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

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5. Summary and conclusions

We presented new measurements of the diffuse radio emission from the Coma cluster at 1.4 GHz using the Effelsberg single-dish telescope. After subtracting contributions from 298 point sources we obtained a radio map exhibiting the large scale characteristics of the diffuse radio halo source Coma C. From the large angular extent down to noise of [FORMULA], one concludes that the radio halo in Coma is a cluster-wide phenomenon which is not restricted to only the cluster core region. This implies the presence of relativistic electrons as well as of a magnetic field throughout a spatial volume with a diameter of [FORMULA] Mpc.

The integrated diffuse flux from Coma C is [FORMULA] mJy at 1.4 GHz. This value fits neatly a power-law extrapolation from lower frequencies, showing that a possible steepening of the integrated spectrum may appear only above 1.4 GHz.

We derived, under some simplifying assumptions, mutual relations between the observationally determined spectral indices of the integrated diffuse flux density, the surface brightness and the halo's scale size, and the spectral index of the synchrotron emission coefficient. These relations allow one i) to achieve a rough estimate concerning the consistency of the presently available data at different frequencies and ii) to place constraints on the emissivity index distribution: i) We conclude that the 2.7 GHz measurements by Schlickeiser et al. (1987) are not consistent to our measurements. We suspect that the value of the integrated flux given by these authors is much too low, which implies that the claimed strong steepening of the spectrum above 1.4 GHz is not real. This has implications on current theories of radio halo formation. Hence, in order to be able to derive constraints on the physics of the formation process of the Coma radio halo, one needs more and improved measurements at frequencies above 1.4 GHz. ii) In the core region of the halo, the emissivity index between 0.3 GHz and 1.4 GHz appears to be in the range 0.4 - 0.75 which implies an energy spectrum index of the electrons of [FORMULA]. From that we conclude that there must be some very effective mechanism for particle acceleration operating in the intracluster medium.

We argued that the observed large extent of Coma C, its regular shape as well as its clear similarity with the X-ray halo support an in-situ acceleration model for radio halo formation, since one may expect a close link between the physical conditions of the relativistic particles and that of the thermal component of the intracluster medium. We showed that in the Coma cluster stochastic reacceleration of the electrons, amplified by turbulent gas motion originating from galaxy motion inside the cluster (Deiss & Just 1996), appears to be sufficiently strong to account for the inferred rather small radio emission index in the core region of Coma C and to sustain a cluster-wide distribution of relativistic particles in that rich cluster.

We suggested that the rarity of the radio halo phenomenon has its origin in that the efficiency of the stochastic reacceleration mechanism may depend on some details of the clusters' structure: diffuse radio halos of otherwise globally similar clusters may have a rather dissimilar appearance. That means, while a cluster-wide distribution of relativistic electrons is likely to be common in rich clusters, in only a few of them the conditions are such that the electrons gain enough energy to produce an extended halo observable at some hundred Megahertz. However, we suspect that at low frequencies (well below 100 MHz) diffuse radio halos of galaxy clusters are much more common than it is suggested by the presently available data.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998