Millimeter imaging of submillimeter galaxies in the COSMOS field:
  Redshift distribution
  We present new IRAM PdBI 1.3mm continuum observations at ~1.5" resolution of
28 SMGs previously discovered with the 870um bolometer LABOCA at APEX within
the central 0.7deg2 of the COSMOS field. 19 out of the 28 LABOCA sources were
detected with the PdBI at a >~3sigma level of ~1.4mJy/b. A combined analysis of
this new sample with existing interferometrically identified SMGs in the COSMOS
field yields the following results: 1) >~15%, and possibly up to ~40% of
single-dish detected SMGs consist of multiple sources, 2) statistical
identifications of multi-wavelength counterparts to the single-dish SMGs yield
that only ~50% of these single-dish SMGs have real radio or IR counterparts, 3)
~18% of interferometric SMGs have only radio or even no multi-wavelength
counterpart at all, and 4) ~50-70% of z>~3 SMGs have no radio counterparts down
to an rms of 7-12uJy at 1.4GHz. Using the exact interferometric positions to
identify proper multi-wavelength counterparts allows us to determine accurate
photometric redshifts for these sources. The redshift distributions of the
combined and the individual 1.1mm and 870um selected samples have a higher mean
and broader width than the redshift distributions derived in previous studies.
Our sample supports the previous tentative trend that on average brighter
and/or mm-selected SMGs are located at higher redshifts. There is a tentative
offset between the mean redshift for the 1.1mm (<z>=3.1+/-0.4) and 870um
(<z>=2.6+/-0.4) selected samples, with the 1.1mm sources lying on average at
higher redshifts. Based on our nearly complete sample of AzTEC 1.1mm SMGs
within a uniform 0.15deg2 area we infer a higher surface density of z>~4 SMGs
than predicted by current cosmological models. In summary, our findings imply
that (sub-)millimeter interferometric identifications are crucial to build
statistically complete and unbiased samples of SMGs.
