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

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1. Introduction

Diffuse cluster-wide radio emission not associated with individual galaxies defines a separate class of extragalactic radio sources: the diffuse radio halos of galaxy clusters. Radio halo sources are observed in the richest and most X-ray luminous clusters of galaxies. However, one of their most enigmatic properties appears to be their obvious rarity. At present, only a few clusters are definitely known to have an extended radio halo: e.g., A754 (Waldthausen 1980), A2255 (Harris et al. 1980), A2256 (Bridle & Fomalont 1976), A2319 (Harris & Miley 1978). The best studied example, however, is the Coma cluster (A1656) with its extended, diffuse halo source Coma C (e.g.,Hanisch 1980; Hanisch & Erickson 1980; Waldthausen 1980; Cordey 1985; Schlickeiser et al. 1987; Henning 1989; Venturi et al. 1990; Kim et al. 1990; Giovannini et al. 1993).

At lower frequencies diffuse radio emission from Coma C has been observed up to an angular distance of [FORMULA] from the cluster center [e.g., Henning (1989) at 30.9 MHz], which corresponds to a radius of 1.5 Mpc in Coma (z = 0.0235; Sarazin et al. 1982).(We adopt [FORMULA] for the Hubble constant throughout the paper; i.e., [FORMULA] kpc). This implies the existence of a cluster-wide presence of relativistic electrons as well as of a magnetic field.

Kim et al. (1990) published a detailed image of Coma C at 1.4 GHz using a synthesis aperture telescope (Dominion Radio Astrophysical Observatory, DRAO). Applying a Gaussian fit, they inferred a [FORMULA] of the radio halo of [FORMULA] which is significantly smaller than the value of the FWHM obtained at lower frequencies, e.g., at 326 MHz (Venturi et al. 1990). From these observations Giovannini et al. (1993) concluded that the spectral index of the diffuse radio emission strongly steepens with increasing radius. The DRAO radio map of Kim et al. (1990) reveals a halo diameter down to noise level of [FORMULA]. However, emission from larger structures could be attenuated due to the synthesis aperture technique. This has implications for the determination of the spectral index distribution (Giovannini et al. 1993) as well as of the integrated flux at 1.4 GHz. In order to obtain informations on the large scale characteristics of the halo at high frequencies, we observed Coma C at 1.4 GHz with the Effelsberg single-dish 100-m-telesope.

The exact shape of the diffuse radio emission spectrum of Coma C is yet unclear. Schlickeiser et al. (1987) claimed that the integrated flux density spectrum strongly steepens at high frequencies ([FORMULA] GHz). We discuss the consistency of their measurements to our observations, and we discuss some implications of our new measurements on current models for radio halo formation which have been proposed in the literature (e.g. Hanisch 1982, Tribble 1993).

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998