High-resolution studies of radio sources in the Hubble Deep and Flanking Fields

18 days of MERLIN data and 42 h of A-array VLA data at 1.4 GHz have been combined to image a 10-arcmin field centred on the Hubble Deep Field (HDF). This area also includes the Hubble Flanking Fields (HFF). A complete sample of 92 radio sources with S-1.4 > 40 mu Jy was detected using the VLA data alone and then imaged with the MERLIN+VLA combination. The combined images offer ( i) higher angular resolution ( synthesized beams of diameter 0.2-0.5 arcsec), (ii) improved astrometric accuracy, and (iii) improved sensitivity compared with VLA-only data. The images are amongst the most sensitive yet made at 1.4 GHz, with rms noise levels of 3.3 mu Jy beam(-1) in the 0.2-arcsec images. Virtually all the sources are resolved, with angular sizes in the range 0.2 to 3 arcsec. The central 3-arcmin square was imaged separately to search for sources down to 27 mu Jy. No additional sources were detected, indicating that sources fainter than 40 mu Jy are heavily resolved with MERLIN and must have typical angular sizes > 0.5 arcsec. Radio sources associated with compact galaxies have been used to align the HDF, the HFF and a larger CFHT optical field to the radio-based International Celestial Reference Frame. The HST optical fields have been registered to < 50 mas in the HDF itself, and to <= 150 mas in the outer parts of the HFF. We find a statistical association of very faint ( >= 2 mu Jy) radio sources with optically bright HDF galaxies down to similar to 23 mag. Of the 92 radio sources above 40 mu Jy, similar to 85 per cent are identified with galaxies brighter than I = 25 mag; the remaining 15 per cent are associated with optically faint systems close to or beyond the HFF ( or even the HDF) limit. The high astrometric accuracy and the ability of radio waves to penetrate obscuring dust have led to the correct identification of several very red, optically faint systems, including the the strongest submillimetre source in the HDF, HDF850.1. On the basis of their radio structures and spectra, 72 per cent ( 66 sources) can be classified as starburst or active galactic nucleus-type systems; the remainder are unclassified. The proportion of starburst systems increases with decreasing flux density; below 100 mu Jy > 70 per cent of the sources are starburst-type systems associated with major disc galaxies 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. The most recent submillimetre results on the HDF and HFF do not provide any unambiguous identifications with these latest radio data, except for HDF 850.1, but suggest at least three strong candidates.