The 6dF Galaxy Survey: Bulk Flows on $50-70 h^{-1}$ Mpc scales

TL;DR

This paper measures the bulk flow of the local Universe using the 6dF Galaxy Survey, providing robust results on scales of 50-70 h^{-1} Mpc that align with Lambda CDM predictions.

Contribution

It presents the largest homogeneous peculiar velocity sample and applies a robust 'Minimum Variance' method to measure bulk flows at large scales.

Findings

Bulk flow amplitude at 50 h^{-1} Mpc: 248±58 km/s

Bulk flow amplitude at 70 h^{-1} Mpc: 243±58 km/s

Results consistent with Lambda CDM predictions

Abstract

We measure the bulk flow of the local Universe using the 6dF Galaxy Survey peculiar velocity sample (6dFGSv), the largest and most homogeneous peculiar velocity sample to date. 6dFGSv is a Fundamental Plane sample of $\sim10^4$ peculiar velocities covering the whole southern hemisphere for galactic latitude $|b| > 10^\circ$, out to redshift ${z=0.0537}$. We apply the `Minimum Variance' bulk flow weighting method, which allows us to make a robust measurement of the bulk flow on scales of $50$ and $70\,h^{-1}{\rm Mpc}$. We investigate and correct for potential bias due to the lognormal velocity uncertainties, and verify our method by constructing $\Lambda{\rm CDM}$ 6dFGSv mock catalogues incorporating the survey selection function. For a hemisphere of radius $50\,h^{-1}{\rm Mpc}$ we find a bulk flow amplitude of $U=248\pm58\,{\rm km}\,{\rm s}^{-1}$ in the direction $(l,b) = (318^\circ\pm20^\circ,40^\circ\pm13^\circ)$, and for $70\,h^{-1}{\rm Mpc}$ we find $U=243\pm58\,{\rm km}\,{\rm s}^{-1}$, in the same direction. Our measurement gives us a constraint on $\sigma_8$ of $1.01^{+1.07}_{-0.58}$. Our results are in agreement with other recent measurements of the direction of the bulk flow, and our measured amplitude is consistent with a $\Lambda{\rm CDM}$ prediction.