2008MNRAS.389.1489G

We present a 1200-µm image of the Great Observatories Origin Deep Survey North (GOODS-N) field, obtained with the Max Planck Millimetre Bolometer array (MAMBO) on the IRAM 30-m telescope. The survey covers a contiguous area of 287arcmin² to a near-uniform noise level of ∼0.7mJy/beam. After Bayesian flux deboosting, a total of 30 sources are recovered (≥3.5σ). An optimal combination of our 1200-µm data and an existing 850-µm image from the Submillimetre Common-User Bolometer Array (SCUBA) yielded 33 sources (≥4σ). We combine our GOODS-N sample with those obtained in the Lockman Hole and ELAISN2 fields [Scott et al. 2002] [Greve et al. 2004] in order to explore the degree of overlap between 1200- and 850-µm-selected galaxies (hereafter SMGs), finding no significant difference between their S_850µm/S_1200µmdistributions. However, a noise-weighted stacking analysis yields a significant detection of the 1200-µm-blank SCUBA sources, S_850µm/S_1200µm= 3.8 ±0.4, whereas no significant 850-µm signal is found for the 850-µm-blank MAMBO sources (S_850µm/S_1200µm= 0.7±0.3). The hypothesis that the S_850µm/S_1200µm distribution of SCUBA sources is also representative of the MAMBO population is rejected at the ∼4σ level, via Monte Carlo simulations. Therefore, although the populations overlap, galaxies selected at 850 and 1200µm are different, and there is compelling evidence for a significant 1200-µm-detected population which is not recovered at 850µm. These are submillimetre dropouts (SDOs), with S_850µm/S_1200µm= 0.7-1.7, requiring very cold dust or unusual spectral energy distributions (T_d≃ 10K; β ≃ 1), unless SDOs reside beyond the redshift range observed for radio-identified SMGs, i.e. at z > 4.