Velocity profiles of matter and biased tracers around voids

ABSTRACT The velocity profile of galaxies around voids is a key ingredient for redshift space distortion (RSD) measurements made using the void–galaxy correlation function. In this paper, we use simulations to test whether the velocity profile of the tracers used to find the voids matches the velocity profile of the dark matter around these voids. A mismatch is expected and found in the inner part of voids, where tracers are very sparse. We discuss how this difference is caused by a selection effect where the void centre positions are correlated to the particular realization of the sparse tracers and their spatial distribution. In turn, this then affects the RSD void–galaxy correlation analysis. We show this by evaluating the Jacobian of the real to redshift space mapping using the tracer or matter velocity profile. Differences of the order of 20 per cent in the velocity profile translate into differences of the order of few per cent in the Jacobian. This small discrepancy propagates to the monopole and quadrupole of the void–tracer correlation function, producing modifications of comparable magnitude to those from changes in fσ8 at the level of the statistical uncertainties from current analyses.