  Eighteen days of MERLIN data and 42 hours of A-array VLA data at 1.4 GHz have
been combined to image a 10-arcmin field centred on the Hubble Deep and
Flanking Fields (HDF and HFF). A complete sample of 92 radio sources with
1.4-GHz flux densities above 40 microJy has been imaged using MERLIN+VLA. The
images are amongst the most sensitive yet made at 1.4 GHz, with rms noise
levels of 3.3 microJy/beam in the 0.2-arcsec images. Virtually all the sources
are resolved, with angular sizes in the range 0.2 to 3 arcsec. No additional
sources were detected down to 23 microJy in the central 3 arcmin, indicating
that sources fainter than 40 microJy are heavily resolved with MERLIN and must
have typical angular sizes greater than 0.5 arcsec. Compact radio sources were
used to align the optical data to the ICRF, to <50 mas in the HDF. We find a
statistical association of very faint (2 microJy and above) radio sources with
optically bright HDF galaxies down to about 23 mag. Of the 92 radio sources
above 40 microJy, about 85 percent are identified with galaxies brighter than
about I = 25 mag; the remaining 15 percent are associated with optically faint
systems. We identify several very red, optically faint systems including the
the strongest sub-mm source in the HDF, HDF850.1. 72 percent of the radio
sources are starburst or AGN-type systems; the remainder are unclassified. The
proportion of starburst systems increases with decreasing flux density; below
100 microJy 70 percent of the sources are starburst-type systems in the
redshift range 0.3 -- 1.3. Chandra detections are associated with 55 of the 92
radio sources but their X-ray flux densities do not appear to be correlated
with the radio flux densities or morphologies.
