2. Radio continuum observations
We have made radio continuum observations at 1.4 GHz with the Effelsberg 100-m telescope in April 1993. At this frequency the telescope has an angular resolution (HPBW) of . A field of centered on the Coma cluster has been mapped twice in orthogonal directions. We used a two channel receiver with cooled HEMT amplifiers. A bandwidth of 20 MHz was centered on 1.4 GHz. The data have been processed using standard procedures for continuum mapping observations with the Effelsberg 100-m telescope (e.g. Reich et al., 1990). The data from both channels have been averaged and the two coverages have been combined using the method described by Emerson and Gräve (1988). The final map is limited by confusion and has an rms-noise of about 7 mJy (or 14.4 mK Tb). We displayed our result in the form of a contour plot in Fig. 1. The map shows numerous compact radio sources, most of them associated with Coma cluster galaxies, superimposed on a weak large scale diffuse emission component.
In order to separate the diffuse extended emission from the contribution of individual sources, we have used the master list of radio sources from the Coma cluster as compiled by Kim (1994). The data for 298 sources are from various observations made with synthesis telescopes and therefore include sources as weak as a few mJy not directly accessible by us due to our larger beam width and resulting higher confusion limit. We have used the spectral fits by Kim et al. (1994) to calculate the flux density of all sources at 1.4 GHz and subracted these contributions assuming a Gaussian source shape at the listed positions. We have in addition subtracted a few sources fitted by a Gaussian at the edge areas of our field which are outside the region where Kim (1994) has listed radio sources. The result of this procedure is shown in Fig. 2 where a weak diffuse radio component is left which is centered on the Coma cluster.
The east-west angular extent of the diffuse radio source is more than . The bridge-like extension of the diffuse radio emission to the south-west is related to the galaxy group associated with the bright galaxy NGC 4839. In order to obtain the integrated flux density of the diffuse radio emission at 1.4 GHz from Coma C, we integrated the diffuse flux (Fig. 2) over a circular area of radius centered at , (1950), but where we subtracted contributions from the region around NGC 4839 and from the narrow halo extension to the north. The latter might be related to the radio source 5C4.109.
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998