Optical validation and characterisation of Planck PSZ1 sources at the Canary Islands observatories

II. Second year of ITP13 observations

¹ Instituto de Astrofísica de Canarias, C/Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
e-mail: rbarrena@iac.es
² Universidad de La Laguna, Departamento de Astrofísica, 38206 La Laguna, Tenerife, Spain
³ University of KwaZulu-Natal, Westville Campus, Private Bag X54001, Durban 4000, South Africa
⁴ Universidad Andrés Bello, Departemento de Ciencias Físicas, 7591538 Santiago de Chile, Chile
⁵ Max-Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁶ Tartu Observatory, University of Tartu, 61602 Tõravere, Tartumaa, Estonia
⁷ Institut d’Astrophysique Spatiale, Universitè Paris-Sud, CNRS, UMR8617, 91405 Orsay Cedex, France
⁸ IRFU, CEA, Université Paris-Saclay, 91191 Gif-sur-Yvette, France
⁹ Université Paris Diderot, AIM, Sorbonne Paris Cité, CEA, CNRS, 91191 Gif-sur-Yvette, France
¹⁰ Max-Planck-Institut für extraterrestrische Physik, 85748 Garching, Germany
¹¹ Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029, Blindern, 0315 Oslo, Norway
¹² Astrophysics Group, Cavendish Laboratory, JJ Thomson Av., Cambridge CB3 0HE, UK
¹³ Kavli Institute for Cosmology, Madingley Road, Cambridge CB3 0HA, UK
¹⁴ Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata”, via della Ricerca Scientifica, 1, 00133 Roma, Italy
¹⁵ Université de Toulouse, UPS-OMP, Institut de Recherche en Astrophysique et Planétologie (IRAP), 31400 Toulouse, France
¹⁶ CNRS, IRAP, 9 avenue Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
¹⁷ IASF-Milano, Istituto Nazionale di Astrofisica, via A. Corti 12, 20133 Milano, Italy

Received: 22 January 2020
Accepted: 1 April 2020

Abstract

We report new galaxy clusters previously unknown included in the first Planck Sunyaev–Zeldovich (SZ) sources catalogue, the PSZ1. The results presented here were achieved during the second year of a two-year observational programme, the ITP13, developed at the Roque de los Muchachos Observatory (La Palma, Spain). Using the 2.5 m Isaac Newton telescope, the 3.5 m Telescopio Nazionale Galileo, the 4.2 m William Herschel telescope and the 10.4 m Gran Telescopio Canarias we characterised 75 SZ sources with low SZ significance, SZ S/N <  5.32. We performed deep optical imaging and spectroscopy in order to associate actual galaxy clusters with the SZ Planck source. We adopted robust criteria, based on the 2D spatial distribution, richness, and velocity dispersions to confirm actual optical counterparts up to z <  0.85. The selected systems are confirmed only if they are well aligned with respect to the PSZ1 coordinate and show high richness and high velocity dispersion. In addition, we also inspected the Compton y-maps and SZ significance in order to identify unrealistic detections. Following this procedure, we identify 26 cluster counterparts associated with the SZ emission, which means that only about 35% of the clusters considered in this low S/N PSZ1 subsample are validated. Forty-nine SZ sources (∼65% of this PSZ1 subset) remain unconfirmed. At the end of the ITP13 observational programme, we have studied 256 SZ sources with Dec ≥ −15° (212 of them completely unknown), finding optical counterparts for 152 SZ sources. The ITP13 validation programme has allowed us to update the PSZ1 purity, which is now more refined, increasing from 72% to 83% in the low SZ S/N regime. Our results are consistent with the predicted purity curve for the full PSZ1 catalogue and with the expected fraction of false detections caused by the non-Gaussian noise of foreground signals. We find a strong correlation between the number of unconfirmed sources and the thermal emission of diffuse galactic dust at 857 GHz, thus increasing the fraction of false Planck SZ detections at low galactic latitudes.