Determining an accurate position for a submillimetre ( submm ) galaxy ( SMG ) is the crucial step that enables us to move from the basic properties of an SMG sample – source counts and 2-D clustering – to an assessment of their detailed , multi-wavelength properties , their contribution to the history of cosmic star formation and their links with present-day galaxy populations .
In this paper , we identify robust radio and/or infrared ( IR ) counterparts , and hence accurate positions , for over two thirds of the SCUBA HAlf-Degree Extragalactic Survey ( SHADES ) Source Catalogue , presenting optical , 24- \mu m and radio images of each SMG .
Observed trends in identification rate have given no strong rationale for pruning the sample .
Uncertainties in submm position are found to be consistent with theoretical expectations , with no evidence for significant additional sources of error .
Employing the submm/radio redshift indicator , via a parameterisation appropriate for radio-identified SMGs with spectroscopic redshifts , yields a median redshift of 2.8 for the radio-identified subset of SHADES , somewhat higher than the median spectroscopic redshift .
We present a diagnostic colour-colour plot , exploiting Spitzer photometry , in which we identify regions commensurate with SMGs at very high redshift .
Finally , we find that significantly more SMGs have multiple robust counterparts than would be expected by chance , indicative of physical associations .
These multiple systems are most common amongst the brightest SMGs and are typically separated by 2–6 arcsec , \sim 15–50/sin i kpc at z \sim 2 , consistent with early bursts seen in merger simulations .