The Radio Halo Coma C

An image of the radio emission in the Coma cluster is given in Fig. 7.5. Coma C is the prototype and best studied example of cluster radio halos. The halo is located at the cluster center, it shows a rather regular shape and a low surface brightness (~ ¿iJy/arcsec2 at 1.4 GHz). The integrated radio spectrum is steep (a=1.34). A possible steepening at frequencies higher than 1.4 GHz could be due to a flux density underestimate because of the small area covered (Deiss et al. 1997). The radio halo scale size is a monotonically decreasing function of the frequency. This supports the suggestion by Giovannini et al. (1993) of a spectrum steepening in the peripheral regions.

Figure 7.5. WSRT radio image of the Coma cluster region at 90 cm, with angular resolution of 55" x 125" (HPBW, RA x DEC). Labels refer to the halo source Coma C and the relic source 1253+275 as well as to some bright Coma cluster radio galaxies. Contour levels are at 2, 3, 5, 7, 10, 30, 50, 100, 300, 500 mJy/beam. The bridge of radio emission connecting Coma C to 1253+275 is resolved at this angular resolution and is visible only as a region with an apparent higher positive noise.

Figure 7.5. WSRT radio image of the Coma cluster region at 90 cm, with angular resolution of 55" x 125" (HPBW, RA x DEC). Labels refer to the halo source Coma C and the relic source 1253+275 as well as to some bright Coma cluster radio galaxies. Contour levels are at 2, 3, 5, 7, 10, 30, 50, 100, 300, 500 mJy/beam. The bridge of radio emission connecting Coma C to 1253+275 is resolved at this angular resolution and is visible only as a region with an apparent higher positive noise.

The Coma C halo is the only radio halo for which a high resolution map of the spectral index has been obtained so far. The spectral index distribution between 327 and 1400 MHz shows a central plateau with a ~0.8, and an outer region with a steeper spectrum, up to a=1.8. This behaviour provides evidence that the source of energy is more efficient at the cluster/halo center, in a region approximately coincident with the optical core radius. The radiative lifetime of the relativistic electrons estimated from the spectrum is ~108 yr. Deiss et al. (1997) obtained a map at 1.4 GHz of the halo, after subtraction of all discrete sources. They pointed out the close similarity between the X-ray and radio images. Both emissions are extended in the E-W direction and towards the NGC 4839 group. This similarity indicates a close link between the physical conditions of the radio source and those of the thermal component.

The equipartition magnetic field is 0.4 h^7 piG and the minimum energy density is erg as estimated by Giovannini et al. (1993). No polarized flux is detected down to a level of ~10% at 1.4 GHz.

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