Multi-wavelength characterisation of z ∼ 2 clustered, dusty star-forming galaxies discovered by Planck

We report the discovery of PHz G95.5-61.6, 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 500 mu m reveal the existence of five sources producing a 500 mu m 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 MeeaCam, and in the near infrared (NIR) H and K,), with WIRCam, plus mid-infrared observations with IRAC/Spitzer (at 3.6 and 4.5 mu m), 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 similar to 0.8 AB-m.ag). Follow-up spectroscopy of 13 galaxies with VLT/X-Shooter establishes the existence of two high-z structures: one at z similar or equal to 1.7 (three confirmed member galaxies), the other at z similar or equal to 2.0 (six confirmed members). The spectroscopic members of each substructure occupy a circular region of comoving radius that is smaller than 1 M.pc, 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 similar or equal to 1.7 and z 2.0 with respect to the surrounding region. These results suggest that PHz G95.5-61.6 corresponds to two accreting nodes, not physically linked to one another, embedded in the large scale structure of the Universe at z similar to 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 he 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.