Redshifts, Sample Purity, and BCG Positions for the Galaxy Cluster
  Catalog from the first 720 Square Degrees of the South Pole Telescope Survey
  We present the results of the ground- and space-based optical and
near-infrared (NIR) follow-up of 224 galaxy cluster candidates detected with
the Sunyaev-Zel'dovich (SZ) effect in the 720 deg^2 of the South Pole Telescope
(SPT) survey completed in the 2008 and 2009 observing seasons. We use the
optical/NIR data to establish whether each candidate is associated with an
overdensity of galaxies and to estimate the cluster redshift. Most photometric
redshifts are derived through a combination of three different cluster redshift
estimators using red-sequence galaxies, resulting in an accuracy of \Delta
z/(1+z)=0.017, determined through comparison with a subsample of 57 clusters
for which we have spectroscopic redshifts. We successfully measure redshifts
for 158 systems and present redshift lower limits for the remaining candidates.
The redshift distribution of the confirmed clusters extends to z=1.35 with a
median of z_{med}=0.57. Approximately 18% of the sample with measured redshifts
lies at z>0.8. We estimate a lower limit to the purity of this SPT SZ-selected
sample by assuming that all unconfirmed clusters are noise fluctuations in the
SPT data. We show that the cumulative purity at detection significance \xi>5
(\xi>4.5) is >= 95 (>= 70%). We present the red brightest cluster galaxy (rBCG)
positions for the sample and examine the offsets between the SPT candidate
position and the rBCG. The radial distribution of offsets is similar to that
seen in X-ray-selected cluster samples, providing no evidence that SZ-selected
cluster samples include a different fraction of recent mergers than
X-ray-selected cluster samples.
