Combination of interferometer and single-dish observations

From February to August 1996 we have observed a small sample of edge-on galaxies, consisting of NGC 891, NGC 3628, and NGC 4565, at 6.2 cm using the Effelsberg 100 m-telescope. The data were reduced in a standard manner using the NOD2 software package available at the MPIfR Bonn. The resulting maps were cleaned from the sidelobes of the telescope pattern, and the instrumental polarization was removed down to a level of 0.3 %.

The spatial resolution of large single-dish telescopes is limited. On the other hand, data from interferometers like the VLA suffer from the so called "missing-spacing problem": because of the lack of visibilities at short baselines they are not sensitive to extended emission. In order to get final maps with both the high resolution of the interferometer data and the short-spacing information provided by the single-dish observations, we combined the maps from the Effelsberg telescope with VLA maps at this wavelength from Sukumar & Allen (1991), Dumke (1994), and Dumke & Krause (in prep.). This was done in the Fourier plane using the SDE task IMERG (which is available from the NRAO) for the total power as well as the polarization data (Holdaway 1992). Remaining interferences in the combined maps around strong sources, which limited the dynamic range, could successfully be removed using a Fourier filter technique.

The combined maps including the short-spacing information differ significantly from the VLA maps. It is obvious that the usage of interferometer data alone may lead - and have probably led in many investigations in the past - to unreliable results concerning the quantitative and qualitative behaviour of radio and polarization properties. Our results also show that even at 20 cm the absence of short spacing information in the interferometer data may be a serious problem. Hence the single-dish data from the Effelsberg telescope are absolutely necessary to investigate the emission of extended radio sources at cm-wavelengths.

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