Cosmic bulk flow and the local motion from Cosmicflows-2

Full sky surveys of peculiar velocity are arguably the best way to map the large-scale structure (LSS) out to distances of a few x 100 h(-1) Mpc. Using the largest and most accurate ever catalogue of galaxy peculiar velocities Cosmicflows-2, the LSS has been reconstructed by means of the Wiener filter (WF) and constrained realizations (CRs) assuming as a Bayesian prior model the Lambda cold dark matter model with the WMAP inferred cosmological parameters. This paper focuses on studying the bulk flow of the local flow field, defined as the mean velocity of top-hat spheres with radii ranging out to R = 500 h(-1) Mpc. The estimated LSS, in general, and the bulk flow, in particular, are determined by the tension between the observational data and the assumed prior model. A pre-requisite for such an analysis is the requirement that the estimated bulk flow is consistent with the prior model. Such a consistency is found here. At R = 50 (150) h(-1) Mpc, the estimated bulk velocity is 250 +/- 21 (239 +/- 38) km s(-1). The corresponding cosmic variance at these radii is 126 (60) km s(-1), which implies that these estimated bulk flows are dominated by the data and not by the assumed prior model. The estimated bulk velocity is dominated by the data out to R approximate to 200 h(-1) Mpc, where the cosmic variance on the individual supergalactic Cartesian components (of the rms values) exceeds the variance of the CRs by at least a factor of 2. The SGX and SGY components of the cosmic microwave background dipole velocity are recovered by the WF velocity field down to a very few km s(-1). The SGZ component of the estimated velocity, the one that is most affected by the zone of avoidance, is off by 126 km s(-1) (an almost 2 sigma discrepancy). The bulk velocity analysis reported here is virtually unaffected by the Malmquist bias and very similar results are obtained for the data with and without the bias correction.