Abstract
The shallow, all-sky Planck surveys at sub-millimetre wavelengths have 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 extraordinary detail the galaxy structure and kinematics in early evolutionary phases through high-resolution imaging and spectroscopic follow-up. This enables us to gain otherwise unaccessible direct information on physical processes in action. However, the extraction of candidate strongly lensed galaxies from Planck catalogues is hindered by the fact that they are generally detected with a poor signal-to-noise ratio, except for the few brightest galaxies. Their photometric properties are therefore strongly blurred, which makes them very difficult to single out. We have devised a method capable of increasing the number of identified Planck-detected strongly lensed galaxies by a factor of about three to four, although with an unavoidably limited efficiency. Our approach exploits the fact that the sub-millimetre colours of strongly lensed galaxies are definitely colder than those of nearby dusty galaxies, which constitute the overwhelming majority of extragalactic sources detected by Planck. The sub-millimetre colours of the 47 confirmed or very likely Planck-detected strongly lensed galaxies have been used to estimate the colour range spanned by objects of this type. Moreover, most nearby galaxies and radio sources can be confirmed by cross-matching with the IRAS and PCNT catalogues, respectively. We present samples of lensed candidates selected at 545, 857, and 353 GHz, comprising 177, 97, and 104 sources, respectively. The efficiency of our approach, tested by exploiting data from the SPT survey covering ≃2500 deg2, is estimated to be in the range 30%−40%. We also discuss stricter selection criteria to increase the estimated efficiency to ≃50%, at the cost of a somewhat lower completeness. Our analysis of SPT data has identified a dozen galaxies that can reliably be considered previously unrecognized Planck-detected strongly lensed galaxies. Extrapolating the number of Planck-detected confirmed or very likely strongly lensed galaxies found within the SPT and H-ATLAS survey areas, we expect ≃150 to ≃190 such sources over the full |b|> 20° sky.
Highlights
This information is absolutely crucial to understand the key processes governing the galaxy formation and early evolution
As mentioned above, picking up strongly lensed galaxies from Planck catalogues is not easy since they are a tiny fraction of detected sources and their flux densities are generally near the detection limit, as expected given the steepness of the bright end of their source counts (Perrotta et al 2002, 2003; Negrello et al 2007; Vieira et al 2010; Mocanu et al 2013; Negrello et al 2017; Everett et al 2020)
The approach adopted is analogous to that described above except that we exploited the Bayesian Extraction and Estimation Package (BeeP) photometry which is available for all sources in the PCCS2 857 GHz list and includes 3000 GHz (100 μm) flux densities extracted from the IRIS map3 at this frequency
Summary
This information is absolutely crucial to understand the key processes governing the galaxy formation and early evolution. Negrello et al (2007) predicted that essentially all high-z galaxies brighter than S 500 μm = 100 mJy detected by Herschel surveys would have been strongly lensed (magnification μ ≥ 2) and pointed out that they could be identified with close to 100% efficiency since the other extragalactic sources above that flux density limit would have been recognizable local galaxies plus a small fraction of radio sources. To put the argument in context, let us remember that essentially all high-z SMGs brighter than 100 mJy at 600 GHz (500 μm) were found to be strongly lensed (Negrello et al 2010, 2017; Wardlow et al 2013; Nayyeri et al 2016), but the brightest candidate strongly lensed galaxy detected over the 602 deg of the Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS; Eales et al 2010) has a flux density of 465.7 mJy at 857 GHz (Negrello et al 2017). H20 W13 H16 H20 H20 H20 H20 C15 C15 C15, H20 H20 H20 C15, H20 F12, He13, N17 H16 N17 D17 H20 N17 H20 C15, Ne16 C15, H20 H16, H20 H16, H20 Sw10 Su21
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