High Confidence Optical Confirmations Among the High Signal-to-Noise Planck Cluster Candidates

TL;DR

This study confirms galaxy clusters from the Planck SZ catalog using optical imaging, identifying 15 new clusters and analyzing reasons for unconfirmed candidates, thereby improving cluster identification methods.

Contribution

It introduces a combined optical and visual approach to confirm high signal-to-noise Planck SZ candidates, discovering new galaxy clusters and analyzing factors affecting confirmation rates.

Findings

15 new galaxy clusters identified among unconfirmed candidates

Low confirmation rate not solely due to cluster redshift or field density

Method improves reliability of optical confirmation of SZ-selected clusters

Abstract

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.