A broad-band spectroscopic search for CO line emission in HDF850.1: the brightest submillimetre object in the Hubble Deep Field-North

Using the 100-m Green Bank Telescope, we have conducted a cm-wavelength search for CO J = 1-0 line emission towards the high-redshift, far-infrared (FIR) luminous object HDF850.1 over the redshift interval 3.3 ≤ z ≤ 5.4. Despite the wealth of existing multiwavelength observations, and the recent identification of a galaxy counterpart in deep AT'-band (2.2 μm) imaging, an unambiguous spectroscopic redshift has not yet been obtained for this object. A FIR-to-radio wavelength photometric redshift technique, however, predicts a ∼90 per cent probability that the redshift is in the range, 3.3 ≤ z ≤ 5.4 (equivalent to an observed redshifted CO J = 1-0 emission line frequency, 26.5 ≤ ν obs ≥ 18.0 GHz), making HDF850.1 a potential occupant of the 'high-redshift tail' of submillimetre (submm)-selected galaxies. We have also conducted a search for CO J = 2-1 line emission over the narrower redshift range, 3.9 ≤ z ≥ 4.3. Although we do not detect any CO line emission in this object, our limits to the CO line luminosity are in broad agreement with the median value measured in the current sample of high-redshift, submm-selected objects detected in high-J CO line emission, but not sufficient to fully test the validity of the photometric redshift technique.