Improving constraints on the growth rate of structure by modelling the density-velocity cross-correlation in the 6dF Galaxy Survey

We present the first simultaneous analysis of the galaxy overdensity and peculiar velocity fields by modelling their cross-covariance. We apply our new maximum-likelihood approach to data from the 6-degree Field Galaxy Survey (6dFGS), which has the largest single collection of peculiar velocities to date. We present a full derivation of the analytic expression for the cross-covariance between the galaxy overdensity and peculiar velocity fields and find direct evidence for a non-zero correlation between the fields on scales up to similar to 50 h(-1) Mpc. When utilizing the cross-covariance, our measurement of the normalized growth rate of structure is f sigma(8)(z = 0) = 0.424(-0.064)(+ 0.067)(15 per cent precision), and our measurement of the redshift-space distortion parameter is beta = 0.341(-0.058) (+0.062) (18 per cent precision). Both measurements improve by similar to 20 per cent compared to only using the autocovariance information. This is consistent with the literature on multiple-tracer approaches, as well as Fisher matrix forecasts and previous analyses of 6dFGS. Our measurement of f sigma(8) is consistent with the standard cosmological model, and we discuss how our approach can be extended to test alternative models of gravity.