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

Abstract

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 ~50 h-1 Mpc. When utilizing the cross-covariance, our measurement of the normalized growth rate of structure is fσ8(z = 0) = 0.424-0.064+0.067 (15 per cent precision), and our measurement of the redshift-space distortion parameter is β = 0.341-0.058+0.062 (18 per cent precision). Both measurements improve by~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 fσ8 is consistent with the standard cosmological model, and we discuss how our approach can be extended to test alternative models of gravity.