Millimeter imaging of submillimeter galaxies in the COSMOS field: redshift distribution⋆⋆⋆

We present new IRAM Plateau de Bure interferometer (PdBI) 1.3 mm continuum observations at ∼1.5′′ resolution of 28 submillimeter galaxies (SMGs), previously discovered with the 870 μm bolometer LABOCA at the APEX telescope from the central 0.7 deg2 of the COSMOS field. Nineteen out of the 28 LABOCA sources were detected with PdBI at a >∼3σ level of ≈1.4 mJy/beam. A combined analysis of this new sample with existing interferometrically identified SMGs in the COSMOS field yields the following results: i) >∼15%, and possibly up to ∼40% of single-dish detected SMGs consist of multiple sources; ii) statistical analysis of multi-wavelength counterparts to single-dish SMGs shows that only ∼50% have real radio or IR counterparts; iii) ∼18% of interferometric SMGs have either no multi-wavelength counterpart or only a radio-counterpart; and iv) ∼50–70% of z >∼ 3 SMGs have no radio counterparts (down to an rms of 7–12 μJy at 1.4 GHz). Using the exact interferometric positions to identify the multi-wavelength counterparts allows us to determine accurate photometric redshifts for these sources. The redshift distributions of the combined and the individual 1.1 mm and 870 μm selected samples shows a higher mean and a broader width than those derived in previous studies. This study finds that on average brighter and/or mmselected SMGs are located at higher redshifts, consistent with previous studies. The mean redshift for the 1.1 mm selected sample (z̄ = 3.1± 0.4) is tentatively higher than that for the 870 μm selected sample (z̄ = 2.6± 0.4). Based on our nearly complete sample of AzTEC 1.1 mm SMGs in a 0.15 deg2 area, we infer a higher surface density of z >∼ 4 SMGs than predicted by current cosmological models. In summary, our findings imply that interferometric identifications at (sub-)millimeter wavelengths are crucial to build statistically complete and unbiased samples of SMGs.