The Hubble deep field north SCUBA super-map -: III.: Optical and near-infrared properties of submillimetre galaxies

We present a new submillimetre(submm) super-map in the Hubble Deep Field North (HDF-N) region (Great Observatories Origins Deep Survey North, GOODS-N, field), containing 40 statistically robust sources at 850 mu m. This map contains additional data, and several new sources, including one of the brightest blank-sky extragalactic submm sources ever detected. We have used the Advanced Camera for Surveys (ACS) Hubble Space Telescope (HST) images and ground-based near-infrared (IR) observations from the Great Observatories Origins Deep Survey (GOODS), along with deep radio observations, to develop a systematic approach for counterpart identification. With the depth achieved by this survey, optical counterparts have been found for all the radio-detected (RD) submm sources. We have used the colours, morphologies and photometric redshifts of these secure identifications to help identify counterparts to the radio-undetected (RU) sources, finding that certain combinations of optical properties can be used to successfully identify the counterpart to a submm source. 72 per cent of our sources with optical coverage have a unique optical counterpart using our new techniques for counterpart identification and an additional 18 per cent have more than one possibility that meet our criteria in the ACS images. Thus, only 10 per cent of our sources lack a plausible optical/near-IR counterpart, meaning that we have the first sample of Submillimetre Common User Bolometer Array (SCUBA) sources that is nearly completely identified in the optical. We have found a much higher extremely red object (ERO) rate than other submm surveys, as a result of the increased depth in the optical images. The median photometric redshift (and quartile range), from optical and near-IR data, is 1.7 (1.3-2.5) for the RD submm sources and rises to 2.3 (1.3-2.7) for the RU subsample. We find interesting correlations between the 850-mu m flux and both the i(775) magnitude and the photometric redshift, from which there appears to be an absence of high-redshift faint counterparts to the lower flux density SCUBA sources. While the quantitative morphologies span a range of values, in general the submm galaxies show larger sizes and a higher degree of asymmetry than other galaxy populations at the same redshifts. In the Appendix, we discuss several improvements in our data analysis procedure, including methods of testing for source reliability.