Aims. The radio halo in the Coma cluster is unique in that its spectrum has been measured over almost two decades in frequency. The current radio data show a steepening of the spectrum at higher frequencies, which has implications for models of the radio halo origin. There is an on-going debate on the possibility that the observed steepening of the spectrum and the apparent shrinking of the halo-size at higher frequencies is not intrinsic to the emitted radiation, but is instead caused by the Sunyaev-Zeldovich (SZ) effect. Methods. Recently, the Planck satellite obtained unprecedented measurements of the SZ signal and its spatial distribution in the Coma cluster, allowing a conclusive testing of this hypothesis. Using the Planck results, we calculated the modification of the radio halo spectrum by the SZ effect in three different ways. With the first two methods we measured the SZ-decrement by adopting self-consistently the aperture radii used for flux measurements of the radio halo at the different frequencies. First we adopted the global compilation of data-points from Thierbach et al. (2003, A&A, 397, 53) and a reference aperture radius consistent with those used by various authors. Second we used the available brightness profiles of the halo at different frequencies to derive the spectrum of the halo within two fixed apertures, corresponding to the size of the halo measured at 2.675 and at 4.85 GHz, and derived the SZ-decrement using these apertures. As a third method we used the quasi-linear correlation between the y-signal and the radio-halo brightness at 330 MHz discovered by the Planck collaboration to derive the modification of the synchrotron spectrum by the SZ-decrement in a way that is almost independent of the adopted aperture radius. Results. We found that the spectral modification induced by the SZ-decrement is negligible and results in values 4-5 times smaller than those necessary to explain the observed steepening at higher frequencies. We also show that, if a spectral steepening is absent from the emitted spectrum, future deep observations at 5 GHz with single dishes are expected to measure a halo flux in a 40 arcmin aperture-radius that would be ~7-8 times higher than currently seen, thus providing a complementary test to our findings. Conclusions. We conclude that according to the current radio data the emitted synchrotron spectrum of the radio halo steepens at higher frequencies, implying a break or cut-off in the spectrum of the emitting electrons at energies of a few GeV.
Bibliographical noteFunding Information:
We thank the referee for useful comments and R. Pizzo and T.Venturi for providing useful information on their observations. L. R. acknowledges support from the U.S. National Science Foundation, under grant AST-1211595 to the University of Minnesota. J.D. acknowledges support by FP7 Marie Curie programme “People” of the European Union. K.D. acknowledges the support by the DFG Cluster of Excellence “Origin and Structure of the Universe”.
- Galaxies: clusters: individual: Coma
- Radiation mechanisms: non-thermal
- Radio continuum: general
- X-rays: galaxies: clusters