A 1200-μm MAMBO survey of the GOODS-N field: a significant population of submillimetre dropout galaxies

We present a 1200-μm image of the Great Observatories Origin Deep Survey North (GOODSN) field, obtained with the Max Planck Millimeter Bolometer array (MAMBO) on the IRAM 30-m telescope. The survey covers a contiguous area of 287 arcmin to a near-uniform noise level of ∼0.7 mJy beam. After Bayesian flux deboosting, a total of 30 sources are recovered (>3.5-σ). 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 (>4-σ). We combine our GOODS-N sample with those obtained in the Lockman Hole and ELAIS N2 fields (Scott et al. 2002; Greve et al. 2004) in order to explore the degree of overlap between 1200-μmand 850-μm-selected galaxies (hereafter SMGs), finding no significant difference between their S850μm/S1200μm distributions. However, a noise-weighted stacking analysis yields a significant detection of the 1200-μm-blank SCUBA sources, S850μm/S1200μm = 3.8 ± 0.4, whereas no significant 850-μm signal is found for the 850-μm-blank MAMBO sources (S850μm/S1200μm = 0.7±0.3). The hypothesis that the S850μm/S1200μm 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 submm drop-outs (SDOs), with S850μm/S1200μm = 0.7 − 1.7, requiring very cold dust or unusual spectral energy distributions (Td ≃ 10 K; β ≃ 1), unless SDOs reside beyond the redshift range observed for radio-identified SMGs, i.e. at z > 4.