(Sub)millimetre interferometric imaging of a sample of COSMOS/AzTEC submillimetre galaxies - I. Multiwavelength identifications and redshift distribution

Abstract

We used the PdBI to map a sample of 15 SMGs in the COSMOS field at the wavelength of 1.3 mm. The target SMGs were originally discovered in the JCMT/AzTEC 1.1 mm continuum survey at S/N=4-4.5. This paper presents, for the first time, interferometric millimetre-wavelength observations of these sources. The angular resolution of our observations, 1.8", allowed us to accurately determine the positions of the target SMGs. Using a detection threshold of S/N>4.5 regardless of multiwavelength counterpart association, and 4<S/N<=4.5 if a multiwavelength counterpart within 1.5" is also present, the total number of detections in our survey is 22. Three of our detected SMGs (AzTEC21, 27, and 28; which corresponds to 20%) are marginally resolved at our angular resolution, and these sources are found to have elongated or clumpy morphologies and/or multiple components. Using optical to NIR photometric redshifts, available spectroscopic redshifts, and redshifts estimated from the radio-to-submm spectral index we infer a median redshift of $\tilde{z}=3.20\pm0.25$ for our sample. To study the overall multiplicity and redshift distribution of flux-limited samples of SMGs we combined these sources with the 15 brightest AzTEC SMGs detected at 1.1 mm, AzTEC1-15, and studied previously. This constitutes a complete, flux- and S/N-limited 1.1-mm selected sample. We find that the median redshift for the 15 brightest AzTEC SMGs ($\tilde{z}=3.05\pm0.44$) is consistent with that for AzTEC16-30. This conforms to recent observational findings that SMGs do not exhibit any significant trend between the redshift and (sub)mm flux density. For the combined AzTEC1-30 sample we derive a median redshift of $\tilde{z}=3.17\pm0.27$, consistent with previous results based on mm-selected samples. We further infer that within the combined AzTEC1-30 sample $\sim25\pm9\%$ of sources separate into multiple components.