The Planck sub-mm surveys detected the brightest strongly gravitationally lensed dusty galaxies in the sky. The combination of their extreme gravitational flux boosting and image stretching offers the unique possibility of measuring in detail, via high-resolution imaging and spectroscopic follow-up, the galaxy structure and kinematics in early evolutionary phases, thus gaining otherwise unaccessible direct information on physical processes in action. The extraction of candidate strongly lensed galaxies (SLGs) from Planck catalogues is hindered by the fact that they are generally detected with poor S/N, except for the few brightest ones, their photometric properties are strongly blurred and they are difficult to single out. We devised a method to increase by a factor of 3 to 4 the number of identified Planck-detected SLGs, although with an unavoidably limited efficiency. Our approach uses the fact that SLGs have sub-mm colours colder than nearby dusty galaxies (the large majority of Planck extragalactic sources). The sub-mm colours of the 47 confirmed or very likely Planck-detected SLGs are used to estimate the colour range of these objects. Moreover, most nearby galaxies and radio sources can be picked up by cross-matching with IRAS and PCNT catalogues, respectively. We present samples of 177, 97, 104 lensed candidates at 545, 857, 353 GHz, respectively. The efficiency of our approach, tested on the SPT survey covering 2,500 sq. deg., is estimated to be of 30%-40%. We also discuss stricter selection criteria increasing efficiency to 50% but with a somewhat lower completeness. Our analysis of SPT data has identified a dozen of galaxies that can be reliably considered previously unrecognized Planck-detected SLGs. Extrapolating the number of Planck-detected confirmed or very likely SLGs found within the SPT and H-ATLAS areas, we expect from 150 to 190 such sources over the|b|>20deg sky.
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