2016A&A...585A..54F

We report the discovery of PHzG95.5-61.6, a complex structure detected in emission in the Planck all-sky survey that corresponds to two over-densities of high-redshift (i.e. z>1) galaxies. This is the first source from the Planck catalogue of high-z candidates (proto-clusters and lensed systems) that has been completely characterised with follow-up observations from the optical to the sub-millimetre (sub-mm) domain. Herschel/SPIRE observations at 250, 350, and 500µm reveal the existence of five sources producing a 500µm emission excess that spatially corresponds to the candidate proto-clusters discovered by Planck. Further observations at the Canada-France-Hawaii Telescope in the optical bands (g and i) with MegaCam, and in the near infrared (NIR) (J, H and K_s), with WIRCam, plus mid-infrared observations with IRAC/Spitzer (at 3.6 and 4.5µm), confirm that the sub-mm red excess is associated with an over-density of colour-selected galaxies (i-K_s∼2.3 and J-K∼0.8AB-mag). Follow-up spectroscopy of 13 galaxies with VLT/X-Shooter establishes the existence of two high-z structures: one at z≃1.7 (three confirmed member galaxies), the other at z≃2.0 (six confirmed members). The spectroscopic members of each substructure occupy a circular region of comoving radius that is smaller than 1Mpc, which supports the existence of a physical bond among them. This double structure is also seen in the photometric redshift analysis of a sample of 127 galaxies located inside a circular region of 1'-radius. This contains the five Herschel/SPIRE sources, where we found a double-peaked excess of galaxies at z≃1.7 and z≃2.0 with respect to the surrounding region. These results suggest that PHzG95.5-61.6 corresponds to two accreting nodes, not physically linked to one another, embedded in the large scale structure of the Universe at z∼2 and along the same line-of-sight. In conclusion, the data, methods and results illustrated in this pilot project confirm that Planck data can be used to detect the emission from clustered, dusty star-forming galaxies at high z, and, thus, to pierce through the early growth of cluster-scale structures.