Calibrating the Planck Cluster Mass Scale with Cluster Velocity
  Dispersions
  We measure the Planck cluster mass bias using dynamical mass measurements
based on velocity dispersions of a subsample of 17 Planck-detected clusters.
The velocity dispersions were calculated using redshifts determined from
spectra obtained at Gemini observatory with the GMOS multi-object spectrograph.
We correct our estimates for effects due to finite aperture, Eddington bias and
correlated scatter between velocity dispersion and the Planck mass proxy. The
result for the mass bias parameter, $(1-b)$, depends on the value of the galaxy
velocity bias $b_v$ adopted from simulations: $(1-b)=(0.51\pm0.09) b_v^3$.
Using a velocity bias of $b_v=1.08$ from Munari et al., we obtain
$(1-b)=0.64\pm 0.11$, i.e, an error of 17% on the mass bias measurement with 17
clusters. This mass bias value is consistent with most previous weak lensing
determinations. It lies within $1\sigma$ of the value needed to reconcile the
Planck cluster counts with the Planck primary CMB constraints. We emphasize
that uncertainty in the velocity bias severely hampers precision measurements
of the mass bias using velocity dispersions. On the other hand, when we fix the
Planck mass bias using the constraints from Penna-Lima et al., based on weak
lensing measurements, we obtain a positive velocity bias $b_v \gtrsim 0.9$ at
$3\sigma$.
