Velocity Segregation and Systematic Biases In Velocity Dispersion
  Estimates With the SPT-GMOS Spectroscopic Survey
  The velocity distribution of galaxies in clusters is not universal; rather,
galaxies are segregated according to their spectral type and relative
luminosity. We examine the velocity distributions of different populations of
galaxies within 89 Sunyaev Zel'dovich (SZ) selected galaxy clusters spanning $
0.28 < z < 1.08$. Our sample is primarily draw from the SPT-GMOS spectroscopic
survey, supplemented by additional published spectroscopy, resulting in a final
spectroscopic sample of 4148 galaxy spectra---2868 cluster members. The
velocity dispersion of star-forming cluster galaxies is $17\pm4$% greater than
that of passive cluster galaxies, and the velocity dispersion of bright ($m <
m^{*}-0.5$) cluster galaxies is $11\pm4$% lower than the velocity dispersion of
our total member population. We find good agreement with simulations regarding
the shape of the relationship between the measured velocity dispersion and the
fraction of passive vs. star-forming galaxies used to measure it, but we find a
small offset between this relationship as measured in data and simulations in
which suggests that our dispersions are systematically low by as much as 3\%
relative to simulations. We argue that this offset could be interpreted as a
measurement of the effective velocity bias that describes the ratio of our
observed velocity dispersions and the intrinsic velocity dispersion of dark
matter particles in a published simulation result. Measuring velocity bias in
this way suggests that large spectroscopic surveys can improve dispersion-based
mass-observable scaling relations for cosmology even in the face of velocity
biases, by quantifying and ultimately calibrating them out.
