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

¹ Université de Toulouse UPS Toulouse III − OMP, IRAP 31062 Toulouse France
e-mail: Genevieve.Soucail@irap.omp.eu
² CNRS, IRAP, Institut de Recherche en Astrophysique et Planétologie, 9 avenue du Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
³ CNRS, IRAP, 14 avenue Édouard Belin, 31400 Toulouse, France
⁴ Institut d’Astrophysique Spatiale, CNRS (UMR 8617) Université Paris-Sud 11, Bâtiment 121, 91440 Orsay, France
⁵ Institut Universitaire de France, 103 bd Saint-Michel, 75005 Paris, France
⁶ Laboratoire AIM, IRFU/Service d’Astrophysique − CEA/DSM − CNRS − Université Paris Diderot, Bât. 709, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
⁷ Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France

Received: 27 October 2014
Accepted: 3 October 2015

Abstract

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 μm reveal the existence of five sources producing a 500 μm 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 4.5 μm), 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 ~ 0.8 AB-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 1 Mpc, 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 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 ~ 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.