An improved model for the nonlinear velocity power spectrum
  The velocity divergence power spectrum is a key ingredient in modelling
redshift space distortion effects on quasi-linear and nonlinear scales. We
present an improved model for the z=0 velocity divergence auto and cross power
spectrum which was originally suggested by Jennings et al. 2011. Using
numerical simulations we measure the velocity fields using a Delaunay
tesselation and obtain an accurate prediction of the velocity divergence power
spectrum on scales k < 1 hMpc^{-1}. We use this to update the model which is
now accurate to 2% for both P_{\theta \theta} and P_{\theta \delta} at z=0 on
scales k <0.7 hMpc^{-1} and k <0.5 hMpc^{-1} respectively. We find that the
formula for the redshift dependence of the velocity divergence power spectra
proposed by Jennings et al. 2011 recovers the measured z>0 P(k) to markedly
greater accuracy with the new model. The nonlinear P_{\theta \theta} and
P_{\theta \delta} at z =1 are recovered accurately to better than 2% on scales
k<0.2 hMpc^{-1}. Recently it was shown that the velocity field shows larger
differences between modified gravity cosmologies and \Lambda CDM compared to
the matter field. An accurate model for the velocity divergence power spectrum,
such as the one presented here, is a valuable tool for analysing redshift space
distortion effects in future galaxy surveys and for constraining deviations
from general relativity.
