Cosmological parameters estimated from velocity -- density comparisons: Calibrating 2M++

Abstract

Cosmological parameters can be measured by comparing peculiar velocities with those predicted from a galaxy density field. Previous work has tested the accuracy of this approach with N-body simulations, but generally on idealised mock galaxy surveys. However, systematic biases may arise solely due to survey selection effects such as flux-limited samples, edge-effects, and complications due to the obscuration of the Galactic plane. In this work, we explore the impact of each of these effects individually well as collectively, using the semi-analytic models from numerical simulations to generate mock catalogues that mimic the 2M++ density field. We find the reconstruction and analysis methods used for our 2M++ mocks produce a value of fsigma_8 that is biased high by a factor 1.04 \pm 0.01 compared to the true value. Moreover, a cosmic volume matching that of 2M++ has a cosmic variance uncertainty in fsigma_8 of ~5%. The systematic bias is a function of distance: it is unbiased close to the origin but is biased slightly high for distances in the range 100-180 Mpc/h. Correcting for this small bias, we find the linear fsigma_8 = 0.362 \pm 0.023. The predicted peculiar velocities from 2M++ have an error of 170 km/s that slowly increases with distance, exceeding 200 km/s only at distances of 180-200 Mpc/h. Finally, the residual bulk flow speeds found in previous work are shown to be not in conflict with those expected in the Lambda cold dark matter model.