Velocity Dispersions of Clusters in the Dark Energy Survey Y3 redMaPPer
  Catalog
  We measure the velocity dispersions of clusters of galaxies selected by the
redMaPPer algorithm in the first three years of data from the Dark Energy
Survey (DES), allowing us to probe cluster selection and richness estimation,
$\lambda$, in light of cluster dynamics. Our sample consists of 126 clusters
with sufficient spectroscopy for individual velocity dispersion estimates. We
examine the correlations between cluster velocity dispersion, richness, X-ray
temperature and luminosity as well as central galaxy velocity offsets. The
velocity dispersion-richness relation exhibits a bimodal distribution. The
majority of clusters follow scaling relations between velocity dispersion,
richness, and X-ray properties similar to those found for previous samples;
however, there is a significant population of clusters with velocity
dispersions which are high for their richness. These clusters account for
roughly 20\% of the $\lambda < 70$ systems in our sample, but more than half of
$\lambda < 70$ clusters at $z>0.5$. A couple of these systems are hot and X-ray
bright as expected for massive clusters with richnesses that appear to have
been underestimated, but most appear to have high velocity dispersions for
their X-ray properties likely due to line-of-sight structure. These results
suggest that projection effects contribute significantly to redMaPPer
selection, particularly at higher redshifts and lower richnesses. The redMaPPer
determined richnesses for the velocity dispersion outliers are consistent with
their X-ray properties, but several are X-ray undetected and deeper data is
needed to understand their nature.
