Optical validation and characterization of

Aims. The second catalogue of Planck Sunyaev–Zeldovich (SZ) sources, hereafter PSZ2, is the largest sample of galaxy clusters selected through their SZ signature in the full sky. At the time of publication, 21% of these objects had no known counterpart at other wavelengths. Using telescopes at the Canary Island observatories, we conducted the long-term observational programme 128-MULTIPLE-16/15B (hereafter LP15), a large and complete optical follow-up campaign of all the unidentified PSZ2 sources in the northern sky, with declinations above −15° and no correspondence in the first Planck catalogue PSZ1. The main aim of LP15 is to validate all those SZ cluster candidates, and to contribute to the characterization of the actual purity and completeness of full Planck SZ sample. In this paper, we describe the full programme and present the results of the first year of observations. Methods. The LP15 programme was awarded 44 observing nights, spread over two years in three telescopes at the Roque de los Muchachos Observatory. The full LP15 sample comprises 190 previously unidentified PSZ2 sources. For each target, we performed deep optical imaging and spectroscopy. Our validation procedure combined this optical information with SZ emission as traced by the publicly available Planck Compton y-maps. The final classification of the new galaxy clusters as optical counterparts of the SZ signal is established according to several quantitative criteria based on the redshift, velocity dispersion, and richness of the clusters. Results. This paper presents the detailed study of 106 objects out of the LP15 sample, corresponding to all the observations carried out during the first year of the programme. We confirmed the optical counterpart for 41 new PSZ2 sources, 31 of them being validated using also velocity dispersion based on our spectroscopic information. This is the largest dataset of newly confirmed PSZ2 sources without any previous optical information. All the confirmed counterparts are rich structures (i.e. they show high velocity dispersion), and are well aligned with the nominal Planck coordinates (i.e. ∼70% of them are located at less than 3′ distance). In total, 65 SZ sources are classified as unconfirmed, 57 of them being due to the absence of an optical over-density, and eight of them due to a weak association with the observed SZ decrement. Most of the sources with no optical counterpart are located at low galactic latitudes and present strong galactic cirrus in the optical images, the dust contamination being the most probable explanation for their inclusion in the PSZ2 catalogue