary and conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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