3 Results

3.1 Morphology

The emission at 2.675 GHz

The 2.675 GHz map shows various point sources, emission by galaxies, extended diffuse emission in the center of the cluster, and the extended diffuse the source 1253+275 (see Sect. 5) southwest of the cluster center. There is no evidence for the bridge of low-brightness emission detected by Kim et al. (1989). Its intensity is obviously below the noise level of the 2.675 GHz map.

The central structure of the cluster emission is consistent with observations presented by Wielebinski (1978) and by SST. The map reveals a couple of point sources, mostly with low-level polarized emission. At the center one finds the radiation of the two galaxies NGC 4869 and NGC 4874 convolved to an extended source by the finite telescope beam. Diffuse emission surrounds the central maximum out to about $20\hbox{$^\prime$ }$. The highest sidelobes of the antenna pattern appear at about 19 dB at a distance of about $7\hbox{$.\mkern-4mu^\prime$ }5$ from the center. Hence, this diffuse structure around the center is not created by sidelobes. Note that we could not apply the CLEAN algorithm (see e.g. Klein & Mack 1995) because of the lack of a deep antenna pattern at this frequency due to the confusion by background sources. Comparing our map with that of the diffuse halo emission presented by Deiss et al. (1997), as done in Fig. 4, one clearly sees the smaller extension of the diffuse emission at 2.675 GHz. Our map is large enough to cover the whole halo source and to ensure emission-free area for baseline fitting. Our observations exhibit a large, frequency dependent extension of the halo source Coma C. Such a behaviour is to be expected in view of the spectral index map of the central region of the Coma cluster of galaxies presented by Giovannini et al. (1993). Those authors found an inner plateau of about $8\hbox{$^\prime$ }$ radius with a spectral index $\alpha = 0.8$, whereas outside that central part $\alpha$ strongly steepens and reaches values larger than 1.8. A comparison of both maps considering the different frequencies and beamsizes and the spectral index shows consistency of the intensity levels of the extended emission in the 2.675 GHz map with the 1.4 GHz map.

Figure 4: Comparison of the new 2.675 GHz map of the center region of the Coma cluster (contours) with the 1.4 GHz map of Deiss et al. (1997) showing the extended diffuse halo emission (greyscale). In both maps the lowest step marks the 1$\sigma$ level. The high-frequency map is smoothed to the 1.4 GHz beamsize ( $9\hbox {$.\mkern -4mu^\prime $ }35$, indicated by the filled circle in the upper right corner) after an approximate (Gaussian) subtraction of the point-like sources. The still visible extensions are remnants of the source removal. Note, we did not remove the two center sources. This overlay shows clearly the smaller extent of the 2.675 GHz emission.

The emission at 4.85 GHz

Figure 3b shows the central part of the Coma cluster of galaxies at 4.85 GHz. Away from the point sources one finds only marginally extended low-brightness emission surrounding the two NGC-galaxies that are resolved at this frequency. In particular, no extended emission above the 3$\sigma$ level is detected.

3.2 Integrated diffuse radio flux

In order to estimate the integrated flux of the halo we first integrated the whole flux surrounding the center down to the zero level using polygons. In case of the 2.675 GHz map we included the bridges of emission towards the east and north of the center, as seen in Fig. 2, at the thinnest parts. We subsequently subtracted the fluxes of the point sources lying within the integration area. We calculated the point source flux densities on the basis of the spectral indices provided by Kim (1994). For those sources without any spectral index information we extrapolated the flux densities based on an assumed uniform spectral index of $\alpha = 0.8$. All the extrapolated values are below 1$\ldots$2 mJy, so that these faint sources sum up to only about 4% of the contaminating flux at 2.675 GHz, and about 5% at 4.85 GHz, respectively. The results are summarized in Table 4.

 

 

Table 4: Flux densities of central part of the Coma cluster ( $S_{\rm tot}$), of the point sources ( $S_{\rm ps}$), and of the diffuse component ( $S_{\rm df}$). The errors of $S_{\rm tot}$ depend on the integration area and the uncertainty of the zero level of the maps, those in $S_{\rm ps}$ follow from the calculations; the quadratic combination of both give the error in $S_{\rm df}$. The point-like sources are given by the numbers according to their positions in the list of Kim (1994). For a text file containing these numbers see http://www.mpifr-bonn.mpg.de/div/konti/kim.txt.

$\nu$	$S_{\rm tot}$	$S_{\rm ps}$	$S_{\rm df}$	point sources
$[{\rm MHz}]$	$[{\rm mJy}]$	$[{\rm mJy}]$	$[{\rm mJy}]$	(position in Kim-list)
				135 137 142 145 150 152
				158 160 163 166 168 170
				171 172 174 183 185 187
2675	$533\pm 21$	$426\pm 18$	$107\pm 28$	191 192 194 196 197 199
				200 205 208 210 219 221
				223 225
				142 145 150 152 158 160
				163 166 168 170 171 172
4850	$256\pm 9$	$230\pm 8$	$26\pm 12$	185 187 191 192 194 197
				200 208 219 221 223

Most of the spectral indices in the Kim list are calculated based on low-frequency (<1.6 GHz) measurements. We expect that the spectral index increases with frequency ( $S\propto\nu^{-\alpha}$), so that our calculated point source fluxes will be overestimated. Such a behaviour is evident in case of the two central galaxies NGC 4869 and NGC 4874 (numbers 187 and 194 in the Kim list). The catalog contains 4.835 GHz fluxes for both sources given by observations. As shown in Table 5 their flux densities are much lower than those calculated with the derived spectral indices (resulting from best fits to the data). In assuming a constant spectral index one overestimates the point source flux and subsequently underestimates the flux density of the diffuse component.

 

 

Table 5: Flux densities of the central galaxies NGC 4869 and NGC 4874. $S_{4.835}^{\rm VLA}$ is the value given by Kim (1994), $S_{4.85}^{\rm calc}$ is that obtained from the spectral index given in the same work, and $S_{4.85}^{\rm Eff}$ is the peak flux of the source in our map. The calculations using the spectral indices, based mainly on low-frequency data, obviously overestimate the fluxes compared to the measurements. The spectral indices will increase with increasing frequency.

N4869:	$S_{4.835}^{\rm VLA}=97.5~{\rm mJy}$	$S_{4.85}^{\rm calc}=119~\;{\rm mJy}$	$S_{4.85}^{\rm Eff}=95.0~{\rm mJy}$
N4874:	$S_{4.835}^{\rm VLA}=77.3~{\rm mJy}$	$S_{4.85}^{\rm calc}=83.7~{\rm mJy}$	$S_{4.85}^{\rm Eff}=72.8~{\rm mJy}$

3.3 The spectrum of Coma C

All data available in the literature together with their references are summarized in Table 6. In contrast to earlier publications about the flux density of Coma C, one will not find the work by Waldthausen (1980). His 5 GHz observation of the region only covers the central part of the cluster and was done to check the morphology of the central sources. In particular Waldthausen did not mention any 5 GHz flux value in his thesis. The often cited value (<52 mJy) is Waldthausen's 2.7 GHz upper limit for the Coma C flux.

The resulting spectrum of the halo source Coma C is displayed in Fig. 5. The new measurements of this work are symbolized with filled squares. Despite the fact that our 2.675 GHz point is slightly higher than that of SST, the break in the spectrum, first mentioned by those authors, is visible. The first 4.85 GHz observation supports this spectral steepening. A more detailed discussion of the spectrum will follow in the next chapter.

 

 

Table 6: Integrated flux densities from Coma C. References: (1) Henning (1989); (2) Hanisch & Erickson (1980); (3) Cordey (1985); (4) Venturi et al. (1990); (5) Kim et al. (1990); (6) Hanisch (1980); (7) Giovannini et al. (1993); (8) Deiss et al. (1997); (9) present paper; (10) Schlickeiser et al. (1987).

frequency	flux density			references
$[{\rm MHz}]$		$[{\rm Jy}]$
30.9	49	$\pm$	10	1
43	51	$\pm$	13	2
73.8	17	$\pm$	12	2
151	7.2	$\pm$	0.8	3
326	3.81	$\pm$	0.03	4
408	2.0	$\pm$	0.2	5
430	2.55	$\pm$	0.28	6
608.5	1.2	$\pm$	0.3	7
1380	0.53	$\pm$	0.05	5
1400	0.64	$\pm$	0.035	8
2675	0.11	$\pm$	0.03	9
2700	0.07	$\pm$	0.02	10
4850	0.03	$\pm$	0.01	9

Figure 5: Integrated radio continuum spectrum of the diffuse radio halo source Coma C. The data and their references are compiled in Table 6. The filled dots represent our new observations.