Herschel-ATLAS: VISTA VIKING near-infrared counterparts in the Phase 1 GAMA 9-h data

We identify near-infrared Ks-band counterparts to Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS) submillimetre (submm) sources, using a preliminary object catalogue from the VISTA Kilo-degree Infrared Galaxy (VIKING) survey. The submm sources are selected from the H-ATLAS Phase 1 catalogue of the Galaxy and Mass Assembly 9-h field, which includes all objects detected at 250, 350 or ? with the instrument. We apply and discuss a likelihood ratio method for VIKING candidates within a search radius of 10 arcsec of the 22 000 SPIRE sources with a 5σ detection at ?. We estimate the fraction of SPIRE sources with a counterpart above the magnitude limit of the VIKING survey to be Q0≈ 0.73. We find that 11 294 (51 per cent) of the SPIRE sources have a best VIKING counterpart with a reliability R≥ 0.8, and the false identification rate of these is estimated to be 4.2 per cent. We expect to miss ˜5 per cent of true VIKING counterparts. There is evidence from Z-J and J-Ks colours that the reliable counterparts to SPIRE galaxies are marginally redder than the field population. We obtain photometric redshifts for ˜68 per cent of all (non-stellar) VIKING candidates with a median redshift of ?. We have spectroscopic redshifts for 3147 (˜28 per cent) of the reliable counterparts from existing redshift surveys. Comparing to the results of the optical identifications supplied with the Phase 1 catalogue, we find that the use of medium-deep near-infrared data improves the identification rate of reliable counterparts from 36 to 51 per cent. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. VISTA is an ESO near-infrared telescope in Chile.