THE LARGE-SCALE STRUCTURE OF THE DIFFUSE RADIO HALO OF THE COMA CLUSTER AT 1.4 GHZ

We present new measurements of the diffuse radio emission from the Com a cluster of galaxies at 1.4 GHz using the Effelsberg 100-m-telescope. Even at that high frequency, the halo source Coma C has an extent dow n to noise of similar to 80' corresponding to 3 Mpc (H-0 = 50kms(-1)Mp c(-1)) in Coma. The radio map reveals clear similarities with images o f the extended X-ray halo of the Coma cluster. However, the radio halo appears to be displaced from the X-ray halo by similar to 3 - 4 arcmi n. After subtracting the contributions from point sources we obtained an integrated diffuse flux density of S-1.4GHz = 640 +/- 35 mJy from C oma C. We derive relations between the various observationally determi ned spectral indices and the spectral index of the synchrotron emissiv ity, which allow one to achieve a rough estimate concerning the consis tency of the presently available data at different frequencies and to place constraints on the emissivity index distribution. In the halo's core region, the inferred emissivity index between 0.3 GHz and 1.4 GHz appears to be in the range 0.4 - 0.75 implying that there must be som e very effective mechanism for particle acceleration operating in the intracluster medium. We discuss implications of our measurements and o f the spectral index information on current theories for radio halo fo rmation. We stress the importance of having more measurements of Coma C at frequencies above 1.4 GHz, in order to be able to derive constrai nts on the physics of the formation process of the radio halo.