The Herschel Multi-tiered Extragalactic Survey: SPIRE-mm photometric redshifts

We investigate the potential of submmmm and submmmmradio photometric redshifts using a sample of mm-selected sources as seen at 250, 350 and 500 mu m by the SPIRE instrument on Herschel. From a sample of 63 previously identified mm sources with reliable radio identifications in the Great Observatories Origins Deep Survey North and Lockman Hole North fields, 46 (73 per cent) are found to have detections in at least one SPIRE band. We explore the observed submm/mm colour evolution with redshift, finding that the colours of mm sources are adequately described by a modified blackbody with constant optical depth tau=(/nu0)beta, where beta=+1.8 and nu 0=c/100 mu m. We find a tight correlation between dust temperature and IR luminosity. Using a single model of the dust temperature and IR luminosity relation, we derive photometric redshift estimates for the 46 SPIRE-detected mm sources. Testing against the 22 sources with known spectroscopic or good quality optical/near-IR photometric redshifts, we find submm/mm photometric redshifts offer a redshift accuracy of vertical bar Delta z vertical bar/(1+z)= 0.16 (=0.51). Including constraints from the radiofar-IR correlation, the accuracy is improved to vertical bar Delta z vertical bar/(1+z)=0.15 (=0.45). We estimate the redshift distribution of mm-selected sources finding a significant excess at z > 3 when compared to similar to 850 mu m selected samples.