High Redshift Starburst Galaxies in the Hubble Deep and Flanking Fields

Abstract

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 Field (HDF). This area also includes the Hubble Flanking Fields (HFF). A complete sample of 92 radio sources with S1.4GHz>40μJy was detected using the VLA data alone and then imaged with the MERLIN+VLA combination. The combined images offer: i) higher angular resolution (synthesised 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 3.3μJy/beam in the 0.2‐arcsec images. Virtually all the sources are resolved, with angular sizes in the range 0.2 to 3 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. Of the 92 radio sources above 40μJy, ≈85% are identified with galaxies brighter than I=25 mag; the remaining 15% 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 has led to the correct identification of several very red, optically faint systems. On the basis of their radio structures and spectra 72% (66 sources) can be classified as starburst or AGN‐type systems; the remainder are unclassified. The proportion of starburst systems increases with decreasing flux density; below 100μJy >70% of the sources are starburst‐type systems associated with major disc galaxies in the redshift range 0.3 – 1.3.