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 _ { \text { v } } adopted from simulations : ( 1 - b ) = ( 0.51 \pm 0.09 ) b _ { \text { v } } ^ { 3 } .
Using a velocity bias of b _ { \text { 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 _ { \text { v } } \gtrsim 0.9 at 3 \sigma .