Brighter galaxy bias: underestimating the velocity dispersions of galaxy
  clusters
  We study the systematic bias introduced when selecting the spectroscopic
redshifts of brighter cluster galaxies to estimate the velocity dispersion of
galaxy clusters from both simulated and observational galaxy catalogues. We
select clusters with Ngal > 50 at five low redshift snapshots from a
semi-analytic model galaxy catalogue, and from a catalogue of SDSS DR8 groups
and clusters across the redshift range 0.021<z<0.098. We employ various
selection techniques to explore whether the velocity dispersion bias is simply
due to a lack of dynamical information or is the result of an underlying
physical process occurring in the cluster, for example, dynamical friction. The
velocity dispersions and stacked particle velocity distributions of the parent
dark matter (DM) halos are compared to the corresponding cluster dispersions
and galaxy velocity distribution. We find a clear bias between the halo and the
semi-analytic galaxy cluster velocity dispersion on the order of sigma gal /
sigma DM = 0.87-0.95 and a distinct difference in the stacked galaxy and DM
particle velocity distribution. We identify a systematic underestimation of the
velocity dispersions when imposing increasing absolute I-band magnitude limits.
This underestimation is enhanced when using only the brighter cluster members
for dynamical analysis on the order of 5-35%, indicating that dynamical
friction is a serious source of bias when using galaxy velocities as tracers of
the underlying gravitational potential. In contrast to the literature we find
that the resulting bias is not only halo mass-dependent but that the nature of
the dependence changes according to the galaxy selection strategy. We make a
recommendation that, in the realistic case of limited availability of spectral
observations, a strictly magnitude-limited sample should be avoided to ensure
an unbiased estimate of the velocity dispersion.
