The combination of the Effelsberg total-intensity map with the mosaic of VLA maps was carried out using the program EFFMERG, a modified version of the SDE task IMERG (Cornwell et al. 1995). IMERG deconvolves each of the two maps with its beam, adds them in the Fourier domain, using the Effelsberg data for small spacings, VLA data for large spacings and a linear interpolation in the overlap domain, and finally convolves the combined map with the VLA beam. The only free parameters are the maximum spacing of the Effelsberg map and the minimum spacing of the VLA map (both in units of ).
The results for extended emission are generally satisfying, but ring-like distortions around point sources limit the dynamic range in the combined map to about 2000 times the rms noise. The performance of this algorithm was demonstrated before (Beck & Hoernes 1996). We did not apply combination tasks based on MEM like VTESS in AIPS because this does not deconvolve the beams. Test runs were unsatisfactory; future improved MEM routines are required.
In the case of M 31 several difficulties had to be overcome. In order to get a combined map with flat baselines and without distortions, we first had to subtract strong point-like sources from the VLA map. The subtracted sources were smoothed to the Effelsberg resolution and subtracted from the Effelsberg map. Then the residual single-dish map was smoothed to a resolution of about 15 in order to weaken the influence of source residuals. This still guarantees full coverage of the UV plane after application of EFFMERG, thus avoiding baseline problems around the residuals of strong background sources. This map was then combined with the residual VLA map by EFFMERG, using a maximum spacing of 250 for the Effelsberg map and a minimum spacing of 190 for the VLA map. After combination, the VLA point sources were added again.
The combined map of total intensities but without point-like sources (see Sect. 4.1) is shown in Fig. 4 (click here).
In polarized intensity, the combination of Effelsberg and VLA maps was impossible due to the strong foreground polarization.