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In order to detect even a handful of radio sources in the HDF-N, noise levels of a few microJy must be achieved. These in turn require integration times ranging from a few days - in the case of the WSRT and VLA, to many days in the case of MERLIN.

Figure 3

Figure 3. The WSRT 1.4 GHz contour map of the HDF (see incomplete rotated square) and part of the HFF superimposed upon a deep CFHT optical image of the HDF region made by the Canada-France-Hawaii telescope (courtesy Amy Barger). Crosses indicate previously known VLA detections, boxes indicate new WSRT detections. The detection of a nearby, extended star forming galaxy is highlighted (upper left).

3.1. VLA 8.3 GHz, VLA-MERLIN & WSRT 1.4 GHz observations

Deep VLA observations of the HDF (including the HFF) have been conducted at both 8.3 and 1.4 GHz (Fomalont et al. 1997, Richards et al. 1998, Richards 2000). The 8.3 GHz observations reach noise levels of a few microJy per beam (several times better than the 1.4 GHz observations) but more sources are actually detected at 1.4 GHz where the source counts are steeper and the VLA field of view wider. Perhaps the most "complete" radio view of the HDF (see Fig. 3) is provided by the WSRT 1.4 GHz observations (Garrett et al. 2000a). These are sensitive to very extended radio structures, although for the WSRT (and indeed the VLA), the vast majority of the microJy radio source population remain barely or completely unresolved at arcsecond resolution. Combined VLA-MERLIN 1.4 GHz observations with a resolution of 0.2" (Muxlow et al. 1999) begin to resolve most of these sources but the detailed morphology of the microJy radio source population still remains unknown. The main results of the VLA, MERLIN and WSRT data can be summarised as follows:

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