Quantification of the multi-streaming effect in Redshift Space
  Distortion
  Both multi-streaming (random motion) and bulk motion cause the Finger-of-God
(FoG) effect in redshift space distortion (RSD). We apply a direct measurement
of the multi-streaming effect in RSD from simulations, proving that it induces
an additional, non-negligible FoG damping to the redshift space density power
spectrum. We show that, including the multi-streaming effect, the RSD modelling
is significantly improved. We also provide a theoretical explanation based on
halo model for the measured effect, including a fitting formula with one to two
free parameters. The improved understanding of FoG helps break the
$f\sigma_8-\sigma_v$ degeneracy in RSD cosmology, and has the potential of
significantly improving cosmological constraints.
