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

We present a 1200-mu m 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 287 arcmin(2) to a near-uniform noise level of similar to 0.7mJy beam(-1). After Bayesian flux deboosting, a total of 30 sources are recovered (>= 3.5 sigma). An optimal combination of our 1200-mu m data and an existing 850-mu m image from the Submillimetre Common-User Bolometer Array (SCUBA) yielded 33 sources (>= 4 sigma). 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-and 850-mu m-selected galaxies (hereafter SMGs), finding no significant difference between their S(850) mu m/S(1200) mu m distributions. However, a noise-weighted stacking analysis yields a significant detection of the 1200-mu m-blank SCUBA sources, S(850) mu m/S(1200) mu m = 3.8 +/- 0.4, whereas no significant 850-mu m signal is found for the 850-mu m-blank MAMBO sources (S(850) mu m/S(1200) mu m = 0.7 +/- 0.3). The hypothesis that the S(850) mu m/S(1200) mu m distribution of SCUBA sources is also representative of the MAMBO population is rejected at the similar to 4 sigma level, via Monte Carlo simulations. Therefore, although the populations overlap, galaxies selected at 850 and 1200 mu m are different, and there is compelling evidence for a significant 1200-mu m-detected population which is not recovered at 850 mu m. These are submillimetre dropouts (SDOs), with S(850) mu m/S(1200) mu m = 0.7-1.7, requiring very cold dust or unusual spectral energy distributions (T(d) similar or equal to 10 K; beta similar or equal to 1), unless SDOs reside beyond the redshift range observed for radio-identified SMGs, i. e. at z > 4.