High Confidence Optical Confirmations Among the High Signal-to-Noise
  Planck Cluster Candidates
  We report on newly identified galaxy clusters from the high signal-to-noise
($>5$-$\sigma$) end of the second all-sky \textit{Planck} Sunyaev-Zel'dovich
(SZ) catalog (PSZ2). The clusters are identified in deep, optical imaging from
the Kitt Peak National Observatory 4m Mayall telescope taken between 2014 and
2017. Here we focus on the highest richness systems, and identify galaxy
clusters through a combination of the maxBCG algorithm and visual image
inspection. Galaxy clusters are considered to be confirmed if they are both
rich and spatially coincident ($\lesssim 6'$) with the reported PSZ2 position.
Of the 85 fields containing unconfirmed PSZ2 candidates observed, we find 15
(17.6% of the observed sample) corresponding galaxy clusters ($ 0.13 < z <
0.78$), 12 of which are previously unrecognized as counterparts. To explain
this low identification fraction, we consider three possible scenarios: that
clusters are (1) mostly at low-z, (2) mostly at high-z, or (3) located in
fields with high object density. None of these scenarios alone can account for
the low purity of rich galaxy clusters among the high signal-to-noise PSZ2
unconfirmed candidates.
