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1979A&A....76..109Gisler & Miley
Astron. Astrophys. 76, 109-119 (1979)

(OCR+proof by H.Andernach 4+10/98)

610 MHz Observations of the Perseus Cluster of Galaxies with the Westerbork Synthesis Radio Telescope

G. R. Gisler and G. K. Miley

Summary. We have made observations of the Perseus cluster with the Westerbork Telescope at 610 MHz. Details of our observations and reduction procedure are given. The dynamic range within our maps exceeds 1000. A catalogue of 50 sources trom the field is presented, together with spectral information, where available. Several newly detected sources are identified with cluster galaxies. A faint extension found close to NGC 1265 suggests that the main body of this tailed source has been bent bv buoyancy forces. On the assumption that this is the case, we derive a conservative lower limit to the mass of the Perseus cluster of 1.4 10^15 M_sun. This reinforces previous arguments from the virial theorem that the cluster is bound. The origin of the extended component surrounding NGC 1275 is discussed and a (610,1415MHz) spectral index distribution for it is presented. Its amorphous structure may be influenced by the presence of the cluster gas, but comparisons with similar resolution X-ray observations are inconclusive. No evidence is found for the existence of the previously reported ~ 65'x25' cluster halo, 3C 84B.
Key words: radio galaxies - clusters - tailed radio sources buoyancy

1. Introduction

The Perseus Cluster of galaxies is one of the most fascinating systems accessible to extragalactic astronomers. It is among the brightest X-ray clusters known and was the first in which iron line emission was discovered (Mitchell & Culhane 1977). Perseus is also distinguished by having the largest velocity dispersion reported for any cluster (Chincarini & Rood, 1971). In the radio domain the cluster contains three head tail galaxies, NGC 1265, IC 310 (Ryle & Windram 1968) and CR 15 (Miley et al. 1972). The Perseus Cluster was the first in which such tailed radio sources were found and to date only Abell 2256 (Bridle et al. 1978) is known to contain a larger number.
At the easterly end of the prominent line of bright galaxies near the cluster center lies NGC 1275, the brightest member of the cluster and one of the most active galaxies known. This unique object is an early-type galaxy with a Seyfert-like spectrum and a system of high velocity knots and filaments which may belong to a superimposed late-type galaxy (Rubin et al. 1977). NGC 1275 is a source of strong X-ray and radio emission in its own right. The radio source associated with NGC 1275 (Perseus A/3C 84) is one of the brightest radio sources known and has very complex structure on scales from milli-arcsec (Pauliny-Toth et al. 1976) to several arcmin (Miley & Perola 1975).
Apart from the radio sources already mentioned which are associated with individual cluster galaxies. a large radio halo with an extent of about one degree has been reported by Ryle & Windram (1968) and designated 3C84B.
Because the radio emission of the Perseus Cluster is so complex, high resolution radio telescopes with extremely good dynamic range properties are needed to map its structure. The excellent gain and phase stability of the Westerbork Synthesis Radio Telescope (WSRT) (Hogbom & Brouw 1974; Baars et al. 1973) makes it one of the most suitable instruments for such studies. Previous investigations have been made with the WSRT at 1415 and 4995 MHz of individual galaxies in the cluster (Miley et al. 1972: Miley 1973; Wellington et al. 1973. Miley et al. 1975; Miley & Perola 1976; Ekers et al. 1976). This paper reports on measurements of the Perseus Cluster made with the WSRT at 610 MHz. At 610 MHz the field of view or 'primary beam' of the WSRT has a half power diameter of 83' and is comparable in size to the total extent of the cluster. Because of their relatively low frequency and large field of view, these observations have provided new information about low brightness radio components in the cluster.

2. Observations and General Reduction

The observations were obtained in four separate twelve-hour periods during early 1975 (see Table 1). The field center was chosen to be to the northwest of NGC 1275 in order that the strong sources associated with NGC 1275, NGC 1265 or IC 310 would not be affected greatly by the attenuation of the primary beam.
As we have previously mentioned. the large flux density of Perseus A (~23 Jy at 610 MHz) results in severe dynamic range requirements. For this reason particular care was taken with the calibration. Initially the observations were calibrated relative to the point sources 3C 48 and 3C147, whose 610 MHz flux densities were assumed to be 15.7 and 21.6 Jy respectively and whose positions were taken from Elsmore & Ryle (1976). Each of the four observations was calibrated identically. The resultant combined map showed very weak residual grating rings whose locations indicated that they were due to slight remaining systematic phase differences between the separate observations. These residual rings had intensities ~0.2% of the peak intensity on the map and were reduced further in the following manner. Separate maps were produced for each of the four observations and individually searched for the positions of the point source 3C 83.1 A and the peak of Perseus A. Tiny but consistent shifts in positions of ~0.1" were obtained between the different observations. We applied these shifts to the data and produced a new combined map in which the residual grating rings were reduced by a factor of two to the ~0.1% level. In addition to standard resolution (70" x 105" HPBW) maps in all four Stokes parameters, we also made convolutions having circular synthesized beams with half power diameters of 3' and 6' respectively. The latter were used to make a more sensitive study of low brightness structure and to compare with various X-ray data. In order to study the spectral distribution of the 5' component around NGC 1275 (Miley & Perola 1976) we constructed additional maps at both 610 and 1415 MHz having resolutions which were as identical as possible. The 1415 MHz observations were also made with the WSRT and calibrated very carefully with the object of achieving maximum dynamic range. Details of the various observations used and parameters of the resulting maps are given in Table 1.
In all cases the cleaning algorithm (Hogbom 1974; Schwartz 1978) was used in order to remove both the real (far) diffraction grating rings and the distorting effects of near sidelobes.