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Description
18 days of MERLIN data and 42 hours of A-array VLA data at 1.4 GHz have been combined to image a 10 arcminute field centred on the Hubble Deep Field. This area includes both the Hubble Deep and Flanking Fields. A complete sample of 92 sources with flux densities above 40µJy have been detected using the VLA data, and imaged with the MERLIN+VLA combination at resolutions of 0.2, 0.3 and 0.5 arcsecond. These are the most sensitive 1.4 GHz images yet made, with rms noise levels of 3.3µJy/beam in the 0.2 arcsecond images. Virtually all the objects are resolved by the MERLIN+VLA combination images showing that they have angular sizes in the range 0.2 to 3 arcseconds, typically smaller than the sizes of the optical galaxy images. In addition to the imaging of 92 sources found with the VLA alone, the central 3 arcminutes, which encloses the HDF, has been separately imaged with the MERLIN+VLA combination at the full 3.3µJy/beam sensitivity to search for compact sources fainter than 40µJy. No additional sources were found that were not previously detected by the VLA, indicating that such sources are heavily resolved with MERLIN and hence must have typical angular sizes >0.5 arcseconds. In addition to the images, high quality astrometry is an equally important product of this work allowing reliable source identification in a crowded field. Radio sources associated with compact galaxies have been used to align both the HST WFPC2 frames and a CFHT optical field to the ICRF. The HST optical field has been aligned to better than 50 mas in the Deep Field itself, and to ∼150 mas in the outer parts of the Flanking Fields. The proportion of starburst systems is found to increase with decreasing source strength. At fluxes below 100µJy in excess of 70% of the µJy sources are found to be starburst type systems associated with major disc galaxies in the redshift range 0.3 - 1.3. Some 40% of the brighter sources are found to be intermediate luminosity AGN systems identified with galxies in a similar redshift range. Around 85% of the sources are identified with galaxies brighter than I=25mag. The remaining 15% are associated with optically faint systems close to or beyond the HDF limit; many of these may be dust-shrouded starburst galaxies at high redshift (z>3). Approaching half the sources in the 10 arcminute field are detected in X-rays by the Chandra Satellite. The X-ray detection rate appears to be uncorrelated with the radio source classification.
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