Diffuse Emission In The Coma Cluster Periphery

Besides the existence of the central Coma C halo, the Coma cluster is characterized by the presence of another diffuse extended source, named 1253+275 (Fig. 7.5), located at ~2.7 Mpc from the cluster center, in the direction of the cluster A1367.

The radio source morphology and the trend of the spectral index suggest that this source is an extended radio source not strictly related to the activity of a single galaxy, but to the cluster.

Its large size, brightness and spectrum are similar to those of Coma C. Unlike Coma C it shows an elongated shape, and is 30% polarized at 1.4 GHz. The high polarization degree in this source is naturally explained by a tangled magnetic field associated with the cluster inter-galactic medium. In this case, a larger number of magnetic field cells along the line of sight is present at the cluster center compared to the outer regions.

Additional diffuse emission is present in the Coma cluster: a bridge of radio emission is detected in the region connecting Coma C to 1253+275 (Kim et al. 1989; Giovannini et al. 1990). The surface brightness of this diffuse emission is very low and it is only enhanced at low frequency and low resolution, so it is not easily visible in Fig. 7.5. In the recent map presented by Deiss et al. (1997), the bridge is visible only as an asymmetric extension of the central halo Coma C.

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