Optical Redshift and Richness Estimates for Galaxy Clusters Selected
  with the Sunyaev-Zel'dovich Effect from 2008 South Pole Telescope
  Observations
  We present redshifts and optical richness properties of 21 galaxy clusters
uniformly selected by their Sunyaev-Zel'dovich signature. These clusters, plus
an additional, unconfirmed candidate, were detected in a 178 square-degree area
surveyed by the South Pole Telescope in 2008. Using griz imaging from the
Blanco Cosmology Survey and from pointed Magellan telescope observations, as
well as spectroscopy using Magellan facilities, we confirm the existence of
clustered red-sequence galaxies, report red-sequence photometric redshifts,
present spectroscopic redshifts for a subsample, and derive R_200 radii and
M_200 masses from optical richness. The clusters span redshifts from 0.15 to
greater than 1, with a median redshift of 0.74; three clusters are estimated to
be at z > 1. Redshifts inferred from mean red-sequence colors exhibit 2% RMS
scatter in sigma_z/(1+z) with respect to the spectroscopic subsample for z < 1.
We show that M_200 cluster masses derived from optical richness correlate with
masses derived from South Pole Telescope data and agree with previously derived
scaling relations to within the uncertainties. Optical and infrared imaging is
an efficient means of cluster identification and redshift estimation in large
Sunyaev-Zel'dovich surveys, and exploiting the same data for richness
measurements, as we have done, will be useful for constraining cluster masses
and radii for large samples in cosmological analysis.
