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.