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

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 $\sim10<sup>4$ peculiar velocities covering the whole southern hemisphere for galactic latitude $|b| &gt; 10<sup>\circ$, 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 $U=248\pm58\,{\rm km}\,{\rm s}<sup>{-1}$ in the direction $$(l,b) = (318<sup>\circ\pm20<sup>\circ,40<sup>\circ\pm13<sup>\circ)$$, and for $70\,h^{-1}{\rm Mpc}$ we find $U=243\pm58\,{\rm km}\,{\rm s}<sup>{-1}$, in the same direction. Our measurement gives us a constraint on $\sigma_8$ of $1.01<sup>{+1.07}_{-0.58}$. 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.