Optical validation and characterization of \Planck\ PSZ2 sources at the
  Canary Islands observatories. I. First year of LP15 observations
  Using telescopes at the Canary Island observatories, we conducted the
long-term observational programme 128-MULTIPLE-16/15B (LP15), a large and
complete optical follow-up campaign of all the unidentified PSZ2 sources in the
northern sky (Dec>-15 deg) and no correspondence in the first Planck catalogue
PSZ1. We perform this optical validation of SZ clusters in order to contribute
to the characterization of the actual purity and completeness of full Our
validation procedure combines this optical information with SZ emission as
traced by the publicly available Planck Compton y-maps. The clusters
counterparts are classified according to redshift, velocity dispersion and
richness of the clusters. 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, being 31 of them 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 (~70% of
them are located at less than 3 arcmin distance). In total, 65 SZ sources are
classified as unconfirmed, being 57 of them due to the absence of an optical
over-density, and 8 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,
being the dust contamination the most probable explanation for their inclusion
in the PSZ2 catalogue.
